Can you list the keyword arguments a function receives?

I have a dict, which I need to pass key/values as keyword arguments.. For example..

d_args = {'kw1': 'value1', 'kw2': 'value2'}
example(**d_args)

This works fine, but if there are values in the d_args dict that are not accepted by the example function, it obviously dies.. Say, if the example function is defined as def example(kw2):

This is a problem since I don’t control either the generation of the d_args, or the example function.. They both come from external modules, and example only accepts some of the keyword-arguments from the dict..

Ideally I would just do

parsed_kwargs = feedparser.parse(the_url)
valid_kwargs = get_valid_kwargs(parsed_kwargs, valid_for = PyRSS2Gen.RSS2)
PyRSS2Gen.RSS2(**valid_kwargs)

I will probably just filter the dict, from a list of valid keyword-arguments, but I was wondering: Is there a way to programatically list the keyword arguments the a specific function takes?

Answers:

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Method 1

A little nicer than inspecting the code object directly and working out the variables is to use the inspect module.

>>> import inspect
>>> def func(a,b,c=42, *args, **kwargs): pass
>>> inspect.getargspec(func)
(['a', 'b', 'c'], 'args', 'kwargs', (42,))

If you want to know if its callable with a particular set of args, you need the args without a default already specified. These can be got by:

def get_required_args(func):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if defaults:
        args = args[:-len(defaults)]
    return args   # *args and **kwargs are not required, so ignore them.

Then a function to tell what you are missing from your particular dict is:

def missing_args(func, argdict):
    return set(get_required_args(func)).difference(argdict)

Similarly, to check for invalid args, use:

def invalid_args(func, argdict):
    args, varargs, varkw, defaults = inspect.getargspec(func)
    if varkw: return set()  # All accepted
    return set(argdict) - set(args)

And so a full test if it is callable is :

def is_callable_with_args(func, argdict):
    return not missing_args(func, argdict) and not invalid_args(func, argdict)

(This is good only as far as python’s arg parsing. Any runtime checks for invalid values in kwargs obviously can’t be detected.)

Method 2

This will print names of all passable arguments, keyword and non-keyword ones:

def func(one, two="value"):
    y = one, two
    return y
print func.func_code.co_varnames[:func.func_code.co_argcount]

This is because first co_varnames are always parameters (next are local variables, like y in the example above).

So now you could have a function:

def get_valid_args(func, args_dict):
    '''Return dictionary without invalid function arguments.'''
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    return dict((key, value) for key, value in args_dict.iteritems() 
                if key in validArgs)

Which you then could use like this:

>>> func(**get_valid_args(func, args))

if you really need only keyword arguments of a function, you can use the func_defaults attribute to extract them:

def get_valid_kwargs(func, args_dict):
    validArgs = func.func_code.co_varnames[:func.func_code.co_argcount]
    kwargsLen = len(func.func_defaults) # number of keyword arguments
    validKwargs = validArgs[-kwargsLen:] # because kwargs are last
    return dict((key, value) for key, value in args_dict.iteritems() 
                if key in validKwargs)

You could now call your function with known args, but extracted kwargs, e.g.:

func(param1, param2, **get_valid_kwargs(func, kwargs_dict))

This assumes that func uses no *args or **kwargs magic in its signature.

Method 3

For a Python 3 solution, you can use inspect.signature and filter according to the kind of parameters you’d like to know about.

Taking a sample function with positional or keyword, keyword-only, var positional and var keyword parameters:

def spam(a, b=1, *args, c=2, **kwargs):
    print(a, b, args, c, kwargs)

You can create a signature object for it:

from inspect import signature
sig =  signature(spam)

and then filter with a list comprehension to find out the details you need:

>>> # positional or keyword
>>> [p.name for p in sig.parameters.values() if p.kind == p.POSITIONAL_OR_KEYWORD]
['a', 'b']
>>> # keyword only
>>> [p.name for p in sig.parameters.values() if p.kind == p.KEYWORD_ONLY]
['c']

and, similarly, for var positionals using p.VAR_POSITIONAL and var keyword with VAR_KEYWORD.

In addition, you can add a clause to the if to check if a default value exists by checking if p.default equals p.empty.

Method 4

In Python 3.0:

>>> import inspect
>>> import fileinput
>>> print(inspect.getfullargspec(fileinput.input))
FullArgSpec(args=['files', 'inplace', 'backup', 'bufsize', 'mode', 'openhook'],
varargs=None, varkw=None, defaults=(None, 0, '', 0, 'r', None), kwonlyargs=[], 
kwdefaults=None, annotations={})

Method 5

Extending DzinX’s answer:

argnames = example.func_code.co_varnames[:func.func_code.co_argcount]
args = dict((key, val) for key,val in d_args.iteritems() if key in argnames)
example(**args)

Method 6

Just use this for a function name ‘myfun’:

myfun.__code__.co_varnames

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0