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kennethreitz merged 2 commits into from
Apr 27, 2012
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Added idioms to style.rst #52

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Update docs/writing/style.rst
epequeno Dec 30, 2011
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Update docs/writing/style.rst
epequeno Dec 30, 2011
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23 changes: 23 additions & 0 deletions docs/writing/style.rst
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Expand Up @@ -7,6 +7,29 @@ Idioms

Idiomatic Python code is often referred to as being *pythonic*.

A common idiom for creating strings is to use `join <http://docs.python.org/library/string.html#string.join>`_ on an empty string.::
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@alex alex Dec 30, 2011

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It's probably easier to set up intersphinx with CPython and then just link in using that: http://sphinx.pocoo.org/latest/ext/intersphinx.html


letters = ['s', 'p', 'a', 'm']
word = ''.join(letters)
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@dcrosta dcrosta Dec 31, 2011

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This probably isn't the best example. This pattern is idiomatic (and better-performing) than += when you're getting chunks of string data from something like a socket, file, template engine, etc, not from a list literal. That might be too complex to convey in the style section, perhaps we need to add a performance tricks area? Could be in recipe format:

Problem

You receive bits of data from one or more sources, but need to concatenate this into a single string for a return value

Solution

Use the string.join method, like:

parts = []
for part in thing_that_gives_strings():
    parts.append(part)
out = ''.join(parts)

(There are probably better examples, this is just off the top of my head)


This will set the value of the variable *word* to 'spam'. This idiom can be applied to lists and tuples.
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@dcrosta dcrosta Dec 31, 2011

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And generators and anything iterable that gives strings, like ''.join(line for line in file('the_filename.txt')

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@alex alex Dec 31, 2011

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That doesn't work, be because that's just a silly way of writing:

with open("filename.txt") as f:
chars = "".join(f)

which is a silly version of: chars = f.read()

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@dcrosta dcrosta Dec 31, 2011

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Definitely a bad example. But there are better ones, I'm sure. Point is, not just lists or tuples are join-able.


Sometimes we need to search through a collection of things. Let's look at two options: lists and dictionaries.

Take the following code for example::

d = {'s': [], 'p': [], 'a': [], 'm': []}
l = ['s', 'p', 'a', 'm']

def lookup_dict(d):
return 's' in d

def lookup_list(l):
return 's' in l

Even though both functions look identical, because *lookup_dict* is utilizing the fact that dictionaries in python are hashtables, the lookup performance between the two is very different.
Python will have to go through each item in the list to find a matching case, which is time consuming. By analysing the hash of the dictionary finding keys in the dict can be done very quickly.
For more information see this `StackOverflow <http://stackoverflow.com/questions/513882/python-list-vs-dict-for-look-up-table>`_ page.
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@dcrosta dcrosta Dec 31, 2011

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This all might also fit better in a performance section.


Zen of Python
-------------
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