I'd like to find out the arity of a method in Python (the number of parameters that it receives).
Right now I'm doing this:
def arity(obj, method):
return getattr(obj.__class__, method).func_code.co_argcount - 1 # remove self
def bar(self, bla):
arity(Foo(), "bar") # => 1
Foo().bar.arity() # => 1
# Traceback (most recent call last):
# File "bla.py", line 10, in <module>
# print arity('foo', 'split') # =>
# File "bla.py", line 3, in arity
# return getattr(obj.__class__, method).func_code.co_argcount - 1 # remove self
# AttributeError: 'method_descriptor' object has no attribute 'func_co
inspect from Python's standard library is your friend -- see the online docs!
inspect.getargspec(func) returns a tuple with four items,
args, varargs, varkw, defaults:
len(args) is the "primary arity", but arity can be anything from that to infinity if you have
None, and some arguments may be omitted (and defaulted) if
defaults is not
None. How you turn that into a single number, beats me, but presumably you have your ideas in the matter!-)
This applies to Python-coded functions, but not to C-coded ones. Nothing in the Python C API lets C-coded functions (including built-ins) expose their signature for introspection, except via their docstring (or optionally via annotations in Python 3); so, you will need to fall back to docstring parsing as a last ditch if other approaches fail (of course, the docstring might be missing too, in which case the function will remain a mystery).