Indentation 缩进

Use 4 spaces per indentation level. 

每个缩进级别使用4个空格

Continuation lines should align wrapped elements either vertically using Python’s implicit line joining inside parentheses, brackets and braces, or using a hanging indent[7]. When using a hanging indent the following should be considered; there should be no arguments on the first line and further indentation should be used to clearly distinguish itself as a continuation line.

连续行需要对齐其所包含的元素，或者垂直的使用Python的隐式缩进，对齐圆括号、方括号及花括号，或者使用悬挂缩进。当使用悬挂缩进时，需考虑以下因素：第一行不应包含参数，后续的行应该增加一级缩进：

Yes 正确:

# Aligned with opening delimiter. 
# 和开放的符号对齐（隐式缩进）
foo = long_function_name(var_one, var_two,
                         var_three, var_four)

# More indentation included to distinguish this from the rest.
# 使用更多的缩进以便与后面的代码区分（悬挂缩进）
def long_function_name(
        var_one, var_two, var_three,
        var_four):
    print(var_one)

# Hanging indents should add a level.
# 悬挂缩进需要增加一级缩进
foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

No 错误:

# Arguments on first line forbidden when not using vertical alignment.
# 如果不使用垂直对齐（隐式缩进），第一行禁止使用参数
foo = long_function_name(var_one, var_two,
    var_three, var_four)

# Further indentation required as indentation is not distinguishable.
# （悬挂缩进）需要进一步缩进以便与后续代码（print）进行区分
def long_function_name(
    var_one, var_two, var_three,
    var_four):
    print(var_one)

The 4-space rule is optional for continuation lines.

对于连续的行，4-空格 规则不是强制的。

Optional 可选方案（此处2-空格）:

# Hanging indents *may* be indented to other than 4 spaces.
foo = long_function_name(
  var_one, var_two,
  var_three, var_four)

When the conditional part of an if-statement is long enough to require that it be written across multiple lines, it’s worth noting that the combination of a two character keyword (i.e. if), plus a single space, plus an opening parenthesis creates a natural 4-space indent for the subsequent lines of the multiline conditional. This can produce a visual conflict with the indented suite of code nested inside the if-statement, which would also naturally be indented to 4 spaces. This PEP takes no explicit position on how (or whether) to further visually distinguish such conditional lines from the nested suite inside the if-statement. Acceptable options in this situation include, but are not limited to: 

当if语句中的条件部分长到需要使用多行时，不值得结合使用一个2字母的关键字（比如 if）、一个空格以及开放的括号独占一行，然后（多）缩进4个空格开始后续的条件语句。由于if语句内部本身要缩进4个空格，再次缩进会造成视觉上的冲突。本PEP没有明确指出怎么（或者是否）要进一步在视觉上区分这些条件部分及if-语句内嵌套的部分。在这种情况下可接受的选择包括但不限于以下几种：

# No extra indentation.
# 无额外缩进
if (this_is_one_thing and
    that_is_another_thing):
    do_something()

# Add a comment, which will provide some distinction in editors
# 添加一个注释，由其提供编辑器内的区分
# supporting syntax highlighting.
if (this_is_one_thing and
    that_is_another_thing):
    # Since both conditions are true, we can frobnicate.
    do_something()

# Add some extra indentation on the conditional continuation line.
# 在条件部分的连续行增加额外缩进
if (this_is_one_thing
        and that_is_another_thing):
    do_something()

(Also see the discussion of whether to break before or after binary operators below.)

（也参见下边关于在二元运算符之前或之后换行的讨论）

The closing brace/bracket/parenthesis on multiline constructs may either line up under the first non-whitespace character of the last line of list, as in:

横跨多行的大括号/方括号/括号可以要么闭合于最后一行的第一个非空格元素，如下：

my_list = [
    1, 2, 3,
    4, 5, 6,
    ]
result = some_function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f',
    )

or it may be lined up under the first character of the line that starts the multiline construct, as in:

或者与这个多行结构的开始行的第一个字符对齐，如下：

my_list = [
    1, 2, 3,
    4, 5, 6,
]
result = some_function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f',
)

Tabs or Spaces? 制表符还是空格？

Spaces are the preferred indentation method.

