Python 语法糖 : function arg aliases

Question

是否有别名函数参数的语法？如果没有，是否有任何 PEP 提案？我不是编程语言理论家，所以我的观点可能是无知的，但我认为实现某种函数 arg 别名可能很有用。

我正在对 libcloud 做一些更改我的想法将帮助我避免在更改 API 时破坏他人。

例如，假设我正在重构并想将函数参数“foo”重命名为“bar”:

原文:

def fn(foo):
    <code (using 'foo')>

我可以:

def fn(foo, bar=None):
    if foo and bar:
        raise Exception('Please use foo and bar mutually exclusively.')
    bar = foo or bar
    <code (using 'bar')>

# But this is undesirable because it changes the method signature to allow
# a new parameter slot.
fn('hello world', 'goodbye world')

我未提炼的语法糖想法:

def fn(bar|foo|baz):
    # Callers can use foo, bar, or baz, but only the leftmost arg name
    # is used in the method code block. In this case, it would be bar.
    # The python runtime would enforce mutual exclusion between foo,
    # bar, and baz.
    <code (using 'bar')>

# Valid uses:
fn(foo='hello world')
fn(bar='hello world')
fn(baz='hello world')
fn('hello world')

# Invalid uses (would raise some exception):
fn(foo='hello world', bar='goodbye world')
fn('hello world', baz='goodbye world')

Answer 1

不，没有这样的语法糖。

您可以使用 **kwargs 来捕获额外的关键字参数并在其中查找已弃用的名称(如果不存在则引发异常)。您甚至可以使用装饰器将其自动化。

from functools import wraps

def renamed_argument(old_name, new_name):
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            if old_name in kwargs:
                if new_name in kwargs:
                    raise ValueError(
                        "Can't use both the old name {} and new name {}. "
                        "The new name is preferred.".format(old_name, new_name))
                kwargs[new_name] = kwargs.pop(old_name)
            return func(*args, **kwargs)
        return wrapper
    return decorator

@renamed_argument('bar', 'foo')
def fn(foo=None):
    <method code>

演示:

>>> @renamed_argument('bar', 'foo')
... def fn(foo=None):
...     return foo
...
>>> fn()  # default None returned
>>> fn(foo='spam')
'spam'
>>> fn(bar='spam')
'spam'
>>> fn(foo='eggs', bar='spam')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in wrapper
ValueError: Can't use both the old name bar and new name foo. The new name is preferred.