Google Python Style Notes (Revision 2.59)

对你的代码运行pylint.

Suppress warnings if they are inappropriate so that other issues are not hidden.

To suppress warnings, you can set a line-level comment:

dict = 'something awful'  # Bad Idea... pylint: disable=redefined-builtin

pylint warnings are each identified by a alphanumeric code (C0112) and a symbolic name (empty-docstring). Prefer the symbolic names in new code or when updating existing code.

If the reason for the suppression is not clear from the symbolic name, add an explanation.

Suppressing in this way has the advantage that we can easily search for suppressions and revisit them.

You can get a list of pylint warnings by doing pylint --list-msgs. To get more information on a particular message, use pylint --help-msg=C6409.

Prefer pylint: disable to the deprecated older form pylint: disable-msg.

Unused argument warnings can be suppressed by using '_' as the identifier for the unused argument or prefixing the argument name with 'unused_'. In situations where changing the argument names is infeasible, you can mention them at the beginning of the function. For example:

def foo(a, unused_b, unused_c, d=None, e=None):
    _ = d, e
    return a

仅对包和模块使用导入

Decision: Use import x for importing packages and modules.

Use from x import y where x is the package prefix and y is the module name with no prefix.

Use from x import y as z if two modules named y are to be imported or if y is an inconveniently long name.

For example the module sound.effects.echo may be imported as follows:

from sound.effects import echo
...
echo.EchoFilter(input, output, delay=0.7, atten=4)

Do not use relative names in imports. Even if the module is in the same package, use the full package name. This helps prevent unintentionally importing a package twice.

使用模块的全路径名来导入每个模块

Decision: All new code should import each module by its full package name.

Imports should be as follows:

# Reference in code with complete name.
import sound.effects.echo

# Reference in code with just module name (preferred).
from sound.effects import echo

允许使用异常, 但必须小心

Decision: Exceptions must follow certain conditions:

• Raise exceptions like this: raise MyException('Error message') or raise MyException. Do not use the two-argument form (raise MyException, 'Error message') or deprecated string-based exceptions (raise 'Error message').
• Modules or packages should define their own domain-specific base exception class, which should inherit from the built-in Exception class. The base exception for a module should be called Error.

  class Error(Exception):
      pass
  

• Never use catch-all except: statements, or catch Exception or StandardError, unless you are re-raising the exception or in the outermost block in your thread (and printing an error message). Python is very tolerant in this regard and except: will really catch everything including misspelled names, sys.exit() calls, Ctrl+C interrupts, unittest failures and all kinds of other exceptions that you simply don't want to catch.
• Minimize the amount of code in a try/except block. The larger the body of the try, the more likely that an exception will be raised by a line of code that you didn't expect to raise an exception. In those cases, the try/except block hides a real error.
• Use the finally clause to execute code whether or not an exception is raised in the try block. This is often useful for cleanup, i.e., closing a file.
• When capturing an exception, use as rather than a comma. For example:

  try:
      raise Error
  except Error as error:
      pass
  

避免全局变量

Decision:

Avoid global variables in favor of class variables. Some exceptions are:

• Default options for scripts.
• Module-level constants. For example: PI = 3.14159. Constants should be named using all caps with underscores; see Naming below.
• It is sometimes useful for globals to cache values needed or returned by functions.
• If needed, globals should be made internal to the module and accessed through public module level functions; see Naming below.

鼓励使用嵌套/本地/内部类或函数

可以在简单情况下使用

Decision: Okay to use for simple cases. Each portion must fit on one line: mapping expression, for clause, filter expression. Multiple for clauses or filter expressions are not permitted. Use loops instead when things get more complicated.

Yes:
  result = []
  for x in range(10):
      for y in range(5):
          if x * y > 10:
              result.append((x, y))

  for x in xrange(5):
      for y in xrange(5):
          if x != y:
              for z in xrange(5):
                  if y != z:
                      yield (x, y, z)

  return ((x, complicated_transform(x))
          for x in long_generator_function(parameter)
          if x is not None)

  squares = [x * x for x in range(10)]

  eat(jelly_bean for jelly_bean in jelly_beans
      if jelly_bean.color == 'black')
No:
  result = [(x, y) for x in range(10) for y in range(5) if x * y > 10]

  return ((x, y, z)
          for x in xrange(5)
          for y in xrange(5)
          if x != y
          for z in xrange(5)
          if y != z)

如果类型支持, 就使用默认迭代器和操作符. 比如列表, 字典及文件等.