推荐使用空格进行缩进。

Tabs should be used solely to remain consistent with code that is already indented with tabs.

制表符仅在需要与已经使用制表符进行缩进的代码保持一致时才使用。

Python 3 disallows mixing the use of tabs and spaces for indentation.

Python 3 不允许在缩进时混合使用制表符和空格。

Python 2 code indented with a mixture of tabs and spaces should be converted to using spaces exclusively.

Python 2 代码中本来混合使用制表符和空格进行缩进的，需要全部转换为使用空格进行缩进。

When invoking the Python 2 command line interpreter with the -t option, it issues warnings about code that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are highly recommended!

当使用 -t 选项运行Python 2 代码时，会对非法混用制表符和空格的代码进行警告。 当使用 -tt 选项时，这些警告会变成报错。非常推荐使用这些选项。

Maximum Line Length 每行最大长度

Limit all lines to a maximum of 79 characters.

每行的最大长度限定于79字符。

For flowing long blocks of text with fewer structural restrictions (docstrings or comments), the line length should be limited to 72 characters.

对于含有较少结构限制的连续大块文字（docstrings或者注释），行的长度应该限定于72字符。

Limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.

对编辑器窗口的宽度进行限定后可以并排打开若干文件，并且便于使用代码检查工具时在相邻的两列展示不同的版本。

The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.

大部分工具的默认换行功能会破坏代码的可视化结构，从而使代码更加难以理解。选择上述代码长度的限制是为了避免编辑窗口宽度为80字符时的自动换行，尽管工具会自动换行时候的最后一列设置一个标记。有些基于网页的工具也许根本就不提供自动换行功能。

Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the nominal line length from 80 to 100 characters (effectively increasing the maximum length to 99 characters), provided that comments and docstrings are still wrapped at 72 characters.

有些团队强烈推荐较长的代码行长度。如果完全或者主要负责维护代码的队伍对此达成一致，那么将代码行长度从80提高到100字符也是可以的（有效的将最大行长度提高的99字符），同时应该确保注释和docstring仍然在72字符处换行。

The Python standard library is conservative and requires limiting lines to 79 characters (and docstrings/comments to 72).

Python的标准库是保守的并且要求行长度限定为最大79字符（并且docstrings和注释最大72字符）。

The preferred way of wrapping long lines is by using Python’s implied line continuation inside parentheses, brackets and braces. Long lines can be broken over multiple lines by wrapping expressions in parentheses. These should be used in preference to using a backslash for line continuation.

比较推荐的续行方式是在Python的圆括号、方括号和大括号内进行隐式续行。括号中的较长代码行可以折成多行。这种换行方式比使用反斜杠进行续行更受推荐。

Backslashes may still be appropriate at times. For example, long, multiple with-statements cannot use implicit continuation, so backslashes are acceptable:

有时使用反斜杠也比较合适。比如，在较长的多行with语句中，不能使用隐式续行的时候，反斜杠也是可以接受的：
with open('/path/to/some/file/you/want/to/read') as file_1, \
     open('/path/to/some/file/being/written', 'w') as file_2:
    file_2.write(file_1.read())

(See the previous discussion on multiline if-statements for further thoughts on the indentation of such multiline with-statements.)

（查看之前讨论的多行if语句深入思考对这种多行with语句的缩进。）

Another such case is with assert statements.

另一种类似情况是对assert语句。

Make sure to indent the continued line appropriately.

