Interpreted Languages: JavaScript, PHP, Python, Ruby (Sheet Two)

a side-by-side reference sheet

sheet one: grammar and invocation | variables and expressions | arithmetic and logic | strings | regexes | dates and time | arrays | dictionaries | functions | execution control | exceptions | concurrency

sheet two: file handles | files | file formats | directories | processes and environment | option parsing | libraries and namespaces | objects | inheritance and polymorphism | reflection | net and web | unit tests | debugging and profiling | java interop

file handles
node php python ruby
standard file handles process.stdin
only set by CLI; not set when reading script from stdin:
sys.stdin sys.stdout sys.stderr $stdin $stdout $stderr
read line from stdin
$line = fgets(STDIN); line = sys.stdin.readline() line = gets
end-of-file behavior returns string without newline or FALSE returns string without newline or '' returns non-empty string without newline or raises EOFError
chop($line); line = line.rstrip('\r\n') line.chomp!
write line to stdout
console.log("Hello, World"); echo "Hello, World!\n"; print('Hello, World!') puts "Hello, World!"
write formatted string to stdout printf("%.2f\n", M_PI); import math

print('%.2f' % math.pi)
printf("%.2f\n", Math::PI)
open file for reading
$f = fopen("/etc/hosts", "r"); f = open('/etc/hosts') f ="/etc/hosts")
open file for writing
var f = fs.openSync("/tmp/test", "w"); $f = fopen("/tmp/test", "w"); f = open('/tmp/test', 'w') f ="/tmp/test", "w")
set file handle encoding import codecs

fin ='/tmp/foo',

fout ='/tmp/bar', 'w',
fin ="/tmp/foo", "r:utf-8")

fout ="/tmp/bar", "w:utf-8")
open file for appending $f = fopen("/tmp/test", "a"); f = open('/tmp/err.log', 'a') f ="/tmp/err.log", "a")
close file
fclose($f); f.close() f.close
close file implicitly none with open('/tmp/test', 'w') as f:
  f.write('lorem ipsum\n')"/tmp/test", "w") do |f|
  f.puts("lorem ipsum")
i/o error return false value and write warning to stderr raise IOError exception raise IOError or subclass of SystemCallError exception
encoding error raise UnicodeDecodeError on read; raise UnicodeEncodeError on write
read line
$line = fgets($f); f.readline() f.gets
iterate over file by line
while (!feof($f)) {
  $line = fgets($f);
  echo $line;
for line in f:
f.each do |line|
read file into array of strings $a = file("/etc/hosts"); a = f.readlines() a = f.lines.to_a
read file into string var fs = require('fs');

var s = fs.readFileSync('/etc/hosts', 'utf8');
$s = file_get_contents(
s = s =
write string
fwrite($f, "lorem ipsum"); f.write('lorem ipsum') f.write("lorem ipsum")
write line
fwrite($f, "lorem ipsum\n"); f.write('lorem ipsum\n') f.puts("lorem ipsum")
flush file handle
CLI output isn't buffered
f.flush() f.flush
end-of-file test
feof($f) none f.eof?
get and set file handle position ftell($f)
fseek($f, 0);

f.pos = 0
open temporary file $f = tmpfile();

fwrite($f, "lorem ipsum\n");

# no way to get file name


file is removed when file handle is closed
import tempfile

f = tempfile.NamedTemporaryFile()

f.write('lorem ipsum\n')

print("tmp file: %s" %


file is removed when file handle is closed
require 'tempfile'

f ='')

f.puts "lorem ipsum"

puts "tmp file: #{f.path}"


file is removed when file handle is garbage-collected or interpreter exits
in memory file $meg = 1024 * 1024;
$mem = "php://temp/maxmemory:$meg";
$f = fopen($mem, "r+");
fputs($f, "lorem ipsum");
$s = fread($f, $meg);
from StringIO import StringIO

f = StringIO()
f.write('lorem ipsum\n')
s = f.getvalue()

Python 3 moved StringIO to the io module
require 'stringio'

f =
f.puts("lorem ipsum")
s =
node php python ruby
file exists test, file regular test
var fs = require('fs');

var qry = fs.existsSync('/etc/hosts');

var stat = fs.statSync('/etc/hosts');
var qry2 = stat.isFile();
file size
var fs = require('fs');

var stat = fs.statSync('/etc/hosts');
var sz = stat.size;
filesize("/etc/hosts") os.path.getsize('/etc/hosts') File.size("/etc/hosts")
is file readable, writable, executable is_readable("/etc/hosts")
os.access('/etc/hosts', os.R_OK)
os.access('/etc/hosts', os.W_OK)
os.access('/etc/hosts', os.X_OK)
set file permissions
var fs = require('fs');

  parseInt('755', 8));
chmod("/tmp/foo", 0755); os.chmod('/tmp/foo', 0755) File.chmod(0755, "/tmp/foo")
last modification time var fs = require('fs');

var stat = fs.statSync('/etc/hosts');
var dt = stat.mtime;
# unix epoch:
$t = stat('/etc/passwd')['mtime'];

# DateTime object:
$t2 = new DateTime('UTC');
from datetime import datetime as dt

# unix epoch:
t = os.stat('/etc/passwd').st_mtime

# datetime object:
t2 = dt.fromtimestamp(t)
# Time object:
t2 = File.stat('/etc/passwd').mtime

# unix epoch:
t = t2.to_i
copy file, remove file, rename file # npm install fs-extra
var fs = require('fs-extra');

fs.copySync('/tmp/foo', '/tmp/bar');
fs.renameSync('/tmp/bar', '/tmp/foo');
copy("/tmp/foo", "/tmp/bar");
rename("/tmp/bar", "/tmp/foo");
import shutil

shutil.copy('/tmp/foo', '/tmp/bar')
shutil.move('/tmp/bar', '/tmp/foo')
require 'fileutils'

FileUtils.cp("/tmp/foo", "/tmp/bar")
FileUtils.rm("/tmp/foo")"/tmp/bar", "/tmp/foo")
create symlink, symlink test, readlink var fs = require('fs');

var stat = fs.statSync('/tmp/hosts');
var path = fs.readlinkSync(
symlink("/etc/hosts", "/tmp/hosts");
generate unused file name $path = tempnam("/tmp", "foo");
$f = fopen($path, "w");
import tempfile

f, path = tempfile.mkstemp(
file formats
node php python ruby
parse csv var fs = require('fs');
# npm install csv
var csv = require('csv');

var s = fs.readFileSync('no-header.csv');
var a;
csv().from.string(s).to.array(function(d) { a = d });
$f = fopen("no-header.csv", "r");
while (($row = fgetcsv($f)) != FALSE) {
  echo implode("\t", $row) . "\n";
import csv

with open('foo.csv') as f:
  cr = csv.reader(f)
  for row in cr:
require 'csv'

CSV.foreach("foo.csv") do |row|
  puts row.join("\t")
generate csv # npm install csv
var csv = require('csv');

var a = [['one', 'une', 'uno'],
['two', 'deux', 'dos']];

var s;
csv().from.array(a).to.string(function (o) { s = o; });
import csv

with open('foo.csv', 'w') as f:
  cw = csv.writer(f)
  cw.writerow(['one', 'une', 'uno'])
  cw.writerow(['two', 'deux', 'dos'])
require 'csv'"foo.csv", "w") do |csv|
  csv << ["one", "une", "uno"]
  csv << ["two", "deux", "dos"]
json generate/parse var d1 = {'t': 1, 'f': 0};
var s = JSON.stringify(d1);
var d2 = JSON.parse(s);
$a = array("t" => 1, "f" => 0);
$s = json_encode($a);
$d = json_decode($s, TRUE);
import json

s = json.dumps({'t': 1, 'f': 0})
d = json.loads(s)
require 'json'

s = {'t' => 1,'f' => 0}.to_json
d = JSON.parse(s)
generate xml # npm install xmlbuilder
var builder = require('xmlbuilder');

var xml = builder.create('a').ele('b', {id: 123}, 'foo').end();
$xml = "<a></a>";
$sxe = new SimpleXMLElement($xml);
$b = $sxe->addChild("b", "foo");
$b->addAttribute("id", "123");

