python代码优化---就喜欢细节

地址：http://www.codeproject.com/Tips/829060/Python-Code-Optimizations-Part-One

转发过来保存一下。喜欢精雕细琢，编程才有乐趣。作者牛。

Introduction

Listed below are some very common day to day usage scenarios, and a way to do them pythonically!

Using the Code

1. Looping over a Range of Numbers

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for i in [0,1,2,3,4,5]:
    print i**2

Better way (looks better):

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for i in range(6):
    print i**2

What Happens in this Loop?

range produces a list in memory and then the for loop loops over the list.
Both create a list of 6 integers in memory and then iterate over each
number, raise it to power 2 and then print. Thus, both the loops do
exactly the same thing in exactly the same way!

Pythonic way: Use xrange()

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#Python 2.x
for i in xrange(6):
    print i**2
 
#Python 3.x
for i in range(6):
    print i**2

What is xrange?

• xrange is a sequence object that evaluates lazily.
• xrange creates an iterator over the range(list) and yields one number at a time, thus consuming less amount of memory than the methods above.

2. Looping Over a Collection

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colours = ['red', 'green', 'blue', 'yellow']
 
for i in range(len(colours)):
    print colours[i]

Pythonic way

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for colour in colours:
    print colour

3. Looping Over a Collection and its Indices

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for i in range(len(colours)):
    print i, '-->', colours[i]

Pythonic way: use enumerate()

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for i, colour in enumerate(colours):
    print i, '-->', colour

4. Looping Backwards

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for i in range(len(colours), -1, -1, -1):
    print colours[i]

Pythonic way: Use reversed()

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for colour in reversed(colours):
    print colour

5. Loooping in Sorted Order

Pythonic way: Use sorted()

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for colour in sorted(colours):
    print colour

Looping Backwards in Sorted Order

Just add reverse=True to the sorted function arguments list.

Pythonic Way

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for colour in sorted(colours, reverse=True):
    print colour

6. Looping Over Two Collections

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names = ['a', 'b', 'c']
colours = ['red', 'green', 'blue', 'yellow']
 
n = min(len(colours), len(names))
 
for i in range(n):
    print names[i], '-->', colours[i]

Better Way

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for name, colour in zip(names, colours):
    print name, '-->', colour    

zip creates a third list in memory which consists of tuples, each of which is its own separate object with pointers back to the original. In other words, it takes far more memory than the original two lists combined.
Most importantly "It doesn't scale!".

Pythonic Way: use izip()

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from itertools import izip
for name, colour in izip(names, colours):
    print name, '-->', colour

For smaller lists, zip is faster, but if you have lists with millions of records, then better use izip, as izip only advances the underlying iterators when needed.