确保对连续的行进行合适的缩进。

Should a line break before or after a binary operator? 应该在二元运算符之前还是之后换行？

For decades the recommended style was to break after binary operators. But this can hurt readability in two ways: the operators tend to get scattered across different columns on the screen, and each operator is moved away from its operand and onto the previous line. Here, the eye has to do extra work to tell which items are added and which are subtracted:

几十年来推荐的风格是在二元运算符之后换行。但是这有可能在两方面影响可读性：运算符会漫布于屏幕上不同的行，另外运算符远离它的算子并且成为较前一行。这样，眼睛就必须弄明白哪些项被加，哪些项被减：

# No: operators sit far away from their operands
# 不要：运算符远离他们的算子
income = (gross_wages +
          taxable_interest +
          (dividends - qualified_dividends) -
          ira_deduction -
          student_loan_interest)

To solve this readability problem, mathematicians and their publishers follow the opposite convention. Donald Knuth explains the traditional rule in his Computers and Typesetting series: “Although formulas within a paragraph always break after binary operations and relations, displayed formulas always break before binary operations” [3].

为了解决这个可读性问题，数学家和出版人追从相反的约定。Donald Knuth在他的Computers and Typesetting系列中解释了传统的原则: 尽管一段之内的公式常在二元运算符和关系之换行，展示出来的公式常常在二元运算符之前换行。

Following the tradition from mathematics usually results in more readable code:

根据数学家的传统常常可以得到更具可读性的代码：

# Yes: easy to match operators with operands
# 正确：便于连接运算符和算子 
income = (gross_wages
          + taxable_interest
          + (dividends - qualified_dividends)
          - ira_deduction
          - student_loan_interest)

In Python code, it is permissible to break before or after a binary operator, as long as the convention is consistent locally. For new code Knuth’s style is suggested.

Blank Lines 空行

Surround top-level function and class definitions with two blank lines.

使用两个空行来分隔最高级的函数和类的定义。

Method definitions inside a class are surrounded by a single blank line.

使用一个空行来分隔一个类之中的方法定义。

Extra blank lines may be used (sparingly) to separate groups of related functions. Blank lines may be omitted between a bunch of related one-liners (e.g. a set of dummy implementations).

额外的空行也许可以（谨慎地）用于分隔不同的某组相关的函数。在一系列单行代码之间（比如一系列dummy）可以省略空行。

Use blank lines in functions, sparingly, to indicate logical sections.

在函数中（谨慎的）使用空行来区分不同的逻辑块。

Python accepts the control-L (i.e. ^L) form feed character as whitespace; Many tools treat these characters as page separators, so you may use them to separate pages of related sections of your file. Note, some editors and web-based code viewers may not recognize control-L as a form feed and will show another glyph in its place.

Python接受control-L (即 ^L)作为空格；许多工具把这些作为分页符，因此你可以用它们来对文件中的相关块进行分页。注意，有些编辑器和基于网页的代码浏览器也许不把control-L识别为换页符，它们也许会被显示为其他符号。 

Source File Encoding 源文件编码

Code in the core Python distribution should always use UTF-8 (or ASCII in Python 2).

Python核心发行版中的代码应该保持使用UTF-8（或者Python2使用ASCII）。

Files using ASCII (in Python 2) or UTF-8 (in Python 3) should not have an encoding declaration.

使用ASCII（Python 2）和UTF-8 （Python 3）的文件不需要编码声明。

In the standard library, non-default encodings should be used only for test purposes or when a comment or docstring needs to mention an author name that contains non-ASCII characters; otherwise, using \x, \u, \U, or \N escapes is the preferred way to include non-ASCII data in string literals.

在标准库中，非默认的编码应该仅用于测试目的或者当注释或docstring需要提及含有非ASCII字符的作者名字时；否则，推荐使用\x, \u, \U 和 \N 等转义符是表达字符串中的非ASCII数据。

For Python 3.0 and beyond, the following policy is prescribed for the standard library (see PEP 3131): All identifiers in the Python standard library MUST use ASCII-only identifiers, and SHOULD use English words wherever feasible (in many cases, abbreviations and technical terms are used which aren’t English). In addition, string literals and comments must also be in ASCII. The only exceptions are (a) test cases testing the non-ASCII features, and (b) names of authors. Authors whose names are not based on the Latin alphabet (latin-1, ISO/IEC 8859-1 character set) MUST provide a transliteration of their names in this character set.

