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PyCon Toronto 2013 notes

James Powell - james@nycpython.com

itertools

  • useful for dealing with iterators & iterables
  • e.g. chain (for appending lists and tuples, instead of +)
  • also includes algorithms, e.g. takewhile (e.g. takewhile fibonacci < 50)

generators

  • memory efficient when you need subset of entire return value
  • generating an instance is expensive, so avoid generators unless the operation is also expensive. tl;dr sometimes generators are faster, but not always
>>> def pairwise(xs):
...     xs = iter(xs)
...     x = next(xs)
...     for y in xs:
...             yield x, y
...             x = y
... 
>>> print list(pairwise(xrange(10)))
[(0, 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 8), (8, 9)]

OR with a single line using itertools using tee, izip, islice (but more difficult to understand).

  • sometimes you have to decide if your algorithm returns first n numbers or returns value that go up to but not exceeding n
    • use generators instead! both are possible and decision left up to the user
def fibonacci(a=1, b=1):
  while True:
    yield a
    a, b = b, a+b

decorator

>>> from contextlib import contextmanager
>>> @contextmanager
... def ctx():
...     print 'setup code'
...     yield
...     print 'teardown code'
... 
>>> with ctx():
...     print 'do stuff'
... 
setup code
do stuff
teardown code

Typical usage:

    @contextmanager
    def some_generator(<arguments>):
        <setup>
        try:
            yield <value>
        finally:
            <cleanup>

This makes this:

    with some_generator(<arguments>) as <variable>:
        <body>

equivalent to this:

    <setup>
    try:
        <variable> = <value>
        <body>
    finally:
        <cleanup>

This is an alternative to

class controlled_execution():
  def __enter__(self):
    print 'setup and return thing'
  def __exit__(*args, **kwargs): # __....__ called context guards
    print 'teardown' # no matter what happens, __exit__ is called

with controlled_execution() as thing:
    print 'use thing'
    # auto-assigns return value of __enter__ to variable thing

This is what file object uses, allowing you to do

with open(filename) as f:
    data = f.read()
    # do something with data

finally

  • don't write classes, write functions (with generators)
    • if that's not good enough, write a class
  • write as little code as you can, each line is a liability
  • python crash course at github.com/pycademy
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