确定Lambda中的参数数量 [英] Determining the number of parameters in a lambda

问题描述

我想知道是否有一种方法(给定包含lambda的变量)确定其中包含的lambda的参数数量.原因是，我希望根据参数的数量有条件地调用函数.

I am wondering if there is a way to determine (given a variable containing a lambda) the number of parameters the lambda it contains. The reason being, I wish to call a function conditionally dependent on the number of parameters.

我在寻找什么

def magic_lambda_parameter_counting_function(lambda_function):
    """Returns the number of parameters in lambda_function

    Args:
        lambda_function - A lambda of unknown number of parameters
    """

所以我可以做类似的事情

So I can do something like

def my_method(lambda_function):

    # ... 
    # (say I have variables i and element)

    parameter_count = magic_lambda_parameter_counting_function(lambda_function)

    if parameter_count == 1:
        lambda_function(i)
    elif parameter_count == 2:
        lambda_function(i, element)

推荐答案

我跳过了有关如何计算参数的部分，因为我不知道您想如何考虑varargs和关键字.但这应该可以帮助您入门.

I'm skipping the part about how to count the arguments, because I don't know how you want to consider varargs and keywords. But this should get you started.

>>> import inspect
>>> foo = lambda x, y, z: x + y + z
>>> inspect.getargspec(foo)
ArgSpec(args=['x', 'y', 'z'], varargs=None, keywords=None, defaults=None)

听起来像 inspect.getargspec早于 Python 3(感谢@JeffHeaton).推荐的解决方案使用 inspect.signature .结果的.parameters成员包含各种结构，具体取决于相关函数的参数排列方式，但是我正在匹配上面Python 2示例中的函数.

It sounds like inspect.getargspec is deprecated as of Python 3 (thanks to @JeffHeaton). The recommend solution uses inspect.signature. The .parameters member of the result contains a variety of structures depending on the arrangement of parameters to the function in question, but I'm matching my function from the Python 2 example above.

>>> import inspect
>>> foo = lambda x, y, z: x + y + z
>>> inspect.signature(foo).parameters
mappingproxy(OrderedDict([('x', <Parameter "x">), ('y', <Parameter "y">), ('z', <Parameter "z">)]))

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