对于Python 3.0以上的版本，标准库遵循以下原则（参见 PEP 3131）：标准库中的所有标识符都必须仅使用ASCII标识符，并且在任何可能的情况下应该使用英语（在很多情况下，缩写和技术名词不是英语）。此外，字符串和注释也必须是ASCII，仅有两种例外情况（a）测试情况下进行非ASCII特征进行测试， （b）作者名字。当作者的名字不是基于拉丁字母时（latin-1, ISO/IEC 8859-1 character set），作者必须提供他们名字的拉丁字符音译。

Open source projects with a global audience are encouraged to adopt a similar policy.

鼓励面向全世界开源的项目使用类似原则。

Imports

• Imports should usually be on separate lines, e.g.: 通常Imports应该在单独的一行，比如：

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

  It’s okay to say this though: 尽管如此，如下语句也可以：

  from subprocess import Popen, PIPE
  

• Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants. Imports语句总是在文件的最顶端，仅在模块注释和docstring之后，并且在全局模块和常量之前：

  Imports should be grouped in the following order: Imports应该按以下顺序进行分组：

  1. standard library imports 标准库imports
  2. related third party imports 相关第三方 imports
  3. local application/library specific imports 本地应用和库的特定imports

  You should put a blank line between each group of imports. 你应该在每组之间加入空行。

• Absolute imports are recommended, as they are usually more readable and tend to be better behaved (or at least give better error messages) if the import system is incorrectly configured (such as when a directory inside a package ends up on sys.path): 推荐使用绝对imports，因为如果import系统正确配置的话（比如包内的路径以sys.path），通常来说使用绝对imports更加具有可读性而且表现更好 （至少会提供更好的报错信息）

  import mypkg.sibling
  from mypkg import sibling
  from mypkg.sibling import example
  

  However, explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose: 然而，除了绝对imports之外，详尽的相对imports也是可以接受的，特别是当使用复杂的包时当绝对imports变得不必要的冗长的时候，

  from . import sibling
  from .sibling import example
  

  Standard library code should avoid complex package layouts and always use absolute imports. 标准库代码应该避免复杂的包并且始终使用绝对imports。

  Implicit relative imports should never be used and have been removed in Python 3. 不明确的相对引用应该从不被使用，并且在Python 3中已经被移除。

• When importing a class from a class-containing module, it’s usually okay to spell this: 当从一个含有类的模块中import一个类的时候，以下语句也可以：

  from myclass import MyClass
  from foo.bar.yourclass import YourClass
  

  If this spelling causes local name clashes, then spell them 如果以下拼写造成了本地变量名冲突的话，则使用：

  import myclass
  import foo.bar.yourclass
  

  and use “myclass.MyClass” and “foo.bar.yourclass.YourClass”. 并且用 “myclass.MyClass” 和 “foo.bar.yourclass.YourClass”

• Wildcard imports (from <module> import *) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools. There is one defensible use case for a wildcard import, which is to republish an internal interface as part of a public API (for example, overwriting a pure Python implementation of an interface with the definitions from an optional accelerator module and exactly which definitions will be overwritten isn’t known in advance). 应该避免使用通配符import， (from <module> import *)，因为会造成命名空间中的名称不清晰，给读者和许多自动化工具造成困扰。在一种情况下可以使用通配符import，也就是当把一个内部接口作为一个公共API重新发布的时候（比如，通过一个可选加速器模块覆盖一个纯Python实现的接口，并且哪些定义会被覆盖提前并不能预知。）

  When republishing names this way, the guidelines below regarding public and internal interfaces still apply. 当这样重复发布名称时，以下关于公共和内部接口的指南依然适用。

Module level dunder names 模块层dunder名称

Module level “dunders” (i.e. names with two leading and two trailing underscores) such as __all__, __author__, __version__, etc. should be placed after the module docstring but before any import statements exceptfrom __future__ imports. Python mandates that future-imports must appear in the module before any other code except docstrings.

模块层的“dunders” （即名称中含有两个前和两个后下划线）比如 __all__, __author__, __version__ 等，应该放置于模块的文档说明之后但是在任何除了from __future__ imports之外的import语句之前。Python要求future-imports必须比除了文档说明之外的任何代码更早出现在模块。

For example: 例如：

"""This is the example module.

