Python 最佳实践

避免名称污染

尽量不要import *

Very bad
[...]
from modu import *
[...]
x = sqrt(4) # Is sqrt part of modu? A builtin? Defined above?

Better
from modu import sqrt
[...]
x = sqrt(4) # sqrt may be part of modu, if not redefined in between

Best
import modu
[...]
x = modu.sqrt(4) # sqrt is visibly part of modu

可变变量和不变变量

可变变量允许改变自身的值，典型的可变变量包括列表和字典。比如，所有的字典都有更改的方法，比如append和pop，可以原地更改。
然而，不变类型没有类似的方法来更改自己的内容。参考如下例子，其中id()函数可以得到变量的地址，我们可以看到，字符串变量的地址发生了变化

>>> c = 'i'
>>> c
'i'
>>> id(c)
140467898860912
>>> c = c + ' am'
>>> c
'i am'
>>> id(c)
140467796846656
>>> dir
<built-in function dir>
>>> dir()
['__builtins__', '__doc__', '__name__', '__package__', 'a', 'b', 'c', 'sys']

所以在处理字符串的时候，不要频繁的修改其值，否则会带来性能损失，参考如下例子。

Bad
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = ""
for n in range(20):
  nums += str(n) # slow and inefficient
print nums

Good
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = []
for n in range(20):
  nums.append(str(n))
print "".join(nums) # much more efficient

Best
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = [str(n) for n in range(20)]
print "".join(nums)

字符串赋值

除了使用join() 和 + 以外，还可以利用 % 以及format函数来初始化字符串

foo = 'foo'
bar = 'bar'

foobar = '%s%s' % (foo, bar) # It is OK
foobar = '{0}{1}'.format(foo, bar) # It is better
foobar = '{foo}{bar}'.format(foo=foo, bar=bar) # It is best

取字典元素

不要使用has_key函数

Bad:
d = {'hello': 'world'}
if d.has_key('hello'):
    print d['hello'] # prints 'world'
else:
    print 'default_value'

Good:
d = {'hello': 'world'}

print d.get('hello', 'default_value') # prints 'world'
print d.get('thingy', 'default_value') # prints 'default_value'

# Or:
if 'hello' in d:
    print d['hello']

列表操作

列表类型提供了一系列强大的操作方法，比如map和filter方法。

Bad:
# Filter elements greater than 4
a = [3, 4, 5]
b = []
for i in a:
    if i > 4:
        b.append(i)

Good:
b = [i for i in a if i > 4]
b = filter(lambda x: x > 4, a)


Bad:
# Add three to all list members.
a = [3, 4, 5]
count = 0
for i in a:
    a[count] = i + 3
    count = count + 1

Good:
a = [3, 4, 5]
a = [i + 3 for i in a]
# Or:
a = map(lambda i: i + 3, a)

使用enumerate函数来对列表进行计数，这种处理更易读，而且更高效。

for i, item in enumerate(a):
    print i + ", " + item
# prints
# 0, 3
# 1, 4
# 2, 5

读文件

使用 with open 语法来读文件，这种方法会自动关闭文件，即使发生了异常。

Bad:
f = open('file.txt')
a = f.read()
print a
f.close()

Good:
with open('file.txt') as f:
    for line in f:
        print line

VIM 设置

set textwidth=79 " lines longer than 79 columns will be broken
set shiftwidth=4  " operation >> indents 4 columns; << unindents 4 columns
set tabstop=4 " an hard TAB displays as 4 columns
set expandtab     " insert spaces when hitting TABs
set softtabstop=4 " insert/delete 4 spaces when hitting a TAB/BACKSPACE
set shiftround    " round indent to multiple of 'shiftwidth'
set autoindent " align the new line indent with the previous line