How does this code work?

Author: Jeff Rush <jeff@taupro.com>

Copyright: 2010 Tau Productions Inc.

License: Creative Commons Attribution-ShareAlike 3.0

Date: August 27, 2011

Duration: 50-minutes

Difficulty: intermediate

Keywords: language, techniques, data structures

A slideshow of a series of code snippets illustrating aspects of Python different fromyour usual procedural C code that may not be well-known in the community. Eachcode snippet will be discussed.

assignment (binding not copying)code block (many per source file)

a callablefunctionmethod - bound vs unboundclass, invokes __init__instance, invokes __call__

initializer method: __init__destructor method: __del__allocator method: __new____ribbed__

modulepackage (__init__.py, multi-file, hierarchy)non-package (single .py file)

distribution (grouping of modules)

What Does This Code Do?

Vocabulary Primer

What Does This Code Do? file:///mnt/flash/talk-What_Does_This_Code_Do/talk-Wha..

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attribute lookupobj.attrnamegetattr(obj, "attrname")setattr(obj, "attrname", value)

mutable vs immutabletuples vs listsstrings, numbers are immutabledicts are mutablesets vs frozen sets(matters for dict keys)

Vocabulary Primer - Basic Objects

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a sequence vs a mappingorderingkey type (int?)dicts, lists, tuples

a set (keys-only dict)subscripting

element fetchslicing (multi-element)key lookup (aspect of entry)

iterationalways forward, never backcan be brittle if modifying

an iterable (not irritable)operations: get, set, del

Vocabulary Primer - Containers

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Any other terms people are puzzled by?

Vocabulary Primer - Open Discussion

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>>> s = "abcdefgh">>> s[3] = 'x' #FAILS

Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: 'str' object does not support item assignment

Strings

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>>> colors = ['red', 'blue', 'green', 'yellow']

>>> result = ''>>> for color in colors:... result += color

>>> result'redbluegreenyellow'

result = ''.join(colors)

More Strings

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What is the output and why it is wrong?

>>> def gimme(length=5):... print length>>>>>> print gimme()

Every callable returns something.

Return Values

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>>> somedict = {'a': 1, 'b': 2}>>> list(somedict) # what do you get?

['a', 'b']

>>> d = { 'person': 'Jack', 'city': 'Conway', 'animal': 'bear' }>>> d = dict(person='Jack', city='Conway', animal='bear')

About Dictionaries

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>>> x = (1, 2, 3)>>> x[0]1

>>> x[0:1]

>>> x[99]

Traceback (most recent call last): File "<stdin>", line 1, in <module>IndexError: string index out of range

x[99:]

x[0] versus x[0:1]

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>>> x = [1, 2, 3]>>> x.append(4)>>> x[1, 2, 3, 4]

>>> x = [1, 2, 3]>>> x += 4

Raises an exception

>>> x += (4, )>>> x

[1, 2, 3, 4]

list.append versus list += x

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>>> x = 1, 2, 3 # what is x?

>>> x(1, 2, 3)

>>> a, b, c = x>>> [a, b, c] = x>>> (a, b, c) = x

>>> person = (37, ('John', 'Thomas'), 190)>>> a, (b1, b2), c = person

a = 37, b1 = 'John', b2 = 'Thomas', c = 190

Iterable Packing/Unpacking

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In other languages:

temp = aa = bb = temp

In Python:

b, a = a, b

b, c, a = a, b, c

Swapping Values

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"sort on the 2nd and 4th column"

def my_compare(item1, item2):return cmp((item1[1], item1[3]),

(item2[1], item2[3]))

container.sort(cmp=my_compare)

def my_key(item):return (item[1], item[3])

container.sort(key=my_key)

Sorting Against a Complex Key

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Write the following C code into a Pythonic style:

if (a == 1 | a == 3 | a == 7)...

if a in (1, 3, 7):...

constants = frozenset((1, 3, 7))...if a in constants:

Testing Against Multiple Values

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if a < b and b < c:... # How many times is b evaluated?

if a() < b() and b() < c():...

Comparison Chaining

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if a < b < c:...

if a() < b() < c():... # Now how many times is b evaluated?

Comparison Chaining cont'd

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if 10 <= x < 20:...

if x == y == z:...

if x is y is z:...

Comparison Chaining cont'd

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>>> x = type(5)(2)>>> x2

>>> x = 5.0...>>> y = type(x)(2.0)>>> y2.0

Making More

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>>> value = True>>> isinstance(value, float)False

>>> isinstance(value, (int, long, float))

But why is the above True?

>>> issubclass(bool, int)True

Global Function: isinstance()

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int.__subclasses__()[<type 'bool'>]

basestring.__subclasses__()[<type 'str'

object.__subclasses__()(long output!)

Discovering Subclasses of a Class

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>>> bool(1)True

>>> bool("")False

>>> bool(5.0)True

>>> bool(0.0)False

>>> bool([])False

>>> bool(dict())False

Many Things are Boolean in Nature

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>>> x = ''

>>> not xTrue

>>> x''

>>> not not xFalse

>>> x is TrueFalse

>>> x is FalseFalse

>>> bool(x)False

Kinds of Comparison

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>>> f = False>>> f and 'enabled'False

>>> f= True>>> f and 'enabled''enabled'

"and" / "or"although they are logical and not bitwise operatorsthey do NOT return a boolean!