This module does stuff.
"""

from __future__ import barry_as_FLUFL

__all__ = ['a', 'b', 'c']
__version__ = '0.1'
__author__ = 'Cardinal Biggles'

import os
import sys

Pet Peeves

Avoid extraneous whitespace in the following situations:

避免在如下情况使用无关的空格：

• Immediately inside parentheses, brackets or braces. 括号，方括号和大括号中直接的空格。

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

• Between a trailing comma and a following close parenthesis. 在尾随逗号和结束括号之间。

  Yes: foo = (0,)
  No:  bar = (0, )
  

• Immediately before a comma, semicolon, or colon: 直接在逗号、分号和冒号之前。

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

• However, in a slice the colon acts like a binary operator, and should have equal amounts on either side (treating it as the operator with the lowest priority). In an extended slice, both colons must have the same amount of spacing applied. Exception: when a slice parameter is omitted, the space is omitted. 然而，在切片中冒号类似于二元运算符，应该在其两边使用相同数量的空格（把它作为拥有最低优先级的运算符）。在扩展切片中，两个冒号必须有相同的空格。特殊情况：如果一个切片参数被省略了，空格也省略。

  Yes:

  ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
  ham[lower:upper], ham[lower:upper:], ham[lower::step]
  ham[lower+offset : upper+offset]
  ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
  ham[lower + offset : upper + offset]
  

  No:

  ham[lower + offset:upper + offset]
  ham[1: 9], ham[1 :9], ham[1:9 :3]
  ham[lower : : upper]
  ham[ : upper]
  

• Immediately before the open parenthesis that starts the argument list of a function call: 直接在一个函数的开始括号之前。

  Yes: spam(1)
  No:  spam (1)
  

• Immediately before the open parenthesis that starts an indexing or slicing: 直接在一个索引或者切片的开始括号之前。

  Yes: dct['key'] = lst[index]
  No:  dct ['key'] = lst [index]
  

• More than one space around an assignment (or other) operator to align it with another. 为了对齐其他参数而使用多于一个空格。

  Yes:

  x = 1
  y = 2
  long_variable = 3
  

  No:

  x             = 1
  y             = 2
  long_variable = 3
  

Other Recommendations 其他推荐

• Avoid trailing whitespace anywhere. Because it’s usually invisible, it can be confusing: e.g. a backslash followed by a space and a newline does not count as a line continuation marker. Some editors don’t preserve it and many projects (like CPython itself) have pre-commit hooks that reject it. 避免在任何地方使用尾空格。因为它是不可见的，这会造成困扰：比如一个反斜杠后面跟了一个空格因此新的一行不被认为是新行。有些编辑器不确保其正确运行，许多项目（比如CPython本身）有pre-commit hooks可以拒绝此类编译。

• Always surround these binary operators with a single space on either side: assignment (=), augmented assignment (+=, -=etc.), comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not), Booleans (and, or, not). 总是在这些二元运算符两侧使用单独空格：等号（=），增量赋值负号 (+=, -=etc.), 比较符号(==, <, >, !=, <>, <=, >=, in, not in, is, is not), 布尔运算符(and, or, not).

• If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies). Use your own judgment; however, never use more than one space, and always have the same amount of whitespace on both sides of a binary operator. 如果使用不同优先级的运算符，考虑给低优先级的运算符增加空格。自行判断，然而，永远不要使用多于一个空格，并且总是在二元运算符的两侧使用相同数量的空格。

  Yes:

  i = i + 1
  submitted += 1
  x = x*2 - 1
  hypot2 = x*x + y*y
  c = (a+b) * (a-b)
  

  No:

  i=i+1
  submitted +=1
  x = x * 2 - 1
  hypot2 = x * x + y * y
  c = (a + b) * (a - b)
  

• Don’t use spaces around the = sign when used to indicate a keyword argument or a default parameter value.不要在用于定义函数关键字默认值的等号两侧使用空格。

  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)
  

• Function annotations should use the normal rules for colons and always have spaces around the -> arrow if present. (SeeFunction Annotations below for more about function annotations.) 函数annotations 应该使用平常的冒号规则并且如果使用 -> 的话需要配合空格。

  Yes:

  def munge(input: AnyStr): ...
  def munge() -> AnyStr: ...
  