# <a><b id="123">foo</b></a>:
echo $sxe->asXML();
import xml.etree.ElementTree as ET

builder = ET.TreeBuilder()
builder.start('a', {})
builder.start('b', {'id': '123'})'foo')
et = builder.close()

# <a><b id="123">foo</b></a>:
# gem install builder
require 'builder'

builder =
xml = builder.a do |child|
  child.b("foo", :id=>"123")

# <a><b id="123">foo</b></a>:
puts xml
parse xml
all nodes matching xpath query; first node matching xpath query
# npm install xmldom xpath
var dom = require('xmldom').DOMParser;
var xpath = require('xpath');

var xml = '<a><b><c ref="3">foo</c></b></a>';
var doc = new dom().parseFromString(xml);
var nodes ='/a/b/c', doc);
$xml = "<a><b><c ref='3'>foo</c></b></a>";

# returns NULL and emits warning if not
# well-formed:

$doc = simplexml_load_string($xml);

$nodes = $doc->xpath("/a/b/c");
echo count($nodes);
echo $nodes[0];

$node = $nodes[0];
echo $node;
echo $node["ref"];
from xml.etree import ElementTree

xml = '<a><b><c ref="3">foo</c></b></a>'

# raises xml.etree.ElementTree.ParseError
# if not well-formed:

doc = ElementTree.fromstring(xml)

nodes = doc.findall('b/c')

node = doc.find('b/c')
require 'rexml/document'
include REXML

xml = "<a><b><c ref='3'>foo</c></b></a>"

# raises REXML::ParseException if
# not well-formed:

doc =

nodes = XPath.match(doc,"/a/b/c")
puts nodes.size
puts nodes[0].text

node = XPath.first(doc,"/a/b/c")
puts node.text
puts node.attributes["ref"]
parse html $html = file_get_contents("foo.html");
$doc = new DOMDocument;
$xpath = new DOMXPath($doc);

$nodes = $xpath->query("//a/@href");
foreach($nodes as $href) {
  echo $href->nodeValue;
# pip install beautifulsoup4
import bs4

html = open('foo.html').read()
doc = bs4.BeautifulSoup(html)

for link in doc.find_all('a'):
# gem install nokogiri
require 'nokogiri'

html ="foo.html").read
doc = Nokogiri::HTML(html)
doc = doc.xpath("//a").each do |link|
  puts link["href"]
node php python ruby
working directory var old_dir = process.cwd();

$old_dir = getcwd();

old_dir = os.path.abspath('.')

old_dir = Dir.pwd

build pathname "/etc" . DIRECTORY_SEPARATOR . "hosts" os.path.join('/etc', 'hosts') File.join("/etc", "hosts")
dirname and basename dirname("/etc/hosts")
absolute pathname
and tilde expansion
# file must exist; symbolic links are
# resolved:

# no function for tilde expansion
# symbolic links are not resolved:
# symbolic links are not resolved:
iterate over directory by file if ($dir = opendir("/etc")) {
  while ($file = readdir($dir)) {
    echo "$file\n";
for filename in os.listdir('/etc'):
  print(filename)"/etc").each do |file|
  puts file
glob paths foreach (glob("/etc/*") as $file) {
  echo "$file\n";
import glob

for path in glob.glob('/etc/*'):
Dir.glob("/etc/*").each do |path|
  puts path
make directory mkdir("/tmp/foo/bar", 0755, TRUE); dirname = '/tmp/foo/bar'
if not os.path.isdir(dirname):
require 'fileutils'

recursive copy none import shutil

require 'fileutils'

remove empty directory rmdir("/tmp/foodir"); os.rmdir('/tmp/foodir') File.rmdir("/tmp/foodir")
remove directory and contents none import shutil

require 'fileutils'

directory test
is_dir("/tmp") os.path.isdir('/tmp')"/tmp")
generate unused directory import tempfile

path = tempfile.mkdtemp(dir='/tmp',
require 'tmpdir'

path = Dir.mktmpdir("/tmp/foo")
system temporary file directory sys_get_temp_dir() import tempfile

require 'tmpdir'

processes and environment
node php python ruby
command line arguments
and script name
// process.argv[0] contains "node"
get and set environment variable

process.env["PATH"] = "/bin";


os.environ['PATH'] = '/bin'

ENV["PATH"] = "/bin"
get pid, parent pid
get user id and name $uid = posix_getuid();
$uinfo = posix_getpwuid($uid);
$username = $uinfo["name"];
import getpass

require 'etc'

process.exit(0); exit(0); sys.exit(0) exit(0)
set signal handler
import signal

def handler(signo, frame):

signal.signal(signal.SIGINT, handler)
  lambda do |signo|
    puts "exiting..."
    exit 1
executable test
is_executable("/bin/ls") os.access('/bin/ls', os.X_OK) File.executable?("/bin/ls")
external command
system("ls -l /tmp", $retval);
if ($retval) {
  throw new Exception("ls failed");
if os.system('ls -l /tmp'):
  raise Exception('ls failed')
unless system("ls -l /tmp")
  raise "ls failed"
escaped external command
$path = chop(fgets(STDIN));
$safe = escapeshellarg($path);
system("ls -l " . $safe, $retval);
if ($retval) {
  throw new Exception("ls failed");
import subprocess

cmd = ['ls', '-l', '/tmp']
  raise Exception('ls failed')
path = gets
unless system("ls", "-l", path)
  raise "ls failed"
$files = `ls -l /tmp`; import subprocess

cmd = ['ls', '-l', '/tmp']
files = subprocess.check_output(cmd)
files = `ls -l /tmp`
unless $?.success?
  raise "ls failed"

files = %x(ls)
unless $?.success?
  raise "ls failed"
option parsing
node php python ruby
command line options
boolean option, option with argument, usage
$usage = "usage: " .
  " [-f FILE] [-v] [ARG ...]\n";

$opts = getopt("f:hv",
  array("file:", "help", "verbose"));

if (array_key_exists("h", $opts) ||
    array_key_exists("help", $opts)) {
  echo $usage;

$file = $opts["f"] ? $opts["f"] :

if (array_key_exists("v", $opts) ||
    array_key_exists("verbose", $opts)) {
  $verbose = TRUE;

# Processing stops at first positional
# argument.
# Unrecognized options are ignored.
# An option declared to have an argument
# is ignored if the argument is not
# provided on the command line.
# getopt() does not modify $argv or
# provide means to identify positional
# arguments.
import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--file', '-f',
parser.add_argument('--verbose', '-v',

args = parser.parse_args()

the_file = args.file
verbose = args.verbose

# The flags -h and --help and the
# usage message are generated
# automatically.
# Positional arguments are in
# args.positional_args
# Options can follow positional arguments.
require 'optparse'

options = {} do |opts|
  opts.banner =
    "usage: #{$0} [OPTIONS] [ARG ...]"

