CODEWITHSUNDEEP
python
vezult
vezult

Reputation: 5243

How to I determine the default value for a given function parameter at runtime?

Using python 2.4, I'm attempting to identify, at runtime, which of an arbitrary function's arguments have default values. Unfortunately, although I can find what the default values are, I can't seem to get a handle on which parameters they correspond to. For example:

def foo(a, b, c=5):
    return a + b + c

import inspect
inspect.getargspec(foo) # output is: (['a', 'b', 'c'], None, None, (5,))

The output of getargspec is clearer in python 2.6, which returns a named tuple:

ArgSpec(args=['a', 'b', 'c'], varargs=None, keywords=None, defaults=(5,))

Python obviously has enough information to accomplish the task during execution. How can I get at it?

Upvotes: 4

Views: 155

Answers (2)

unutbu
unutbu

Reputation: 879143

Arguments with default values must follow arguments without default values. So if there are any defaults, they must correspond to the arguments at the tail end of args.

In your case, args=['a','b','c'], and defaults=(5,). So the default must correspond to c.

import inspect

def foo(a, b, c=5):
    return a + b + c
def show_defaults(f):
    args, varargs, varkw, defaults = inspect.getargspec(f) 
    if defaults:
        for arg, default in zip(args[-len(defaults):], defaults):
            print('{a} = {d}'.format(a=arg,d=default))

show_defaults(foo)
# c = 5

Upvotes: 6

Gary Kerr
Gary Kerr

Reputation: 14410

From the documentation, it discusses the value for defaults:

if this tuple has n elements, they correspond to the last n elements listed in args.

In your case defaults contains one value and corresponds to the last element listed in args, which is c.

Upvotes: 3

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