  No:

  def munge(input:AnyStr): ...
  def munge()->PosInt: ...
  

• When combining an argument annotation with a default value, use spaces around the = sign (but only for those arguments that have both an annotation and a default). 当同时使用annotation和默认值的时候，给等号两侧加上空格 （但是只给同时有annotation和默认值的等号加空格）。

  Yes:

  def munge(sep: AnyStr = None): ...
  def munge(input: AnyStr, sep: AnyStr = None, limit=1000): ...
  

  No:

  def munge(input: AnyStr=None): ...
  def munge(input: AnyStr, limit = 1000): ...
  

• Compound statements (multiple statements on the same line) are generally discouraged. 通常不推荐使用混合语句（同一行多个语句）。

  Yes:

  if foo == 'blah':
      do_blah_thing()
  do_one()
  do_two()
  do_three()
  

  Rather not:

  if foo == 'blah': do_blah_thing()
  do_one(); do_two(); do_three()
  

• While sometimes it’s okay to put an if/for/while with a small body on the same line, never do this for multi-clause statements. Also avoid folding such long lines! 尽管有时可以将if/for/while和一小部分语句放在同一行，永远不要将含有多语句的指令放在同一行。同时也避免折叠这样的很长的行 [注：我理解是避免从这样比较长的行的某一部分进行折叠]。

  Rather not最好不要:

  if foo == 'blah': do_blah_thing()
  for x in lst: total += x
  while t < 10: t = delay()
  

  Definitely not 绝对不要:

  if foo == 'blah': do_blah_thing()
  else: do_non_blah_thing()
  
  try: something()
  finally: cleanup()
  
  do_one(); do_two(); do_three(long, argument,
                               list, like, this)
  
  if foo == 'blah': one(); two(); three()

Block Comments 注释块

Block comments generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment).

注释块通常应用于紧随它的部分（或者所有）代码，并且他们应该和这些代码有相同的缩进。注释块的每一行由 # 和一个空格开始（除非是注释中已经缩进的文字）

Paragraphs inside a block comment are separated by a line containing a single #.

注释块内部的各段由一个单个的 # 进行分隔。

Inline Comments 行内注释

Use inline comments sparingly.

小心地使用行内注释。

An inline comment is a comment on the same line as a statement. Inline comments should be separated by at least two spaces from the statement. They should start with a # and a single space.

行内注释是指与指令在同一行的注释。行内注释需要与指令至少相隔两个空格。他们应该由一个#和一个空格开始。

Inline comments are unnecessary and in fact distracting if they state the obvious. Don’t do this:

行内注释不必要，而且事实上很让人闹心，如果它们只是陈述明显的事实。不要像这样：

x = x + 1                 # Increment x

But sometimes, this is useful:

但是有时候也是有用的：

x = x + 1                 # Compensate for border

[注：个人觉得这个自己把握，不需要用行内注释说明运算本身的作用，而可以用行内注释说明这行指令的潜在意图，尤其是一些特殊的意图和想法。对于有些想法，也许过了一段时间再看代码就不明白为什么当时要这么做了，那就应该注释下来。]

Documentation Strings 文档字符串

Conventions for writing good documentation strings (a.k.a. “docstrings”) are immortalized in PEP 257.