  opts.on("-f", "--file FILE") do |arg|
    options[:file] = arg

  opts.on("-v", "--verbose") do |arg|
    options[:verbose] = arg

file = options[:file]
verbose = options[:verbose]

# The flags -h and --help and the
# usage message are generated
# automatically.
# After calling OptionParser.parse! only
# positional arguments are in ARGV.
# Options can follow positional args.
libraries and namespaces
node php python ruby
load library
require_once("foo.php"); # searches sys.path for foo.pyc or
import foo
require 'foo.rb'

# searches $LOAD_PATH for foo.rb,,
# foo.o, foo.dll:

require 'foo'
load library in subdirectory require_once('foo/bar.php'); # foo must contain file
require 'foo/bar.rb'

require 'foo/bar'
hot patch
require("foo.php"); reload(foo) load 'foo.rb'
load error require and require_once raise fatal error if library not found; include and include_once emit warnings raises ImportError if library not found; exceptions generated when parsing library propagate to client raises LoadError if library not found; exceptions generated when parsing library propagate to client
main routine in library none if __name__ == '__main__':
if $0 == __FILE__
library path $libpath = ini_get("include_path");

  $libpath . ":/some/path");

# $: is synonym for $LOAD_PATH:

<< "/some/path"
library path environment variable none $ PYTHONPATH=~/lib python $ RUBYLIB=~/lib ruby foo.rb
library path command line option none none $ ruby -I ~/lib foo.rb
simple global identifiers variables defined outside of functions or with global keyword built-in functions variables which start with $
multiple label identifiers classes, interfaces, functions, and constants modules constants, classes, and modules
label separator
\Foo\Bar\baz(); Foo::Bar.baz
root namespace definition \foo none # outside of class or module; only
# constants in root namespace:

FOO = 3

# inside class or module:
::FOO = 3
namespace declaration
namespace Foo; put declarations in class Foo
  # class definition

module Foo
  # module definition
child namespace declaration namespace Foo\Bar; foo must be in sys.path:
$ mkdir foo
$ touch foo/
$ touch foo/
module Foo::Bar
  # module definitions

module Foo
  module Bar
    # module definitions

# classes can nest inside classes or
# modules; modules can nest in classes
namespace alias use Foo as Fu; import foo as fu Fu = Foo.dup

include Fu
unqualified import of namespace
none, but a long module name can be shortened from foo import * # inside class or module:
include Foo
unqualified import of all subnamespaces # subnamespaces in list __all__ of
# foo/ are imported

from foo import *
unqualified import of definitions
only class names can be imported from foo import bar, baz none
list installed packages, install a package
$ pear list
$ pear install Math_BigInteger
$ pip freeze
$ pip install jinja2
$ gem list
$ gem install rails
package specification format in

#!/usr/bin/env python

from distutils.core import setup

  author='Joe Foo',
  description='a package',
in foo.gemspec:

spec = do |s| = "foo"
  s.authors = "Joe Foo"
  s.version = "1.0"
  s.summary = "a gem"
  s.files = Dir["lib/*.rb"]
javascript php python ruby
define class
class Int
  public $value;
  function construct($int=0)
    $this->value = $int;
class Int(object):
  def __init__(self, v=0):
    self.value = v
class Int
  attr_accessor :value
  def initialize(i=0)
    @value = i
create object
$i = new Int();
$i2 = new Int(7);
i = Int()
i2 = Int(7)
i =
i2 =
instance variable visibility visibility must be declared public; attributes starting with underscore private by convention private by default; use attr_reader, attr_writer, attr_accessor to make public
get and set instance variable
$v = $i->value;
$i->value = $v + 1;
v = i.value
i.value = v + 1
v = i.value
i.value = v + 1
define method
function plus($i)
  return $this->value + $i;
def plus(self,v):
  return self.value + v
def plus(i)
  value + i
invoke method
define class method @classmethod
def get_instances(cls):
  return Counter.instances
class Foo
    puts "one"
invoke class method
Counter::getInstances() Counter.get_instances()
define class variable class Foo:
  instances = 0
class Foo
  @@instances = 1
get and set class variable class Foo:
  def __init(self):
    Foo.instances += 1
class Foo
  def initialize
    @@instances += 1
handle undefined method invocation
function __call($name, $args)
  $argc = count($args);
  echo "no def: $name " .
    "arity: $argc\n";
def __getattr__(self, name):
  s = 'no def: ' + name + ' arity: %d'

  return lambda *a: print(s % len(a))
def method_missing(name, *a)
  puts "no def: #{name}" +
    " arity: #{a.size}"
alias method class Point

  attr_reader :x, :y, :color

  alias_method :colour, :color

  def initialize(x, y, color=:black)
    @x, @y = x, y
    @color = color
function __destruct()
  echo "bye, $this->value\n";
def __del__(self):
  print('bye, %d' % self.value)
val = i.value
ObjectSpace.define_finalizer(int) {
  puts "bye, #{val}"
inheritance and polymorphism
javascript php python ruby
class Counter extends Int
  private static $instances = 0;
  function __construct($int=0)
    Counter::$instances += 1;
  function incr()
  static function getInstances()
    return $instances;
class Counter(Int):

  instances = 0

  def __init__(self, v=0):
    Counter.instances += 1
    Int.__init__(self, v)

  def incr(self):
    self.value += 1
class Counter < Int

  @@instances = 0

  def initialize
    @@instances += 1

  def incr
    self.value += 1

  def self.instances
overload operator class Fixnum
  def /(n)
javascript php python ruby
object id
none id(o) o.object_id
inspect type
typeof([]) === 'object' gettype(array()) == "array"

returns object for objects
type([]) == list [].class == Array
basic types number

# these evaluate as 'object':
unknown type
inspect class // returns prototype object:
returns FALSE if not an object:
get_class($o) == "Foo"
o.__class__ == Foo
isinstance(o, Foo)
o.class == Foo
inspect class hierarchy var pa = Object.getPrototypeOf(o)
//prototype's of prototype object:
var grandpa = Object.getPrototypeOf(pa)
get_parent_class($o) o.__class__.__bases__ o.class.superclass
has method?
o.reverse && typeof(o.reverse) === 'function' method_exists($o, "reverse") hasattr(o, 'reverse') o.respond_to?("reverse")
message passing
not a standard feature for ($i = 1; $i <= 10; $i++) {
    "phone$i"), NULL);
for i in range(1,10):
  getattr(o, 'phone'+str(i))(None)
(1..9).each do |i|
  o.send("phone#{i}=", nil)
eval('1 + 1') eval evaluates to argument of return statement or NULL:
while ($line = fgets(STDIN)) {
  echo eval($line) . "\n";
argument of eval must be an expression:
while True:
loop do
  puts eval(gets)
list object methods
get_class_methods($o) [m for m in dir(o)
  if callable(getattr(o,m))]
list object attributes
get_object_vars($o) dir(o) o.instance_variables
list loaded libraries # relative to directory in lib path:
list loaded namespaces dir() do |c|
  Module.const_get(c).class == Class
inspect namespace import urlparse

require 'uri'

pretty print
$d = array("lorem"=>1,

import pprint

d = {'lorem':1, 'ipsum':[2,3]}

require 'pp'

d = {"lorem"=>1, "ipsum"=>[2,3]}

pp d
source line number and file name __LINE__
import inspect

cf = inspect.currentframe()
command line documentation none $ pydoc math
$ pydoc math.atan2
$ ri -c
$ ri Math
$ ri Math.atan2
net and web
javascript php python ruby
get local hostname, dns lookup, reverse dns lookup $host = gethostname();
$ip = gethostbyname($host);
$host2 = gethostbyaddr($ip);
import socket

