问题描述

在 Python 3.4 的 help 输出中，/ 在右括号之前是什么意思?

What does the / mean in Python 3.4's help output for range before the closing parenthesis?

>>> help(range)
Help on class range in module builtins:

class range(object)
 |  range(stop) -> range object
 |  range(start, stop[, step]) -> range object
 |  
 |  Return a virtual sequence of numbers from start to stop by step.
 |  
 |  Methods defined here:
 |  
 |  __contains__(self, key, /)
 |      Return key in self.
 |  
 |  __eq__(self, value, /)
 |      Return self==value.

                                        ...

推荐答案

它表示 仅位置参数，您不能用作关键字参数的参数.在 Python 3.8 之前，此类参数只能在 C API 中指定.

It signifies the end of the positional only parameters, parameters you cannot use as keyword parameters. Before Python 3.8, such parameters could only be specified in the C API.

表示__contains__的key参数只能按位置传入(range(5).__contains__(3))，不是作为关键字参数 (range(5).__contains__(key=3))，您可以在纯 Python 函数中使用位置参数.

It means the key argument to __contains__ can only be passed in by position (range(5).__contains__(3)), not as a keyword argument (range(5).__contains__(key=3)), something you can do with positional arguments in pure-python functions.

另请参阅 Argument Clinic 文档:

要在 Argument Clinic 中将所有参数标记为仅位置参数，请在最后一个参数之后的一行上单独添加一个 /，缩进与参数行相同.

To mark all parameters as positional-only in Argument Clinic, add a / on a line by itself after the last parameter, indented the same as the parameter lines.

以及(最近添加的)Python 常见问题解答:

函数参数列表中的斜杠表示它之前的参数是仅位置参数.仅位置参数是没有外部可用名称的参数.在调用接受仅位置参数的函数时，参数仅根据其位置映射到参数.

A slash in the argument list of a function denotes that the parameters prior to it are positional-only. Positional-only parameters are the ones without an externally-usable name. Upon calling a function that accepts positional-only parameters, arguments are mapped to parameters based solely on their position.

语法现在是 Python 语言规范的一部分，as版本 3.8，请参阅 PEP 570 - Python Positional-Only 参数.在 PEP 570 之前，该语法已被保留以备将来可能包含在 Python 中，请参阅 PEP 457 -仅位置参数的语法.

The syntax is now part of the Python language specification, as of version 3.8, see PEP 570 – Python Positional-Only Parameters. Before PEP 570, the syntax was already reserved for possible future inclusion in Python, see PEP 457 - Syntax For Positional-Only Parameters.

仅位置参数可以使 API 更简洁明了，使纯 Python 实现的其他纯 C 模块更加一致且更易于维护，并且由于仅位置参数需要很少的处理，它们会导致更快的 Python 代码.

Positional-only parameters can lead to cleaner and clearer APIs, make pure-Python implementations of otherwise C-only modules more consistent and easier to maintain, and because positional-only parameters require very little processing, they lead to faster Python code.

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