关于怎么写好文档字符串（a.k.a. “docstrings”）的约定详见 PEP 257。

• Write docstrings for all public modules, functions, classes, and methods. Docstrings are not necessary for non-public methods, but you should have a comment that describes what the method does. This comment should appear after the def line. 为所有的公共模块、函数和方法写文档字符串。对于非公共的方法，文档字符串不是必须的，但是你应该有一个描述这个方法的注释。这个注释应该出现在def行下方。

• PEP 257 describes good docstring conventions. Note that most importantly, the """ that ends a multiline docstring should be on a line by itself, e.g.:   PEP 257 阐述了好的文档字符串的相关约定。注意最重要的是，用以结束一个多行的文档字符串的"""应该自成一行，比如：

  """Return a foobang
  
  Optional plotz says to frobnicate the bizbaz first.
  """  [注：第一个"""开始了一段文档字符串，可以不自成一行。但是第二个"""用以结束文档字符串，因此必须自成一行。]

• For one liner docstrings, please keep the closing """ on the same line. 对于一样的文档字符串，请把用以结束的"""写在同一行。

Python的命名约定有些混乱， 所以我们从没就这点达成完全的一致 — 虽然如此，这有些目前推荐的命名标准。 新的模块和包 （包括第三方框架）应该遵从这些标准，但是对于已经存在的具有不同风格的库，建议保持内部的一致性。

Overriding Principle 首要原则

Names that are visible to the user as public parts of the API should follow conventions that reflect usage rather than implementation.

Descriptive: Naming Styles 描述：命名风格

There are a lot of different naming styles. It helps to be able to recognize what naming style is being used, independently from what they are used for.

现存很多不同的命名风格。如果能不依赖于它们的用途而分辨出使用了哪种命名风格，会很有帮助。

The following naming styles are commonly distinguished:

通常可以区分如下命名风格：

• b (single lowercase letter) 单个小写字母

• B (single uppercase letter) 单个大写字母

• lowercase 小写字母

• lower_case_with_underscores 带下划线的小写字母

• UPPERCASE 大写字母

• UPPER_CASE_WITH_UNDERSCORES 带下划线的大写字母

• CapitalizedWords (or CapWords, or CamelCase — so named because of the bumpy look of its letters [4]). This is also sometimes known as StudlyCaps.单词首字母大写 （或者叫CapWord， 或者CamelCase — 这样叫是因为看起来像驼峰 [4]）。这种方法有时也叫StudlyCaps. 

   

  Note: When using acronyms in CapWords, capitalize all the letters of the acronym. Thus HTTPServerError is better than HttpServerError.

  注意：当在CapWords中使用缩写的时候，缩写的所有字母需要大写。 因此HTTPServerError 好于 HttpServerError。

   

• mixedCase (differs from CapitalizedWords by initial lowercase character!) 混合方式（不同于CapitalizedWords中默认为小写字母）

• Capitalized_Words_With_Underscores (ugly!) 单词首字母大写加下划线（难看！）

There’s also the style of using a short unique prefix to group related names together. This is not used much in Python, but it is mentioned for completeness. For example, the os.stat() function returns a tuple whose items traditionally have names like st_mode, st_size, st_mtime and so on. (This is done to emphasize the correspondence with the fields of the POSIX system call struct, which helps programmers familiar with that.)

也有一种风格是使用一个短的前缀将相关的名称分组。这种方法在Python中不常用，为了完整起见提及一下。例如，os.stat()函数返回一个tuple，这个tuple的元素传统上命名为st_mode, st_size, st_mtime等。（这样做是为了强调与POSIX系统调用结构之间的关联，藉此帮助程序员对它熟悉。）

The X11 library uses a leading X for all its public functions. In Python, this style is generally deemed unnecessary because attribute and method names are prefixed with an object, and function names are prefixed with a module name.

X11库使用X作为它所有公共函数的开头。在Python中，这种风格通常不是很必要，因为属性和方法都已经有了对象的前缀，而函数名有模块名作为前缀。

In addition, the following special forms using leading or trailing underscores are recognized (these can generally be combined with any case convention):

补充一下，如下特殊格式可以使用开头或结尾的下划线（这些可以和所有的上述约定结合使用）

• _single_leading_underscore: weak “internal use” indicator. E.g. from M import * does not import objects whose name starts with an underscore.

• 单下划线开头: “内部使用”的弱标识。比如，from M import * 不能import名字以下划线开头的对象

• single_trailing_underscore_: used by convention to avoid conflicts with Python keyword, e.g.