host = socket.gethostname()
ip = socket.gethostbyname(host)
host2 = socket.gethostbyaddr(ip)[0]
require 'socket'

hostname = Socket.gethostname

ip = Socket.getaddrinfo(

host2 = Socket.gethostbyaddr(ip)[0]
http get
$url = '';
$s = file_get_contents($url);
import httplib

url = ''
conn = httplib.HTTPConnection(url)
conn.request("GET", '/')
resp = conn.getresponse()
if resp.status == httplib.OK:
  s =
require 'net/http'

url = ""
r = Net::HTTP.start(url, 80) do |f|
if r.code == "200"
  s = r.body
http post
import httplib
import urllib

url = ''
conn = httplib.HTTPConnection(url)
data = urllib.urlencode({
  'item': 'anvil',
  'qty': 1})
conn.request('POST', '/orders', data)
resp = conn.getresponse()
if resp.status == httplib.OK:
  s =
serve working directory $ php -S localhost:8000 $ python -m SimpleHTTPServer 8000 $ ruby -rwebrick -e \
' => 8000, '\
':DocumentRoot => Dir.pwd).start'
absolute url
from base and relative url
none import urlparse

require 'uri'

URI.join("", "analytics")
parse url $url = "";
$up = parse_url($url);

$protocol = $up["scheme"];
$hostname = $up["host"];
$port = $up["port"];
$path = $up["path"];
$query_str = $up["query"];
$fragment = $up["fragment"];

# $params is associative array; if keys
# are reused, later values overwrite
# earlier values

parse_str($query_str, $params);
# Python 3 location: urllib.parse
import urlparse

url = ''
up = urlparse.urlparse(url)

protocol = up.scheme
hostname = up.hostname
port = up.port
path = up.path
query_str = up.query
fragment = up.fragment

# returns dict of lists:
params = urlparse.parse_qs(query_str)
require 'uri'

url = ""
up = URI(url)

protocol = up.scheme
hostname =
port = up.port
path = up.path
query_str = up.query
fragment = up.fragment

# Ruby 1.9; returns array of pairs:
params = URI.decode_www_form(query_str)
url encode/decode
urlencode("lorem ipsum?")
# Python 3 location: urllib.parse
import urllib

urllib.quote_plus("lorem ipsum?")
require 'cgi'

CGI::escape("lorem ipsum?")
html escape
escape character data, escape attribute value, unescape html entities
$s = htmlspecialchars("<>&");

$s2 = htmlspecialchars("<>&\"'",

$s3 = htmlspecialchars_decode($s2);
import cgi
from HTMLParser import HTMLParser

s = cgi.escape('<>&')
s2 = cgi.escape('<>&"', True)
s3 = HTMLParser().unescape(
require 'cgi'

s2 = CGI.escapeHTML('<>&"')
s3 = CGI.unescapeHTML(s2)
base64 encode/decode $s = file_get_contents("foo.png");
$b64 = base64_encode($s);
$s2 = base64_decode($b64);
import base64

s = open('foo.png').read()
b64 = base64.b64encode(s)
s2 = base64.b64decode(b64)
require 'base64'

s ="foo.png").read
b64 = Base64.encode64(s)
s2 = Base64.decode64(b64)
unit tests
node php python ruby
test class // npm install -g nodeunit

exports.testFoo = function(test) {
  test.ok(true, 'not true!.');
# pear install


Class FooTest extends
  public function test_01()
      "not true!");
import unittest

class TestFoo(unittest.TestCase):
  def test_01(self):
    self.assertTrue(True, 'not True!')

if __name__ == '__main__':
require 'test/unit'

class TestFoo < Test::Unit::TestCase
  def test_01
    assert(true, "not true!")
run tests, run test method $ nodeunit test_foo.js

$ nodeunit -t testFoo test_foo.js
$ phpunit test_foo.php

$ phpunit --filter test_01 test_foo.php
$ python
$ python TestFoo.test_01
$ ruby test_foo.rb
$ ruby test_foo.rb -n test_01
equality assertion var s = 'do re mi';
test.equals(s, 'do re mi');
$s = "do re me";
$this->assertEquals($s, "do re mi");

# also asserts args have same type:
$this->assertSame($s, "do re mi");
s = 'do re me'
self.assertEqual('do re me',
  's: {}'.format(s))
s = "do re me"
assert_equal("do re me", s)
approximate assertion $x = 10.0 * (1.0 / 3.0);
$y = 10.0 / 3.0;

$this->assertEquals($x, $y,
  "not within delta",
  pow(0.1, 6));
x = 10.0 * (1.0 / 3.0)
y = 10.0 / 3.0

# default for delta is 0.1**7
self.assertAlmostEqual(x, y, delta=0.1**6)
x = 10.0 * (1.0 / 3.0)
y = 10.0 / 3.0

# default for delta is 0.001
assert_in_delta(x, y, 0.1**6)
regex assertion $s = "lorem ipsum";
$this->assertRegExp("/lorem/", $s);
s = 'lorem ipsum'
# uses, not re.match:
self.assertRegexpMatches(s, 'lorem')
s = "lorem ipsum"
assert_match(/lorem/, s)
exception assertion class Bam extends Exception {};

public function test_exc {
  throw new Bam("bam!");
a = []
with self.assertRaises(IndexError):
assert_raises(ZeroDivisionError) do
  1 / 0
mock method $mock = $this->getMock('Foo', ['foo']);

# pip install mock
import mock

foo = Foo() = mock.MagicMock(return_value=7)

# gem install mocha
require 'mocha'

foo = mock()
setup exports.setUp = function(callback) {
  console.log('setting up...');
public function setUp()
  echo "setting up\n";
# in class TestFoo:
def setUp(self):
  print('setting up')
# in class TestFoo:
def setup
  puts "setting up"
teardown exports.tearDown = function(callback) {
  console.log('tearing down...');
public function tearDown()
  echo "tearing down\n";
# in class TestFoo:
def tearDown(self):
  print('tearing down')
# in class TestFoo:
def teardown
  puts "tearing down"
debugging and profiling
javascript php python ruby
check syntax
$ php -l foo.php import py_compile

# precompile to bytecode:
$ ruby -c foo.rb
flags for stronger and strongest warnings none $ python -t
$ python -3t
$ ruby -w foo.rb
$ ruby -W2 foo.rb
lint $ sudo pip install pylint
$ pylint
source cleanup $ sudo pip install pep8
$ pep8
run debugger $ python -m pdb $ sudo gem install ruby-debug
$ rdebug foo.rb
debugger commands h l n s b c w u d p q h l n s b c w u down p q
benchmark code import timeit

timeit.timeit('i += 1',
  'i = 0',
require 'benchmark'

n = 1_000_000
i = 0
puts Benchmark.measure do
  n.times { i += 1 }
profile code $ python -m cProfile $ sudo gem install ruby-prof
$ ruby-prof foo.rb
java interoperation
javascript php python ruby
Jython 2.5
compatible with Python 2.5 :(
JRuby 1.7
compatible with Ruby 1.9
$ jython $ jirb
$ jython $ jruby
none $ jrubyc
import java none
rnd = java.util.Random() rnd =
rnd.nextFloat() rnd.next_float
from java.util import Random

rnd = Random()
java_import java.util.Random
rnd =
non-bundled java libraries
import sys

import MyClass
require 'path/to/mycode.jar'
shadowing avoidance
import as javaio module JavaIO
  include_package ""
convert native array to java array
import jarray

jarray.array([1, 2, 3], 'i')
[1, 2, 3].to_java(Java::int)
are java classes subclassable?
yes yes
are java class open?
no yes
____________________________________________ ____________________________________________ ____________________________________________ ____________________________________________

File Handles

standard file handles

The names for standard input, standard output, and standard error.

read line from stdin

How to read a line from standard input.