Decision: Use default iterators and operators for types that support them, like lists, dictionaries, and files. The built-in types define iterator methods, too. Prefer these methods to methods that return lists, except that you should not mutate a container while iterating over it.

Yes:  for key in adict: ...
      if key not in adict: ...
      if obj in alist: ...
      for line in afile: ...
      for k, v in dict.iteritems(): ...
No:   for key in adict.keys(): ...
      if not adict.has_key(key): ...
      for line in afile.readlines(): ...

按需使用生成器.

Decision:

Fine. Use "Yields:" rather than "Returns:" in the doc string for generator functions.

适用于单行函数

Decision:

Okay to use them for one-liners. If the code inside the lambda function is any longer than 60–80 chars, it's probably better to define it as a regular (nested) function.

For common operations like multiplication, use the functions from the operator module instead of lambda functions. For example, prefer operator.mul to lambda x, y: x * y.

适用于单行.

Conditional expressions are mechanisms that provide a shorter syntax for if statements. For example: x = 1 if cond else 2

适用于大部分情况.

Decision:

Okay to use with the following caveat:

Do not use mutable objects as default values in the function or method definition.

Yes: def foo(a, b=None):
         if b is None:
             b = []
No:  def foo(a, b=[]):
         ...
No:  def foo(a, b=time.time()):  # The time the module was loaded???
         ...
No:  def foo(a, b=FLAGS.my_thing):  # sys.argv has not yet been parsed...
         ...

访问和设置数据成员时, 你通常会使用简单, 轻量级的访问和设置函数. 建议用属性（properties）来代替它们.

Use properties in new code to access or set data where you would normally have used simple, lightweight accessor or setter methods. Read-only properties should be created with the @property decorator.

Inheritance with properties can be non-obvious if the property itself is not overridden. Thus one must make sure that accessor methods are called indirectly to ensure methods overridden in subclasses are called by the property (using the Template Method DP).

Yes: import math

     class Square(object):
         """A square with two properties: a writable area and a read-only perimeter.

         To use:
         >>> sq = Square(3)
         >>> sq.area
         9
         >>> sq.perimeter
         12
         >>> sq.area = 16
         >>> sq.side
         4
         >>> sq.perimeter
         16
         """

         def __init__(self, side):
             self.side = side

         def __get_area(self):
             """Calculates the 'area' property."""
             return self.side ** 2

         def ___get_area(self):
             """Indirect accessor for 'area' property."""
             return self.__get_area()

         def __set_area(self, area):
             """Sets the 'area' property."""
             self.side = math.sqrt(area)

         def ___set_area(self, area):
             """Indirect setter for 'area' property."""
             self.__set_area(area)

         area = property(___get_area, ___set_area,
                         doc="""Gets or sets the area of the square.""")

         @property
         def perimeter(self):
             return self.side * 4

尽可能使用隐式false

Use the "implicit" false if at all possible, e.g., if foo: rather than if foo != []:. There are a few caveats that you should keep in mind though:

• Never use = or ! to compare singletons like None. Use is or is not.
• Beware of writing if x: when you really mean if x is not None:—e.g., when testing whether a variable or argument that defaults to None was set to some other value. The other value might be a value that's false in a boolean context!
• Never compare a boolean variable to False using ==. Use if not x: instead. If you need to distinguish False from None then chain the expressions, such as if not x and x is not None:.
• For sequences (strings, lists, tuples), use the fact that empty sequences are false, so if not seq: or if seq: is preferable to if len(seq): or if not len(seq):.
• When handling integers, implicit false may involve more risk than benefit (i.e., accidentally handling None as 0). You may compare a value which is known to be an integer (and is not the result of len()) against the integer 0.

