Anthony Sottile Anthony Sottile - 6 months ago 19
Python Question

Determining the number of parameters in a lambda

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.

What I'm looking for

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
"""


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)

Answer

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)

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">)]))