The illustrated function read the standard input stream until a end-of-line marker is found or the end of the stream is encountered. Only in the former case will the returned string be terminated by an end-of-line marker.


fgets takes an optional second parameter to specify the maximum line length. If the length limit is encountered before a newline, the string returned will not be newline terminated.


gets takes an optional parameter to specify the maximum line length. If the length limit is encountered before a newline, the string returned will not be newline terminated.

end-of-file behavior

What happens when attempting to read a line and the seek point is after the last newline or at the end of the file.


Remove a newline, carriage return, or carriage return newline pair from the end of a line if there is one.


chop removes all trailing whitespace. It is an alias for rtrim.


Python strings are immutable. rstrip returns a modified copy of the string. rstrip('\r\n') is not identical to chomp because it removes all contiguous carriage returns and newlines at the end of the string.


chomp! modifies the string in place. chomp returns a modified copy.

write line to stdout

How to write a line to standard out. The line will be terminated by an operating system appropriate end of line marker.


print appends a newline to the output. To suppress this behavior, put a trailing comma after the last argument. If given multiple arguments, print joins them with spaces.

In Python 2 print parses as a keyword and parentheses are not required:

print "Hello, World!"


puts appends a newline to the output. print does not.

write formatted string to stdout

How to format variables and write them to standard out.

The function printf from the C standard library is a familiar example. It has a notation for format strings which uses percent signs %. Many other languages provide an implementation of printf.

open file for reading

How to open a file for reading.


When is given a block, the file is closed when the block terminates.

open file for writing

How to open a file for writing. If the file exists its contents will be overwritten.

set file handle encoding

How to open a file and specify the character encoding.


The encoding of a file handle can be changed after it is opened:

find.encoding = 'UTF-8'

If the encoding is set to UTF-8, but the file contains byte sequences that are not a possible UTF-8 encoding, Python will raise a UnicodeDecodeError.

open file for appending

How to open a file with the seek point at the end of the file. If the file exists its contents will be preserved.

close file

How to close a file.

close file implicitly

How to have a file closed when a block is exited.


File handles are closed when the variable holding them is garbage collected, but there is no guarantee when or if a variable will be garbage collected.


File handles are closed when the variable holding them is garbage collected, but there is no guarantee when or if a variable will be garbage collected.

i/o error

How I/O errors are treated.

encoding error

read line

How to read up to the next newline in a file.

iterate over file by line

How to iterate over a file line by line.

read file into array of string

How to put the lines of a file into an array of strings.

read file into string

How to put the contents of a file into a single string.

write string

How to write a string to a file handle.

write line

How to write a line to a file handle. An operating system appropriate end-of-line marker is appended to the output.


Newlines in strings are translated to the operating system appropriate line terminator unless the file handle was opened with a mode string that contained 'b'.


When file handles are opened with the mode strings 'r', 'w', or 'a', the file handle is in text mode. In text mode the operating system line terminator is translated to '\n' when reading and '\n' is translated back to the operating system line terminator when writing. The standard file handles sys.stdin, sys.stdout, and sys.stderr are opened in text mode.

When file handles are opened with the mode strings 'rb', 'rw', or 'ra', the file handle is in binary mode and line terminator translation is not performed. The operating system line terminator is available in os.linesep.

flush file handle

How to flush a file handle that has been written to.

end-of-file test

How to test whether the seek point of a file handle is at the end of the file.

get and set file handle position

How to get or set the file handle seek point.

The seek point is where the next read on the file handle will begin. The seek point is measured in bytes starting from zero.

open temporary file

How to get a file handle to a file that will be removed automatically sometime between when the file handle is closed and the interpreter exits.

The file is guaranteed not to have existed before it was opened.

The file handle is opened for both reading and writing so that the information written to the file can be recovered by seeking to the beginning of the file and reading from the file handle.

On POSIX operating systems it is possible to unlink a file after opening it. The file is removed from the directory but continues to exist as long as the file handle is open. This guarantees that no other process will be able to read or modify the file contents.


Here is how to create a temporary file with a name:

$path = tempnam(sys_get_temp_dir(), "");
$f = fopen($path, "w+");


To unlink a temporary file on open, used TemporaryFile instead of NamedTemporaryFile:

import tempfile

f = tempfile.TemporaryFile()

in memory file

How to create a file descriptor which writes to an in-memory buffer.


StringIO also supports the standard methods for reading input. To use them the client must first seek to the beginning of the in-memory file:

f = StringIO()
f.write('lorem ipsum\n')


file exists test, file regular test

How to test whether a file exists; how to test whether a file is a regular file (i.e. not a directory, special device, or named pipe).

file size

How to get the file size in bytes.

is file readable, writable, executable

How to test whether a file is readable, writable, or executable.


The flags can be or'ed to test for multiple permissions:

os.access('/etc/hosts', os.R_OK | os.W_OK | os.X_OK)

set file permissions

How to set the permissions on the file.

For Perl, Python, and Ruby, the mode argument is in the same format as the one used with the Unix chmod command. It uses bitmasking to get the various permissions which is why it is normally an octal literal.

The mode argument should not be provided as a string such as "0755". Python and Ruby will raise an exception if a string is provided. Perl will convert "0755" to 755 and not 0755 which is equal to 493 in decimal.

last modification time

How to get the last modification time of a file.

For a regular file, the last modification time is the most recent time that the contents were altered.

For a directory, the last modification time is the most recent time that a file in the directory was added, removed, or renamed.

copy file, remove file, rename file

How to copy a file; how to remove a file; how to rename a file.

create symlink, symlink test, readlink

How to create a symlink; how to test whether a file is a symlink; how to get the target of a symlink.

generate unused file name

How to generate an unused file name. The file is created to avoid a race condition with another process looking for an unused file name.

The file is not implicitly deleted.

File Formats

parse csv

How to parse a CSV file and iterate through the rows.

generate csv

How to generate a CSV file from an array of tuples.

json generate/parse

How to encode JSON data in a JSON string; how to decode such a string.

JSON data consists of objects, arrays, and JSON values. Objects are dictionaries in which the keys are strings and the values are JSON values. Arrays contain JSON values. JSON values can be objects, arrays, strings, numbers, true, false, or null.

A JSON string is JSON data encoded using the corresponding literal notation used by JavaScript source code.

JSON strings are sequences of Unicode characters. The following backslash escape sequences are supported:

  \" \\ \/ \b \f \n \r \t \uhhhh.

generate xml

How to build an XML document.

An XML document can be constructed by concatenating strings, but the techniques illustrated here guarantee the result to be well-formed XML.

parse xml

How to parse XML and extract nodes using XPath.


Another way of handling an XPath expression which matches multiple nodes:

XPath.each(doc,"/a/b/c") do |node|
  puts node.text

parse html

How to parse an HTML document.


working directory

How to get and set the working directory.

build pathname

How to construct a pathname without hard coding the system file separator.

dirname and basename

How to extract the directory portion of a pathname; how to extract the non-directory portion of a pathname.

absolute pathname

How to get the get the absolute pathname for a pathname. If the pathname is relative the working directory will be appended.