Yes: if not users:
         print 'no users'

     if foo == 0:
         self.handle_zero()

     if i % 10 == 0:
         self.handle_multiple_of_ten()
No:  if len(users) == 0:
         print 'no users'

     if foo is not None and not foo:
         self.handle_zero()

     if not i % 10:
         self.handle_multiple_of_ten()

• Note that '0' (i.e., 0 as string) evaluates to true.

尽可能使用字符串方法取代字符串模块. 使用函数调用语法取代apply(). 使用列表推导, for循环取代filter()和map()当函数参数是个内联的lamdba. 使用for取代reduce().

We do not use any Python version which does not support these features, so there is no reason not to use the new styles.

Yes: words = foo.split(':')

     [x[1] for x in my_list if x[2] == 5]

     map(math.sqrt, data)    # Ok. No inlined lambda expression.

     fn(*args, **kwargs)
No:  words = string.split(foo, ':')

     map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))

     apply(fn, args, kwargs)

可以使用

如果好处很显然, 就明智的使用装饰器

Use decorators judiciously when there is a clear advantage. Decorators should follow the same import and naming guidelines as functions. Decorator pydoc should clearly state that the function is a decorator. Write unit tests for decorators.

Avoid external dependencies in the decorator itself (e.g. don't rely on files, sockets, database connections, etc.), since they might not be available when the decorator runs (at import time, perhaps from pydoc or other tools). A decorator that is called with valid parameters should (as much as possible) be guaranteed to succeed in all cases.

Decorators are a special case of "top level code" - see main for more discussion.

不要依赖内建类型的原子性.

While Python's built-in data types such as dictionaries appear to have atomic operations, there are corner cases where they aren't atomic (e.g. if hash or eq are implemented as Python methods) and their atomicity should not be relied upon. Neither should you rely on atomic variable assignment (since this in turn depends on dictionaries).

Use the Queue module's Queue data type as the preferred way to communicate data between threads. Otherwise, use the threading module and its locking primitives. Learn about the proper use of condition variables so you can use threading.Condition instead of using lower-level locks.

避免使用这些特性

不要在行尾加分号, 也不要用分号将两条命令放在同一行.

每行不超过80个字符

Exceptions:

• Long import statements.
• URLs in comments.

Do not use backslash line continuation.

Make use of Python's implicit line joining inside parentheses, brackets and braces. If necessary, you can add an extra pair of parentheses around an expression.

Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
             emphasis=None, highlight=0)

     if (width == 0 and height == 0 and
         color == 'red' and emphasis == 'strong'):

When a literal string won't fit on a single line, use parentheses for implicit line joining.

x = ('This will build a very long long '
     'long long long long long long string')

Within comments, put long URLs on their own line if necessary.

Yes:  # See details at
      # http://www.example.com/us/developer/documentation/api/content/v2.0/csv_file_name_extension_full_specification.html
No:  # See details at
     # http://www.example.com/us/developer/documentation/api/content/\
     # v2.0/csv_file_name_extension_full_specification.html

Make note of the indentation of the elements in the line continuation examples above; see the indentation section for explanation.

保守的使用括号

Do not use them in return statements or conditional statements unless using parentheses for implied line continuation. (See above.) It is however fine to use parentheses around tuples.

Yes: if foo:
         bar()
     while x:
         x = bar()
     if x and y:
         bar()
     if not x:
         bar()
     return foo
     for (x, y) in dict.items(): ...
No:  if (x):
         bar()
     if not(x):
         bar()
     return (foo)

用4个空格来缩进代码

Never use tabs or mix tabs and spaces. In cases of implied line continuation, you should align wrapped elements either vertically, as per the examples in the line length section; or using a hanging indent of 4 spaces, in which case there should be no argument on the first line.

Yes:   # Aligned with opening delimiter
       foo = long_function_name(var_one, var_two,
                                var_three, var_four)

       # Aligned with opening delimiter in a dictionary
       foo = {
           long_dictionary_key: value1 +
                                value2,
           ...
       }

       # 4-space hanging indent; nothing on first line
       foo = long_function_name(
           var_one, var_two, var_three,
           var_four)

       # 4-space hanging indent in a dictionary
       foo = {
           long_dictionary_key:
               long_dictionary_value,
           ...
       }
No:    # Stuff on first line forbidden
       foo = long_function_name(var_one, var_two,
           var_three, var_four)

       # 2-space hanging indent forbidden
       foo = long_function_name(
         var_one, var_two, var_three,
         var_four)

       # No hanging indent in a dictionary
       foo = {
       long_dictionary_key:
       long_dictionary_value,
               ...
       }

顶级定义之间空两行, 方法定义之间空一行

One blank line between method definitions and between the class line and the first method. Use single blank lines as you judge appropriate within functions or methods.