Short-Circuit Logical Operations

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>>> f = False>>> f and 'enabled' or 'disabled''disabled'

>>> f = True>>> f and 'enabled' or 'disabled''enabled'

>>> statelabel = 'enabled' if globalEnableFlag else 'disabled'

>>> statelabel = ('disabled', 'enabled')[globalEnableFlag]

Short-Circuit Logical Operations (cont'd)

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>>> x = ('xxxx'... 'vvvvv'... 'xxxxx')

>>> x'xxxxvvvvvxxxxx'

>>> x = ('xxxx',... 'vvvvv',... 'xxxxx')

>>> x('xxxx', 'vvvvv', 'xxxxx')

Fun with Literals

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>>> cc_number = "1234-5678-9012-3456">>> ''.join(filter(str.isdigit, cc_number))'1234567890123456'

>>> ''.join(c for c in cc_number if c.isdigit())'1234567890123456'

>>> ss = (1, 2), (6, 7), (8, 3), (1, 8), (12, 8)>>> max(x for x, y in subsampling)12

Where Generators are Useful

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>>> s = "01234567ABCDEFGHabcdefgh">>> [ s[i:i+8] for i in xrange(0, len(s), 8) ]

['01234567', 'ABCDEFGH', 'abcdefgh']

What happens if the length is not a multiple of 8?

Chopping a Sequence into Fixed-Length Pieces

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hash = '308b1f248f914d1483281df98c296e85'sig = ':'.join(map(lambda x: hash[x:x+2], range(0, len(hash), 2)))

sig = ':'.join(hash[x:x+2] for x in range(0, len(hash), 2))

from cStringIO import StringIO

s = StringIO(hash)sig = ':'.join( iter( lambda: s.read(2), '' ) )

More Chopping a Sequence into Pieces

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>>> for x in range(3):... print x012

>>> values = [None] * 3>>> for values[0] in range(3):... for values[1] in range(3):... for values[2] in range(3):... print values

[0, 0, 0][0, 0, 1][0, 0, 2][0, 1, 0]...

About Iteration

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>>> x = "abcdefghijklmnopqrstuvwxyz"

>>> for i in range( len(x) ):... print x[i] # not Pythonic

>>> for c in x:... print x # more Pythonic

>>> for i in range( len(x) ):... print i, x[i]

>>> for i, c in enumerate(x):... print i, x

More About Iteration

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>>> round(2.129128, 3)2.129

>>> round(2.1667000000000001, 3)2.1669999999999998

>>> from decimal import Decimal>>> Decimal('2.1667000000000001').quantize(Decimal('.001'))Decimal('2.167')

Fractional Numbers

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class Container(object):

def __getitem__(self, key):print repr(key)

>>> c = Container()>>> c[3]3>>> c['orange']'orange'

>>> c[3:4]slice(3, 4, None)

>>> c[3:4:3]slice(3, 4, 3)

Object Behavior: Subscripting

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>>> c['apple':'lollipop']slice('apple', 'lollipop', None)

>>> c[:'lollipop']slice(None, 'lollipop', None)

>>> c[:'lollipop':100]slice(None, 'lollipop, 100)

>>> c['beagle':'terrier':'ignorecase']slice('beagle', 'terrier', 'ignorecase')

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>>> c[1, 2](1, 2)

>>> c[(1, 2), (3, 4)]((1, 2), (3, 4))

>>> print [1, 2, 3, 4, 5, 6, 7, 8][::2](1, 3, 5, 7)

>>> print [1, 2, 3, 4, 5, 6, 7, 8][1::2](2, 4, 6, 8)

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>>> x = [1, 2, 3, 4, 5, 6, 7, 8]>>> x[::2] = ['A', 'B', 'C', 'D']>>> x['A', 2, 'B', 4, 'C', 6, 'D', 8]

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class Alpha(object):

def fn(self):if sys.platform == 'linux2':

print xif sys.platform == 'win32':

print x

class Alpha(object):

if sys.platform == 'linux2':def fn(self):

print xif sys.platform == 'win32':

def fn(self):print x

Conditionally Creating Methods

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>>> class Alpha(object):...... for x in range(3):...... def fn(self):... print x...... locals()['fn_%d' % x] = fn

>>> a = Alpha()>>> a.fn_0()0>>> a.fn_1()1>>> a.fn_2()2

Dynamically Creating Methods

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import signalimport pdbimport time

signal.signal(signal.SIGUSR1, lambda x, y: pdb.set_trace())

while 1:print "Hello"time.sleep(5)

# note - on OSX, use signal.SIGINT instead of signal.SIGUSR1

Debugger-On-Demand

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Any Questions?

EAFP It's Easier to Ask Forgiveness than PermissionLBYL Look Before You LeapYAGNI You Ain't Gonna Need ItDRY Don't Repeat Yourself

Discussion

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