In the examples provided, if /foo/bar is the working directory and .. is the relative path, then the return value is foo

iterate over directory by file

How to iterate through the files in a directory.

In PHP, Perl, and Ruby, the files representing the directory itself . and the parent directory .. are returned.


The code in the example will stop if a filename which evaluates as FALSE is encountered. One such filename is "0". A safer way to iterate through the directory is:

if ($dir = opendir("/etc")) {
  while (FALSE !== ($file = readdir($dir))) {
    echo "$file\n";


file() is the file handle constructor. file can be used as a local variable name but doing so hides the constructor. It can still be invoked by the synonym open(), however.

os.listdir() does not return the special files . and .. which represent the directory itself and the parent directory.

glob paths

How to iterate over files using a glob pattern.

Glob patterns employ these special characters:

* matches zero or more characters, the first of which is not . and none of which is /
? matches one character
[ ] matches one character from the list inside the brackets
\ escapes one of the previous characters

Use glob patterns instead of simple directory iteration when

  • dot files, including the directory itself (.) and the parent directory (..), should skipped
  • a subset of the files in a directory, where the subset can be specified with a glob pattern, is desired
  • files from multiple directories, where the directories can be specified with a glob pattern, are desired
  • the full pathnames of the files is desired


glob takes a second argument for flags. The flag GLOB_BRACE enables brace notation.


glob.glob returns a list. glob.iglob accepts the same arguments and returns an iterator.


Ruby globs support brace notation.

A brace expression matches any of the comma separated strings inside the braces.

Dir.glob("/{bin,etc,usr}/*").each do |path|
  puts path

make directory

How to create a directory.

If needed, the examples will create more than one directory.

No error will result if a directory at the pathname already exists. An exception will be raised if the pathname is occupied by a regular file, however.

recursive copy

How to perform a recursive copy. If the source is a directory, then the directory and all its contents will be copied.

remove empty directory

How to remove an empty directory. The operation will fail if the directory is not empty.

remove directory and contents

How to remove a directory and all its contents.

directory test

How to determine if a pathname is a directory.

generate unused directory

How to generate an unused directory. The directory is created to avoid a race condition with another process looking for an unused directory.

The directory is not implicitly deleted.


When Dir.mktmpdir is provided with a block the directory is deleted after the block finishes executing:

require 'tmpdir'
require 'fileutils'

Dir.mktmpdir("/tmp/foo") do |path|
  puts path
  FileUtils.cp("/etc/hosts", "#{path}/hosts")

system temporary file directory

The name of the system provided directory for temporary files.

On Linux the directory is often /tmp, and the operating system is often configured to delete the contents of /tmp at boot.

Processes and Environment

command line arguments

How to access arguments provided at the command line when the script was run; how to get the name of the script.

get and set environment variable

How to get and set an environment variable. If an environment variable is set the new value is inherited by child processes.


putenv returns a boolean indicating success. The command can fail because when PHP is running in safe mode only some environment variables are writable.

get pid, parent pid

How to get the process id of the interpreter process; how to get the id of the parent process.


The process pid is also available in the global variable $$.

get user id and name

How to get the user id of the interpreter process; how to get the username associated with the user id.

When writing a setuid application on Unix, there is a distinction between the real user id and the effective user id. The code examples return the real user id.

The process may be able to determine the username by inspecting environment variables. A POSIX system is required to set the environment variable LOGNAME at login. Unix systems often set USER at login, and Windows systems set %USERNAME%. There is nothing to prevent the user from altering any of these environment variables after login. The methods illustrated in the examples are thus more secure.


How to get the effective user id:



How to get the effective user id:




It is possible to register code to be executed upon exit:

import atexit
atexit.register(print, "goodbye")

It is possible to terminate a script without executing registered exit code by calling os._exit.


It is possible to register code to be executed upon exit:

at_exit { puts "goodbye" }

The script can be terminated without executing registered exit code by calling exit!.

set signal handler

How to register a signal handling function.

executable test

How to test whether a file is executable.

external command

How to execute an external command.

escaped external command

How to prevent shell injection.


How to invoke an external command and read its output into a variable.

The use of backticks for this operation goes back to the Bourne shell (1977).


A more concise solution is:

file = os.popen('ls -l /tmp').read()

os.popen was marked as deprecated in Python 2.6 but it is still available in Python 2.7 and Python 3.2.


%x can be used with any delimiter. If the opening delimiter is (, [, or {, the closing delimiter must be ), ], or }.

Option Parsing

command line options

How to process command line options.

We describe the style used by getopt_long from the C standard library. The characteristics of this style are:

  • Options can be short or long. Short options are a single character preceded by a hyphen. Long options are a word preceded by two hyphens.
  • A double hyphen by itself can be used to terminate option processing. Arguments after the double hyphen are treated as positional arguments and can start with a hyphen.
  • Options can be declared to be with or without argument. Options without argument are used to set a boolean value to true.
  • Short options without argument can share a hyphen.
  • Long options can be separated from their argument by a space or an equals sign (=). Short options can be separated from their argument by nothing, a space, or an equals sign (=).

The option processing function should identify the positional arguments. These are the command line arguments which are not options, option arguments, or the double hyphen used to terminate option processing. getopt_long permits options to occur after positional arguments.


The type of an argument can be specified using the named parameter type:

parser.add_argument('--count', '-c', dest='count', type=int)
parser.add_argument('--ratio', '-r', dest='ratio', type=float)

If the argument cannot be converted to the type, the script prints out a usage statement and exits with a non-zero value.

The default value is None, but this can be changed using the named parameter default:

parser.add_argument('--file', '-f', dest='file', default='tmpfile')
parser.add_argument('--count', '-c', dest='count', type=int, default=1)
parser.add_argument('--ratio', '-r', dest='ratio', type=float, default=0.5)

Libraries and Namespaces

Terminology used in this sheet:

  • library: code in its own file that can be included, loaded, or linked by client code.
  • client: code which calls code in a separate file.
  • top-level file or top-level script: the file containing the code in the program which executes first.
  • load: to add definitions in a file to the text of a running process.
  • namespace: a set of names that can be imported as a unit.
  • import: to add definitions defined elsewhere to a scope.
  • unqualified import: to add definitions to a scope using the same identifiers as where they are defined.
  • qualified import: to add definitions to a scope. The identifiers in the scope are derived from the original identifiers in a formulaic manner. Usually the name of the namespace is added as a prefix.
  • label: one of the parts of a qualified identifier.
  • alias import: to add a definition to a scope under an identifier which is specified in the import statement.
  • package: one or more libraries that can be installed by a package manager.

load library

Execute the specified file. Normally this is used on a file which only contains declarations at the top level.


include_once behaves like require_once except that it is not fatal if an error is encountered executing the library.

load library in subdirectory

How to load a library in a subdirectory of the library path.

hot patch

How to reload a library. Altered definitions in the library will replace previous versions of the definition.


Also include.

load error

How errors which are encountered while loading libraries are handled.

main routine in library

How to put code in a library which executes only when the file is run as a top-level script.

library path

The library path is a list of directory paths which are searched when loading libraries.

library path environment variable

How to augment the library path by setting an environment variable before invoking the interpreter.

library path command line option

How to augment the library path by providing a command line option when invoking the interpreter.

simple global identifiers

multiple label identifiers

label separator

The punctuation used to separate the labels in the full name of a subnamespace.

root namespace definition

namespace declaration

How to declare a section of code as belonging to a namespace.

subnamespace declaration

How to declare a section of code as belonging to a subnamespace.

import namespace

import subnamespace

import all definitions in namespace

How to import all the definitions in a namespace.

import definitions

How to import specific definitions from a namespace.

list installed packages, install a package

How to show the installed 3rd party packages, and how to install a new 3rd party package.