按照标准的排版规范来使用标点两边的空格

No whitespace inside parentheses, brackets or braces.

Yes: spam(ham[1], {eggs: 2}, [])
No:  spam( ham[ 1 ], { eggs: 2 }, [ ] )

No whitespace before a comma, semicolon, or colon. Do use whitespace after a comma, semicolon, or colon except at the end of the line.

Yes: if x == 4:
         print x, y
     x, y = y, x
No:  if x == 4 :
         print x , y
     x , y = y , x

No whitespace before the open paren/bracket that starts an argument list, indexing or slicing.

Yes: spam(1)
No:  spam (1)
Yes: dict['key'] = list[index]
No:  dict ['key'] = list [index]

Surround binary operators with a single space on either side for assignment (=), comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not), and Booleans (and, or, not). Use your better judgment for the insertion of spaces around arithmetic operators but always be consistent about whitespace on either side of a binary operator.

Yes: x == 1
No:  x<1

Don't use spaces around the '=' sign when used to indicate a keyword argument or a default parameter value.

Yes: def complex(real, imag=0.0): return magic(r=real, i=imag)
No:  def complex(real, imag = 0.0): return magic(r = real, i = imag)

Don't use spaces to vertically align tokens on consecutive lines, since it becomes a maintenance burden (applies to :, #, =, etc.):

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

  dictionary = {
      'foo': 1,
      'long_name': 2,
  }
No:
  foo       = 1000  # comment
  long_name = 2     # comment that should not be aligned

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

大部分.py文件不要以#!行开头。以#!/usr/bin/env python开头

This line is used by the kernel to find the Python interpreter, but is ignored by Python when importing modules. It is only necessary on a file that will be executed directly.

#!/usr/bin/env python2.4

确保对模块, 函数, 方法和行内注释使用正确的风格

如果一个类不继承自其它类, 就显式的从object继承. 嵌套类也一样.

Yes: class SampleClass(object):
         pass


     class OuterClass(object):

         class InnerClass(object):
             pass


     class ChildClass(ParentClass):
         """Explicitly inherits from another class already."""

No: class SampleClass:
        pass


    class OuterClass:

        class InnerClass:
            pass

Inheriting from object is needed to make properties work properly, and it will protect your code from one particular potential incompatibility with Python 3000. It also defines special methods that implement the default semantics of objects including __new__,__init__, __delattr__, __getattribute__, __setattr__, __hash__, __repr__, and __str__.

用format方法或%操作符格式化字符串, 即使参数都是字符串. 然而你也需要在+和%(或%)之间好好判定.

Yes: 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)
No: x = '%s%s' % (a, b)  # use + in this case
    x = '{}{}'.format(a, b)  # use + in this case
    x = imperative + ', ' + expletive + '!'
    x = 'name: ' + name + '; score: ' + str(n)

Avoid using the + and += operators to accumulate a string within a loop. Since strings are immutable, this creates unnecessary temporary objects and results in quadratic rather than linear running time. Instead, add each substring to a list and ''.join the list after the loop terminates (or, write each substring to a io.BytesIO buffer).

Yes: items = ['<table>']
     for last_name, first_name in employee_list:
         items.append('<tr><td>%s, %s</td></tr>' % (last_name, first_name))
     items.append('</table>')
     employee_table = ''.join(items)
No: employee_table = '<table>'
    for last_name, first_name in employee_list:
        employee_table += '<tr><td>%s, %s</td></tr>' % (last_name, first_name)
    employee_table += '</table>'

Be consistent with your choice of string quote character within a file. Pick ' or " and stick with it. It is okay to use the other quote character on a string to avoid the need to \ escape within the string. GPyLint enforces this.