  Tkinter.Toplevel(master, class_='ClassName')

• 单下划线结尾：用于避免与Python关键字的约定，比如Tkinter.Toplevel(master, class_='ClassName')

• __double_leading_underscore: when naming a class attribute, invokes name mangling (inside class FooBar, __boo becomes_FooBar__boo; see below). 

• 双下划线开头：当命名一个类的属性时，触发命名修饰（在FooBar类之内，__boo 成为_FooBar__boo；见下方）

• __double_leading_and_trailing_underscore__: “magic” objects or attributes that live in user-controlled namespaces. E.g. __init__, __import__ or __file__. Never invent such names; only use them as documented.
• 双下划线开头双下划线结尾：“魔术”对象或者属性，存在于用户控制的命名空间。例如__init__, __import__ 或者 __file__。永远不要自己创造这样的名称；请按照文档说明进行使用。

Prescriptive: Naming Conventions

from typing import TypeVar

VT_co = TypeVar('VT_co', covariant=True)
KT_contra = TypeVar('KT_contra', contravariant=True)

Public and internal interfaces

Any backwards compatibility guarantees apply only to public interfaces. Accordingly, it is important that users be able to clearly distinguish between public and internal interfaces.

Documented interfaces are considered public, unless the documentation explicitly declares them to be provisional or internal interfaces exempt from the usual backwards compatibility guarantees. All undocumented interfaces should be assumed to be internal.

To better support introspection, modules should explicitly declare the names in their public API using the __all__ attribute. Setting __all__ to an empty list indicates that the module has no public API.

Even with __all__ set appropriately, internal interfaces (packages, modules, classes, functions, attributes or other names) should still be prefixed with a single leading underscore.

An interface is also considered internal if any containing namespace (package, module or class) is considered internal.

Imported names should always be considered an implementation detail. Other modules must not rely on indirect access to such imported names unless they are an explicitly documented part of the containing module’s API, such as os.path or a package’s __init__ module that exposes functionality from submodules.

Function Annotations

With the acceptance of PEP 484, the style rules for function annotations are changing.

• In order to be forward compatible, function annotations in Python 3 code should preferably use PEP 484 syntax. (There are some formatting recommendations for annotations in the previous section.)

• The experimentation with annotation styles that was recommended previously in this PEP is no longer encouraged.

• However, outside the stdlib, experiments within the rules of PEP 484 are now encouraged. For example, marking up a large third party library or application with PEP 484 style type annotations, reviewing how easy it was to add those annotations, and observing whether their presence increases code understandability.

• The Python standard library should be conservative in adopting such annotations, but their use is allowed for new code and for big refactorings.

• For code that wants to make a different use of function annotations it is recommended to put a comment of the form:

  # type: ignore
  

  near the top of the file; this tells type checker to ignore all annotations. (More fine-grained ways of disabling complaints from type checkers can be found in PEP 484.)

• Like linters, type checkers are optional, separate tools. Python interpreters by default should not issue any messages due to type checking and should not alter their behavior based on annotations.

• Users who don’t want to use type checkers are free to ignore them. However, it is expected that users of third party library packages may want to run type checkers over those packages. For this purpose PEP 484 recommends the use of stub files: .pyi files that are read by the type checker in preference of the corresponding .py files. Stub files can be distributed with a library, or separately (with the library author’s permission) through the typeshed repo [5].

• For code that needs to be backwards compatible, type annotations can be added in the form of comments. See the relevant section of PEP 484 [6].

Variable annotations

PEP 526 introduced variable annotations. The style recommendations for them are similar to those on function annotations described above:

• Annotations for module level variables, class and instance variables, and local variables should have a single space after the colon.

• There should be no space before the colon.

• If an assignment has a right hand side, then the equality sign should have exactly one space on both sides.

• Yes:

  code: int
  
  class Point:
      coords: Tuple[int, int]
      label: str = '<unknown>'
  

• No:

  code:int  # No space after colon
  code : int  # Space before colon
  
  class Test:
      result: int=0  # No spaces around equality sign
  

• Although the PEP 526 is accepted for Python 3.6, the variable annotation syntax is the preferred syntax for stub files on all versions of Python (see PEP 484 for details).

Footnotes