Two ways to list the installed modules and the modules in the standard library:

$ pydoc modules
$ python
>>> help('modules')

Most 3rd party Python code is packaged using distutils, which is in the Python standard library. The code is placed in a directory with a file. The code is installed by running the Python interpreter on

package specification format

The format of the file used to specify a package.


distutils.core reference

How to create a Python package using distutils. Suppose that the file contains the following code:

def add(x, y):
    return x+y

In the same directory as create with the following contents:

#!/usr/bin/env python

from distutils.core import setup


Create a tarball of the directory for distribution:

$ tar cf foo-1.0.tar foo
$ gzip foo-1.0.tar

To install a tar, perform the following:

$ tar xf foo-1.0.tar.gz
$ cd foo
$ sudo python install

If you want people to be able to install the package with pip, upload the tarball to the Python Package Index.


gemspec attributes

For an example of how to create a gem, create a directory called foo. Inside it create a file called lib/foo.rb which contains:

def add(x, y)
  x + y

Then create a file called foo.gemspec containing:

spec = do |s| = 'foo'
  s.authors = 'Joe Foo'
  s.version = '1.0'
  s.summary = 'a gem'
  s.files = Dir['lib/*.rb']

To create the gem, run this command:

$ gem build foo.gemspec

A file called foo-1.0.gem is created. To install foo.rb run this command:

$ gem install foo-1.0.gem


An object is a set of functions called methods which have shared access to the object's instance variables. An object's methods and instance variables are collectively called its members. If a member of an object can be accessed or invoked by code which is not in a member of the object, it is public. Otherwise it is private.

A class is a set of objects which have the same method definitions. The objects in the set are instances of the class. Functions defined in the class namespace which are not object methods are called class methods. A class method which returns instances of the class is called a factory method. If there is class method which is responsible for creating all instances, it is called a constructor. The existence of a constructor does not preclude the existence of other factory methods since they can invoke the constructor and return its return value.

A class may contain class variables. These are global variables defined in the namespace of the class.

A method which returns the value of an instance variable is called a getter. A method which sets the value of an instance variable is called a setter. Getters and setters and seem pointless at first blush as one could make the underlying instance variable public instead. In practice getters and setters make code more maintainable. Consistent use of getters and setters conforms with the Uniform Access Principle and makes the API presented by an object to its clients simpler.

Perl instance variables are private, so Perl enforces a good practice at the cost of requiring boilerplate code for defining getters and setters.

Python instance variables are public. Although this permits concise class definitions, a maintainer of a Python class may find it difficult to replace an instance variable with a derived value when clients are accessing the instance variable directly. With an old-style Python class, the maintainer can't make the change without breaking the client code. With a new-style class the maintainer can replace an instance variable with a getter and setter and mark them with the @property decorator.

Ruby, like Perl, has private instance variables. It has the directives attr_reader, attr_writer, and attr_accessor for defining getters and setters. Ruby classes are objects and in particular they are instances of the Module class. The directives attr_reader, attr_writer, and attr_accessor are instance methods defined in the Module class which execute when the class block executes.

define class


Properties (i.e. instance variables) must be declared public, protected, or private. Methods can optionally be declared public, protected, or private. Methods without a visibility modifier are public.


As of Python 2.2, classes are of two types: new-style classes and old-style classes. The class type is determined by the type of class(es) the class inherits from. If no superclasses are specified, then the class is old-style. As of Python 3.0, all classes are new-style.

New-style classes have these features which old-style classes don't:

  • universal base class called object.
  • descriptors and properties. Also the __getattribute__ method for intercepting all attribute access.
  • change in how the diamond problem is handled. If a class inherits from multiple parents which in turn inherit from a common grandparent, then when checking for an attribute or method, all parents will be checked before the grandparent.

create object

How to create an object.

get and set attribute

How to get and set an attribute.


Defining explicit setters and getters in Python is considered poor style. Extra logic can be achieved without disrupting the clients of the class by creating a property:

def getValue(self):
  print("getValue called")
  return self.__dict__['value']

def setValue(self,v):
  print("setValue called")
  self.__dict__['value'] = v

value = property(fget=getValue, fset = setValue)

instance variable visibility

How instance variable access works.

define method

How to define a method.

invoke method

How to invoke a method.


How to define a destructor.


A Python destructor is not guaranteed to be called when all references to an object go out of scope, but apparently this is how the CPython implementations work.


Ruby lacks a destructor. It is possible to register a block to be executed before the memory for an object is released by the garbage collector. A ruby interpreter may exit without releasing memory for objects that have gone out of scope and in this case the finalizer will not get called. Furthermore, if the finalizer block holds on to a reference to the object, it will prevent the garbage collector from freeing the object.

method missing

How to handle when a caller invokes an undefined method.


Define the method __callStatic to handle calls to undefined class methods.


__getattr__ is invoked when an attribute (instance variable or method) is missing. By contrast, __getattribute__, which is only available in Python 3, is always invoked, and can be used to intercept access to attributes that exist. __setattr__ and __delattr__ are invoked when attempting to set or delete attributes that don't exist. The del statement is used to delete an attribute.


Define the method self.method_missing to handle calls to undefined class methods.

define class method

invoke class method

How to invoke a class method.

method alias

How to create an alias for a method.


Ruby provides the keyword alias and the method alias_method in the class Module. Inside a class body they behave idenitically. When called from inside a method alias has no effect but alias_method works as expected. Hence some recommend always using alias_method.

Inheritance and Polymorphism

A subclass is a class whose objects contain all of the methods from another class called the superclass. Objects in the subclass should in principle be usable anywhere objects in the superclass can be used. The subclass may have extra methods which are not found in the superclass. Moreover it may replace method definitions in the superclass with its own definitions provided the signature remains the same. This is called overriding.

It is sometimes useful to define superclass which is never instantiated. Such a class is called an abstract class. An abstract class is way to share code between two or more subclasses or to define the API that two or more subclasses should implement.


How to use inheritance.

operator overloading

How to define the behavior of the binary operators.


object id

How to get an identifier for an object or a value.

inspect type


The PHP manual says that the strings returned by gettype are subject to change and advises using the following predicates instead:


All possible return values of gettype are listed.

basic types

inspect class

How to get the class of an object.


inspect class hierarchy

has method?


hasattr(o,'reverse') will return True if there is an instance variable named 'reverse'.

message passing


The following works in Firefox:

var o = {}
o.__noSuchMethod__ = function(name) { alert('you called ' + name) }


How to interpret a string as code and return its value.


The value of the string is the value of of the return statement that terminates execution. If execution falls off the end of the string without encountering a return statement, the eval evaluates as NULL.


The argument of eval must be an expression or a SyntaxError is raised. The Python version of the mini-REPL is thus considerably less powerful than the versions for the other languages. It cannot define a function or even create a variable via assignment.

list object methods

list object attributes


dir(o) returns methods and instance variables.

pretty print

How to display the contents of a data structure for debugging purposes.

source line number and file name

How to get the current line number and file name of the source code.

command line documentation

How to get documentation from the command line.


