python 我如何强制一些函数参数仅为位置参数

gz5pxeao  于 4个月前  发布在  Python
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我想在Python 3.7中模仿Python 3.8的这种行为
仅位置参数/是引入的语法,用于指示某些函数参数必须按位置指定,并且不能用作关键字参数。

#Python3.8
def f(a,b,/,**kwargs):
    print(a,b,kwargs)

>>> f(1,2,**{'a':100,'b':200,'c':300})
# 1 2 {'a': 100, 'b': 200, 'c': 300}

字符串
ab仅用作位置参数。
我如何在Python 3.7中做同样的事情

#Python3.7

def f(a,b,**kwargs):
    print(a,b,kwargs)

>>> f(1,2,**{'a':1,'b':2})
# TypeError: f() got multiple values for argument 'a'


如何使ab仅为位置参数。/在Python 3.8中不起作用
在Python3.7中可以模仿/语法吗?

olqngx59

olqngx591#

您可以创建一个自定义的装饰器来声明仅限位置的参数,返回一个 Package 器来解析它自己的*args, **kwargs,使它们适合装饰函数的签名。不能使用关键字-参数-打包(**)(这是唯一的限制)。封装的保留字参数必须宣告为最后一个positional-or-keyword参数,或是宣告为第一个keyword-only参数。以下是两个范例:

def foo(a, b, kwargs):  # last positional-or-keyword parameter
    pass

def foo(a, *args, kwargs):  # first keyword-only parameter
    pass

字符串
变量kwargs将从 Package 函数中接收剩余的**kwargs,也就是说,它可以像**kwargs直接用在修饰函数中一样使用(就像在Python 3.8+中一样)。
以下装饰器的实现主要基于inspect.Signature.bind的实现,并进行了一些小的调整,以便通过装饰器声明的名称处理仅限位置的参数,并处理附加的(人工)kwargs参数。

import functools
import inspect
import itertools

def positional_only(*names, kwargs_name='kwargs'):
    def decorator(func):
        signature = inspect.signature(func)

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            new_args = []
            new_kwargs = {}            

            parameters = iter(signature.parameters.values())
            parameters_ex = ()
            arg_vals = iter(args)

            while True:
                try:
                    arg_val = next(arg_vals)
                except StopIteration:
                    try:
                        param = next(parameters)
                    except StopIteration:
                        break
                    else:
                        if param.name == kwargs_name or param.kind == inspect.Parameter.VAR_POSITIONAL:
                            break
                        elif param.name in kwargs:
                            if param.name in names:
                                msg = '{arg!r} parameter is positional only, but was passed as a keyword'
                                msg = msg.format(arg=param.name)
                                raise TypeError(msg) from None
                            parameters_ex = (param,)
                            break
                        elif param.default is not inspect.Parameter.empty:
                            parameters_ex = (param,)
                            break
                        else:
                            msg = 'missing a required argument: {arg!r}'
                            msg = msg.format(arg=param.name)
                            raise TypeError(msg) from None
                else:
                    try:
                        param = next(parameters)
                    except StopIteration:
                        raise TypeError('too many positional arguments') from None
                    else:
                        if param.name == kwargs_name or param.kind == inspect.Parameter.KEYWORD_ONLY:
                            raise TypeError('too many positional arguments') from None

                        if param.kind == inspect.Parameter.VAR_POSITIONAL:
                            new_args.append(arg_val)
                            new_args.extend(arg_vals)
                            break

                        if param.name in kwargs and param.name not in names:
                            raise TypeError(
                                'multiple values for argument {arg!r}'.format(
                                    arg=param.name)) from None

                        new_args.append(arg_val)

            for param in itertools.chain(parameters_ex, parameters):
                if param.name == kwargs_name or param.kind == inspect.Parameter.VAR_POSITIONAL:
                    continue

                try:
                    arg_val = kwargs.pop(param.name)
                except KeyError:
                    if (param.kind != inspect.Parameter.VAR_POSITIONAL
                            and param.default is inspect.Parameter.empty):
                        raise TypeError(
                            'missing a required argument: {arg!r}'.format(
                                arg=param.name)) from None
                else:
                    if param.name in names:
                        raise TypeError(
                            '{arg!r} parameter is positional only, '
                            'but was passed as a keyword'.format(arg=param.name))

                    new_kwargs[param.name] = arg_val

            new_kwargs.update(kwargs=kwargs)
            return func(*new_args, **new_kwargs)
        return wrapper
    return decorator


下面是如何使用它的示例:

@positional_only('a')
def foo(a, *args, kwargs, b=9, c):
    print(a, args, b, c, kwargs)

foo(1, **dict(a=2), c=3)  # ok
foo(1, 2, 3, 4, 5, c=6)  # ok
foo(1, b=2, **dict(a=3), c=4)  # ok
foo(a=1, c=2)  # error
foo(c=1)  # error

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