The Python Programming Language

http://www.python.org/

Joshua Sled

http://asynchronous.org/jsled

http://asynchronous.org/preso/vague/python

overview

• simple, readable high-level language
• interpreted; cached compiled bytecode
• garbage collected
• whitespace significant for block structure
• implementations
  • CPython, jython, IronPython, PyPy
  • Nokia 770, Nokia "Series 60" phones
  • Unix/Linux, Windows, Mac, PalmOS, WindowsCE, RiscOS, VxWorks, QNX, OS/2, OS/390, AS/400, PlayStation, Sharp Zaurus, BeOS, VMS
• Guido van Rossum - Benevolent Dictator For Life (BDFL)
• Python Enhancement Proposals (PEPs)

type

• hybrid: procedural, object-oriented, with functional concepts
• dynamically typed: variables are whatever they contain; no definitons of their type
• strongly typed: numbers are numbers, strings are strings. 1 + "1" fails, no automatic coersion
• duck-typed: no explicit (object) interface checks or failures; "if it (behaves) like a duck..."

datatypes

• None, integers, long, boolean, float, string
• no sigils on identifiers; `global` keyword
• tuple - (x,y,z), (x,) - foo[0]
• list - [1,2,3] - foo[0], foo[0:1], foo[0:1:2]
• dictionary - {'a': 1, 'b': 2} - foo['a']
• set
• callables/functions
• classes

quoting

• single: 'jsled', "jsled"
• triple: '''josh 'bob' sled'''
  • newlines automagically quoted, nice for multi-string text.
• raw: r'\n' → '\\n'
• unicode

multiple assignment

>>> a,b,c = 1,2,3
>>> foo = 1,2,3
>>> a,b,c = foo
>>> foo = {'a': 1, 'b': 2, 'c': 3}
>>> for key,val in foo.iteritems():
    print key,val
a 1
b 2
c 3

control structures

• pass
  • no-op filler ... ';' in other languages.
• if condition:/elif condition:/else:
• for var in sequence:, else:

      for ($i = 0; $i < 10; $i++)
  
        ...
  
      for i in range(10): ...
  

• while condition:, else
• break, continue

function definition

def foo(spam, eggs):
    rtn = spam + eggs
    print 'about to return',rtn
    return rtn

foo(1,2)

bar = foo
bar(3,4)

lambda x,y: x + y

advanced functions

• default arguments

  def foo(spam, eggs=42):
    ....
  foo('jsled')
  

• variable, keyword arguments

  def foo(foo, *more, **by_name):
      ...
  
  >>> foo(1, 3, bar=2)
  foo: 1
  more: (3,)
  by_name: {'bar':2}
  

class definition

• constructor: __init__(self, args)
• super-class calls (no syntactic sugar): <super.method>(self, args)
• 'this' object is first argument to methods; "self" by convention only.

Class example

class Foo (Bar):
    def __init__(self, name):
        self.name = name
        Bar.__init__(self)
    def get_name(self):
        return self.name
    def bars_thing(self, arg):
        return Bar.bars_thing(self, arg)

x = Foo('jsled')
x.get_name()

list comprehensions

• language support for common list processing idioms
• transformation
  • [f(x) for x in [1,2,3]]
• filtering
  • [x for x in [1,2,3] if x % 2 == 0]

• bar = \
    dict([(k,f(v)) for k,v in foo.iteritems() \
          if k in ['these', 'are', 'allowed']])

iterators

• for x in obj: ...
  • iter = obj.__iter__()
• iter.next()
  • raise StopIteration

generators

def gen_odd_numbers(limit):
    i = 1
    while i < limit:
        yield i
        i += 2

>>> [x for x in gen_odd_numbers(10)]
[1, 3, 5, 7, 9]
>>> [x for x in (x**2 for x in range(10))]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

standard methods

• str(foo) → foo.__str__()
• len(foo) → foo.__len__()
• indexing
  • foo['a'] → foo.__getitem__('a')
  • foo['a'] = 'b' → foo.__setitem__('a', 'b')
  • del foo['a'] → foo.__delitem__('a')
• Most operators translate into __operation__s.

idiom overloading

class WorldWideWeb:
    def __getitem__(url):
        page = ''
        http_connection = urllib2.open(url)
        while (fragment = http_connection.read(1024)):
           page = page + fragment
        return page

web = WorldWideWeb()
page = web['http://uvm.org/vague/']

doc strings

• module eg.py:

  __doc__ = 'Example Code.'
  
  def foo():
       "foo does awesome things"
       return do_awesome_thing()
  
  

>>> import eg
>>> eg.foo.__doc__ 
>>> 'foo does awesome things'
>>> help(eg)
Help on module eg:

NAME
    eg - Example code.

FILE
    /home/user/path/to/eg.py

FUNCTIONS
    foo()
        foo does awesome things

decorators

def memoize(fn):
    cache = {}
    def _memoized(*args, **kwargs):
        memo = repr((args,kwargs))
        if cache.has_key(memo):
            return cache[memo]
        rtn = fn(*args, **kwargs)
        print 'saving', rtn
        cache[memo] = rtn
        return rtn
    return _memoized

decorators

@memoize
def foo(arg):
    return arg

>>> foo(42)
saving 42
42
>>> foo(42)
42

• python 2.4

dir()

• lists names that a thing defines
  • thing = module, object, callable
• >>> dir('')
  ['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__str__', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'replace', 'rfind', 'rindex', 'rjust', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

try:
    block-1 ...
except Exception1:
    handler-1 ...
except Exception2:
    handler-2 ...
else:
    else-block
finally:
    final-block

from distutils.core import setup
setup(name='Distutils',
      version='1.0',
      description='Python Distribution Utilities',
      author='Greg Ward',
      author_email='gward@python.net',
      url='http://www.python.org/sigs/distutils-sig/',
      packages=['distutils', 'distutils.command'],
     )

import gtk
   
win = gtk.Window()
win.add(box)
box = gtk.VBox()
button = gtk.Button('Click me!')
def clicked_cb(button):
    print 'I was clicked!!!'
button.connect('clicked', clicked_cb)
box.add(button)
win.connect('destroy', lambda win: gtk.main_quit())
win.show_all()
gtk.main()