Searching for Math.atan2 will return either class method or instance method documentation. If there is documentation for both one can be specific with the following notation:

$ ri Math::atan2
$ ri Math#atan2

Net and Web

get local hostname, dns lookup, reverse dns lookup

How to get the hostname and the ip address of the local machine without connecting to a socket.

The operating system should provide a method for determining the hostname. Linux provides the uname system call.

A DNS lookup can be performed to determine the IP address for the local machine. This may fail if the DNS server is unaware of the local machine or if the DNS server has incorrect information about the local host.

A reverse DNS lookup can be performed to find the hostname associated with an IP address. This may fail for the same reasons a forward DNS lookup might fail.

http get

How to make an HTTP GET request and read the response into a string.

http post

serve working directory

A command line invocation to start a single process web server which serves the working directory at http://localhost:8000.

$ sudo cpan -i IO::All

$ perl -MIO::All -e 'io(":8000")->fork->accept->(sub { $_[0] < io(-x $1 ? "./$1 |" : $1) if /^GET \/(.*) / })'

absolute url

How to construct an absolute URL from a base URL and a relative URL as documented in RFC 1808.

When constructing the absolute URL, the rightmost path component of the base URL is removed unless it ends with a slash /. The query string and fragment of the base URL are always removed.

If the relative URL starts with a slash / then the entire path of the base URL is removed.

If the relative URL starts with one or more occurrences of ../ then one or more path components are removed from the base URL.

The base URL and the relative URL will be joined by a single slash / in the absolute URL.


Here is a PHP function which computes absolute urls.

parse url

How to extract the protocol, host, port, path, query string, and fragment from a URL. How to extract the parameters from the query string.


urlparse can also be used to parse FTP URLs:

up = urlparse.urlparse(';type=binary')

# 'foo'

# 'bar'

# 'type=binary'


How to parse an FTP URL:

up = URI(';type=binary')

# "foo"

# up.password

# "binary"

url encode/decode

How to URL encode and URL unencode a string.

URL encoding, also called percent encoding, is described in RFC 3986. It replaces all characters except for the letters, digits, and a few punctuation marks with a percent sign followed by their two digit hex encoding. The characters which are not escaped are:

A-Z a-z 0-9 - _ . ~

URL encoding can be used to encode UTF-8, in which case each byte of a UTF-8 character is encoded separately.

When form data is sent from a browser to a server via an HTTP GET or an HTTP POST, the data is percent encoded but spaces are replaced by plus signs + instead of %20. The MIME type for form data is application/x-www-form-urlencoded.


In Python 3 the functions quote_plus, unquote_plus, quote, and unquote moved from urllib to urllib.parse.

urllib.quote replaces a space character with %20.

urllib.unquote does not replace + with a space character.

html escape

How to escape special characters in HTML character data; how to escape special characters in HTML attribute values; how to unescape HTML entities.

In character data, such as what occurs in between a start and end tag, the characters <, >, and & must be replaced by &lt;, &gt;, and &amp;.

Attribute values in HTML tags must be quoted if they contain a space or any of the characters "'`=<>. Attribute values can be double quoted or single quoted. Double quotes and single quotes can be escaped by using the HTMl entities &quot; and &apos;. It is not necessary to escape the characters <, >, and & inside quoted attribute values.


The flag ENT_NOQUOTES to the function htmlspecialchars causes double quotes " to be escaped.

The flag ENT_QUOTES causes single quotes ' to be escaped.

base64 encode/decode

How to encode binary data in ASCII using the Base64 encoding scheme.

A popular Base64 encoding is the one defined by RFC 2045 for MIME. Every 3 bytes of input is mapped to 4 of these characters: [A-Za-z0-9/+].
If the input does not consist of a multiple of three characters, then the output is padded with one or two hyphens: =.

Whitespace can inserted freely into Base64 output; this is necessary to support transmission by email. When converting Base64 back to binary whitespace is ignored.

Unit Tests

test class

How to define a test class and make a truth assertion.

The argument of a truth assertion is typically an expression. It is a good practice to include a failure message as a second argument which prints out variables in the expression.

run tests; run test method

How to run all the tests in a test class; how to run a single test from the test class.

equality assertion

How to test for equality.


Note that assertEquals does not print the values of its first two arguments when the assertion fails. A third argument can be used to provide a more informative failure message.

approximate assertion

How to assert that two floating point numbers are approximately equal.

regex assertion

How to test that a string matches a regex.

exception assertion

How to test whether an exception is raised.

mock method

How to create a mock method.

A mock method is used when calling the real method from a unit test would be undesirable. The method that is mocked is not in the code that is being tested, but rather a library which is used by that code. Mock methods can raise exceptions if the test fails to invoke them or if the wrong arguments are provided.


assert_called_once_with can takes the same number of arguments as the method being mocked.

If the mock method was called multiple times, the method assert_called_with can be used in place of asert_called_once_with to make an assertion about the arguments that were used in the most recent call.

A mock method which raises an exception:

foo = Foo() = mock.Mock(side_effect=KeyError('foo'))

with self.assertRaises(KeyError):


The with method takes the same number of arguments as the method being mocked.

Other methods are available for use in the chain which defines the assertion. The once method can be replaced by never or twice. If there is uncertainty about how often the method will be called one can used at_least_once, at_least(m), at_most_once, at_most(n) to set lower or upper bounds. times(m..n) takes a range to set both the lower and upper bound.

A mock method which raises an exception:

    foo = mock()
      raises(exception = RuntimeError, message = 'bam!').

    assert_raises(RuntimeError) do

There is also a method called yields which can be used in the chain which defines the assertion. It makes the mock method yield to a block. It takes as arguments the arguments it passes to the block.


How to define a setup method which gets called before every test.


How to define a cleanup method which gets called after every test.

Debugging and Profiling

check syntax

How to check the syntax of code without executing it.

flags for stronger and strongest warnings

Flags to increase the warnings issued by the interpreter.


The -t flag warns about inconsistent use of tabs in the source code. The -3 flag is a Python 2.X option which warns about syntax which is no longer valid in Python 3.X.


A lint tool.

source cleanup

A tool which detects or removes semantically insignificant variation in the source code.

run debugger

How to run a script under the debugger.

debugger commands

A selection of commands available when running the debugger. The gdb commands are provided for comparison.

cmd perl -d python -m pdb rdebug gdb
help h h h h
list l [first, last] l [first, last] l [first, last] l [first, last]
next statement n n n n
step into function s s s s
set breakpoint b b [file:]line
b function
b [file:]line
b class[.method]
b [file:]line
list breakpoints L b info b i b
delete breakpoint B num cl num del num d num
continue c c c c
show backtrace T w w bt
move up stack u u u
move down stack d down do
print expression p expr p expr p expr p expr
(re)run R restart [arg1[, arg2 …]] restart [arg1[, arg2 …]] r [arg1[, arg2 …]]
quit debugger q q q q

benchmark code

How to run a snippet of code repeatedly and get the user, system, and total wall clock time.

profile code

How to run the interpreter on a script and get the number of calls and total execution time for each function or method.

Java Interoperation


Version of the scripting language JVM implementation used in this reference sheet.


Command line name of the repl.


Command line name of the interpreter.


Command line name of the tool which compiles source to java byte code.


Code necessary to make java code accessible.


How to create a java object.


How to invoke a java method.


How to import names into the current namespace.

import non-bundled java library

How to import a non-bundled Java library

shadowing avoidance

How to import Java names which are the same as native names.

convert native array to java array

How to convert a native array to a Java array.

are java classes subclassable?

Can a Java class be subclassed?

are java classes open?

Can a Java array be monkey patched?

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