Google Python Style Guide

阅读Google Python Style Guide的时候，意识到有一些问题确实会很容易犯错。记录下来以备复习。

关于字符串的使用

使用format和%操作符来格式化和拼接字符串。不要使用+。因为+会产生很多不必要零时对象，凭白增加了系统的负载。

下面这些是正确的做法：

     x = a + b
     x = '%s, %s!' % (imperative, expletive)
     x = '{}, {}!'.format(imperative, expletive)
     x = 'name: %s; score: %d' % (name, n)
     x = 'name: {}; score: {}'.format(name, n)

当然，单纯的两个字符串拼接还是支持用a+b的。其他的都应该使用format和%。

注释的正确写法

下面是方法的DocString :

def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
    """Fetches rows from a Bigtable.

    Retrieves rows pertaining to the given keys from the Table instance
    represented by big_table.  Silly things may happen if
    other_silly_variable is not None.

    Args:
        big_table: An open Bigtable Table instance.
        keys: A sequence of strings representing the key of each table row
            to fetch.
        other_silly_variable: Another optional variable, that has a much
            longer name than the other args, and which does nothing.

    Returns:
        A dict mapping keys to the corresponding table row data
        fetched. Each row is represented as a tuple of strings. For
        example:

        {'Serak': ('Rigel VII', 'Preparer'),
         'Zim': ('Irk', 'Invader'),
         'Lrrr': ('Omicron Persei 8', 'Emperor')}

        If a key from the keys argument is missing from the dictionary,
        then that row was not found in the table.

    Raises:
        IOError: An error occurred accessing the bigtable.Table object.
    """
    pass

类的DocString :

class SampleClass(object):
    """Summary of class here.

    Longer class information....
    Longer class information....

    Attributes:
        likes_spam: A boolean indicating if we like SPAM or not.
        eggs: An integer count of the eggs we have laid.
    """

    def __init__(self, likes_spam=False):
        """Inits SampleClass with blah."""
        self.likes_spam = likes_spam
        self.eggs = 0

    def public_method(self):
        """Performs operation blah."""

对于一句话的注释，应该给出两个空格。

# We use a weighted dictionary search to find out where i is in
# the array.  We extrapolate position based on the largest num
# in the array and the array size and then do binary search to
# get the exact number.

if i & (i-1) == 0:        # true iff i is a power of 2

符号间隔

python比较特殊的地方在于，这个符号','后面要加一个空格。比如：

spam(ham[1], {eggs: 2}, [])
x, y = y, x

还有就是，在给参数赋默认值的时候，等号无需空格分开：

def complex(real, imag=0.0): return magic(r=real, i=imag)

不用刻意去对齐注释，下面这样是不对的，会带来维护上的麻烦。

  foo       = 1000  # comment
  long_name = 2     # comment that should not be aligned

  dictionary = {
      'foo'      : 1,
      'long_name': 2,
  }

空行

对于类和顶级的方法，需要用双行空行包围，对于类中的成员方法，用单行包围。注意，是包围哦。

括号

python里面括号是有意义的，代表一个元组，不要随便在return语句里套括号，不然你返回的就是也一个元组了。

参数值

如果有默认参数值的情况，使用none会比较好:

    def foo(a, b=None):
         if b is None:
             b = []

当你使用“可变”的对象作为函数中作为默认参数时会往往引起问题。因为在这种情况下参数可以在不创建新对象的情况下进行修改，例如 list dict。

>>> def function(data=[]):  
...     data.append(1)  
...     return data  
...  
>>> function()  
[1]  
>>> function()  
[1, 1]  
>>> function()  
[1, 1, 1]  

像你所看到的那样，list变得越来越长。如果你仔细地查看这个list。你会发现list一直是同一个对象。

原因很简单: 在每次函数调用的时候，函数一直再使用同一个list对象。这么使用引起的变化，非常“sticky”

像其他人所提到的那样，用一个占位符来替代可以修改的默认值。None

def myfunc(value=None):  
    if value is None:  
        value = []  
    # modify value here