Introspecting Python function parameters#

For https://github.com/simonw/datasette/issues/581 I want to be able to inspect a Python function to determine which named parameters it accepts and send only those arguments.

Python 3.3 added an inspect.signature() function that can be used for this.

Here’s a function I wrote to take advantage of that and solve my problem:

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def call_with_supported_arguments(fn, **kwargs):
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    parameters = inspect.signature(fn).parameters.keys()
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    call_with = []
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    for parameter in parameters:
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        if parameter not in kwargs:
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            raise TypeError("{} requires parameters {}".format(fn, tuple(parameters)))
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        call_with.append(kwargs[parameter])
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    return fn(*call_with)

And here’s an illustrative unit test:

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def test_call_with_supported_arguments():
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    def foo(a, b):
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        return "{}+{}".format(a, b)
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    assert "1+2" == utils.call_with_supported_arguments(foo, a=1, b=2)
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    assert "1+2" == utils.call_with_supported_arguments(foo, a=1, b=2, c=3)
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    with pytest.raises(TypeError):
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        utils.call_with_supported_arguments(foo, a=1)