Passing Default Arguments To A Decorator In Python
Solution 1:
Default arguments are part of the function's signature. They do not exist in the decorator call.
To access them in the wrapper you need to get them out of the function, as shown in this question.
import inspect
from functools import wraps
def get_default_args(func):
signature = inspect.signature(func)
return {
k: v.default
for k, v in signature.parameters.items()
if v.default is not inspect.Parameter.empty
}
def my_decorator(f):
@wraps(f)
def wrapper(*args, **kwds):
print('Calling decorated function')
print('args:', args)
kwargs = get_default_args(f)
kwargs.update(kwds)
print('kwargs:', kwargs)
return f(*args, **kwds)
return wrapper
@my_decorator
def example(i, j=0):
"""Docstring"""
print('Called example function')
example(i=1)
Output:
Calling decorated function
args: ()
kwargs: {'i': 1, 'j': 0}
Called example function
Solution 2:
You can get default argument values by using __defaults__
special attribute.
def my_decorator(f):
@wraps(f)
def wrapper(*args, **kwds):
print('def args values', f.__defaults__)
return f(*args, **kwds)
return wrapper
Reference: look for __defaults__
in https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy
A tuple containing default argument values for those arguments that have defaults, or None if no arguments have a default value
Solution 3:
Getting the exact list of args and kwargs is a little tricky, since you can pass positional args as a kwarg, or vice versa. Newer versions of python also add positional-only or keyword only arguments.
However, inspect.signature
has a mechanism which can apply defaults: calling .bind(*args, **kwargs)
followed by .apply_defaults()
. This can give you a dictionary of effectively what all the arguments are to the function. In the example in OP, this becomes:
from functools import wraps
import inspect
def my_decorator(f):
sig = inspect.signature(f)
@wraps(f)
def wrapper(*args, **kwds):
bound = sig.bind(*args, **kwds)
bound.apply_defaults()
print('Calling decorated function')
print('called with:', bound.arguments)
return f(*args, **kwds)
return wrapper
@my_decorator
def example(i, j=0):
"""Docstring"""
print('Called example function')
example(i=1)
This outputs the following on Python 3.9:
Calling decorated function
called with: OrderedDict([('i', 1), ('j', 0)])
Called example function
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