Yes:
  Python('Why are you hiding your eyes?')
  Gollum("I'm scared of lint errors.")
  Narrator('"Good!" thought a happy Python reviewer.')
No:
  Python("Why are you hiding your eyes?")
  Gollum('The lint. It burns. It burns us.')
  Gollum("Always the great lint. Watching. Watching.")

Prefer """ for multi-line strings rather than '''. Projects may choose to use ''' for all non-docstring multi-line strings if and only if they also use ' for regular strings. Doc strings must use """ regardless. Note that it is often cleaner to use implicit line joining since multi-line strings do not flow with the indentation of the rest of the program:

Yes:
  print ("This is much nicer.\n"
         "Do it this way.\n")
  No:
    print """This is pretty ugly.
Don't do this.
"""

明确的关系文件和套接子当使用完。

Furthermore, while files and sockets are automatically closed when the file object is destructed, tying the life-time of the file object to the state of the file is poor practice, for several reasons:

• There are no guarantees as to when the runtime will actually run the file's destructor. Different Python implementations use different memory management techniques, such as delayed Garbage Collection, which may increase the object's lifetime arbitrarily and indefinitely.
• Unexpected references to the file may keep it around longer than intended (e.g. in tracebacks of exceptions, inside globals, etc).

The preferred way to manage files is using the "with" statement:

with open("hello.txt") as hello_file:
    for line in hello_file:
        print line

For file-like objects that do not support the "with" statement, use contextlib.closing():

import contextlib

with contextlib.closing(urllib.urlopen("http://www.python.org/")) as front_page:
    for line in front_page:
        print line

Legacy AppEngine code using Python 2.5 may enable the "with" statement using "from future import with_statement".

为临时代码使用TODO注释, 它是一种短期解决方案. 不算完美, 但够好了.

TODOs should include the string TODO in all caps, followed by the name, e-mail address, or other identifier of the person who can best provide context about the problem referenced by the TODO, in parentheses. A colon is optional. A comment explaining what there is to do is required. The main purpose is to have a consistent TODO format that can be searched to find the person who can provide more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO, it is almost always your name that is given.

# TODO(kl@gmail.com): Use a "*" here for string repetition.
# TODO(Zeke) Change this to use relations.

If your TODO is of the form "At a future date do something" make sure that you either include a very specific date ("Fix by November 2009") or a very specific event ("Remove this code when all clients can handle XML responses.").

每个导入应该独占一行

E.g.:

Yes: import os
     import sys
No:  import os, sys

Imports are always put at the top of the file, just after any module comments and doc strings and before module globals and constants.

Imports should be grouped with the order being most generic to least generic:

• standard library imports
• third-party imports
• application-specific imports

Within each grouping, imports should be sorted lexicographically, ignoring case, according to each module's full package path.

import foo
from foo import bar
from foo.bar import baz
from foo.bar import Quux
from Foob import ar

通常每个语句应该独占一行

However, you may put the result of a test on the same line as the test only if the entire statement fits on one line. In particular, you can never do so with try/except since the try and except can't both fit on the same line, and you can only do so with an if if there is no else.

Yes:

  if foo: bar(foo)
No:

  if foo: bar(foo)
  else:   baz(foo)

  try:               bar(foo)
  except ValueError: baz(foo)

  try:
      bar(foo)
  except ValueError: baz(foo)

在Python中, 对于琐碎又不太重要的访问函数, 你应该直接使用公有变量来取代它们, 这样可以避免额外的函数调用开销. 当添加更多功能时, 你可以用属性(property)来保持语法的一致性.

On the other hand, if access is more complex, or the cost of accessing the variable is significant, you should use function calls (following the Naming guidelines) such as get_foo() and set_foo(). If the past behavior allowed access through a property, do not bind the new accessor functions to the property. Any code still attempting to access the variable by the old method should break visibly so they are made aware of the change in complexity.

即使是一个打算被用作脚本的文件,仅仅的导入不应该对脚本的主功能造成副作用。 主功能应该在一个main()函数中。

In Python, pydoc as well as unit tests require modules to be importable. Your code should always check if __name__ == '__main__': before executing your main program so that the main program is not executed when the module is imported.

def main():
      ...

if __name__ == '__main__':
    main()