WHAT IS PYTHON?by Gyuri Horak

WHY PYTHON?

WHY PYTHON!

Monty Python!

HISTORY

● 1989, Guido van Rossum, BDFL● BDFL: benevolent dictator for life

○ successor of ABC○ capable of exception handling

● 2000, Python 2○ garbage collector○ unicode support○ community-backed development process

● 2008, Python 3 (not yet widely used)

WHAT IS PYTHON?● open source● object-oriented● Everything is an object.● structured● You don't need to define any class if you don't want to.● So both o-o and structured programming are supported, you can

even mix these techniques.● a bit functional programming, AOP, metaprogramming● Like list comprehensions: sqs = [x**2 for x in range(0, 10)]● New classes can be created runtime, methods can be easily

replaced (monkey-patching)...● dynamic typed, duck typing● memory management (reference counting and cycle-detecting gc)

WHERE CAN I USE IT?

● CPython○ reference implementation○ wide range of C modules○ Stackless, Unladen Swallow

● Jython○ Python on JVM

● IronPython○ Python on .NET

● PyPy○ Python in Python○ uses JIT, focuses on speed

WHO USES IT?

● Google (search, Youtube, GAE)● Yahoo (maps, groups)● Mozilla (addons website)● Video games (Battlefield 2, Crystal Space 3D engine,

Civilization 4, ...)● Movies (Industrial Light & Magic, Walt Disney, Blender,

...)● Science (NASA, National Weather Service, ...)● ...

THE ZEN OF PYTHON

The Zen of Python, by Tim Peters

Beautiful is better than ugly.Explicit is better than implicit.

Simple is better than complex.Complex is better than complicated.

Flat is better than nested.Sparse is better than dense.

Readability counts....

HOW DOES IT LOOK LIKE?

● Indentation, no semicolon● Style guide: PEP-8● Docstrings: PEP-257, reST: PEP-287

def hello(name="World"): """Says hello to `name`

name -- the name to greet (default "World")

"""

if name is "Janos": print "...hi!" else: print "Hello %s!" % name # Python 3: print statement -> print() function

DATA TYPES● None● bool (True/False)● int, long, float, complex [int, float, complex, Fraction]● str, unicode [bytes, str] (!)● list● tuple● dict● set, frozenset● ...

FLOW CONTROL STATEMENTS - IFif a > b: passelif b < a: print "b<a"else: print "b=a"

pass is an empty statement, it does nothing.No switch ... case, but you can have as much elif as you want. There's no a = b ? c : d operator neither, but you can use if in the following way:

a = c if b else d

FLOW CONTROL STATEMENTS - WHILEwhile a < b: a += 1

FLOW CONTROL STATEMENTS - FORfor x in l: print x

It's not the conventional for, it's a foreach, l has to be an iterable.Iterable objects are capable of returning its members one at a time, like iterators or generators.It's easy to simulate the conventional behavior with the range() function, but do not do the following:

for i in range(0, 10): print l[i]

BREAK, CONTINUE, ELSE● break and continue work as usual● you can define else block for for and while loops, they will be executed when

the loop terminates normally

for n in range(2, 100): for x in range(2, n): if n % x == 0: print "%d == %d * %d" % (n, x, n/x) break else: print "%d is a prime number" % n

EXCEPTION HANDLINGtry: result = x / yexcept ZeroDivisionError as e: print "y should not be 0!"except: traceback.print_exc()else: print "x / y =", resultfinally: print "we're in finally block"

FUNCTIONSdef fname(*args, **kwargs): # function body return returnvalue

No function/method overloading, but argument list can vary:def example(a, b=12, c=False): return b if c else a

>>> example(42)42>>> example(1, 20)1>>> example(1, 2, True)2>>> example(1, c=True)12

CLASSESclass MyClass(object):

a = 42

def say_hello(self, name="World"): print "Hello %s!" % name return self.a

def __init__(self, a = None): if a: self.a = a

>>> i = MyClass(1)>>> x = i.say_hello("Everyone")Hello Everyone!>>> x1

● no private attributes/methods (_, __)● methods are bound to instances:● >>> sh = a.say_hello

>>> sh<bound method MyClass.say_hello of <...(a)...>>

INHERITANCE● Class A (old) <=> class A(object) (new)● Don't use the old way, esp. not when considering multiple inheritance.

class MyClass(BC1, BC2, BC3):

def __init__(self): if wrongway: BC1.__init__(self) BC2.__init__(self) BC3.__init__(self) else: super(MyClass, self).__init__()

● method lookup order is well defined (BC1, BC1 bases, BC2, BC2 bases, ...)

MODULES, PACKAGES● Module = file● Package = directory containing __init__.py

Importing a module/package:

import os # the whole os packageimport os.path # the os.path module# the os.path.join functionimport os.path.join as pathjoin# everything imported into the current namespacefrom sys import * # do not do this!

PYTHON 3

● some backward incompatible changes:○ print statement -> print() function○ b"..." for bytes, "..." for (unicode) strings○ 1/2 = 0.5○ classic classes removed○ int/long -> int○ ... lot of smaller changes/improvements

● standardization is not yet finalized● libraries migrate quite slowly