I first want to stress that I have searched both the web generally and the Python documentation + StackOverflow specifically very extensively and did not manage to find an answer to this question. I also want to thank anyone taking the time to read this.
As the title suggests, I am writing a decorator in Python, and I want it to add keyword arguments to the wrapped function (please note: I know how to add arguments to the decorator itself, that's not what I'm asking).
Here is a working example of a piece of code I wrote that does exactly that for Python 3 (specifically Python 3.5). It uses decorator arguments, adds keyword arguments to the wrapped function and also defines and adds a new function to the wrapped function.
from functools import wraps
def my_decorator(decorator_arg1=None, decorator_arg2=False):
# Inside the wrapper maker
def _decorator(func):
# Do Something 1
@wraps(func)
def func_wrapper(
*args,
new_arg1=False,
new_arg2=None,
**kwds):
# Inside the wrapping function
# Calling the wrapped function
if new_arg1:
return func(*args, **kwds)
else:
# do something with new_arg2
return func(*args, **kwds)
def added_function():
print("Do Something 2")
func_wrapper.added_function = added_function
return func_wrapper
return _decorator
Now this decorator can be used in the following manner:
@my_decorator(decorator_arg1=4, decorator_arg2=True)
def foo(a, b):
print("a={}, b={}".format(a,b))
def bar():
foo(a=1, b=2, new_arg1=True, new_arg2=7)
foo.added_function()
Now, while this works for Python 3.5 (and I assume for any 3.x), I have not managed to make it work for Python 2.7. I'm getting a SyntaxError: invalid syntax on the first line that tries to define a new keyword argument for the func_wrapper, meaning the line stating new_arg1=False,, when importing the module containing this code.
Moving the new keywords to the start of the argument list of func_wrapper solves the SyntaxError but seems to screw with the wrapped function's signature; I'm now getting the error TypeError: foo() takes exactly 2 arguments (0 given) when calling foo(1, 2). This error disappears if I assign the arguments explicitly, as in foo(a=1, b=2), but that is obviously not enough - unsurprisingly, my new keyword arguments seem to be "stealing" the first two positional arguments sent to the wrapped function. This is something that did not happen with Python 3.
I would love to get your help on this. Thank you for taking the time to read this.
Shay
If you only ever specify the additional arguments as keywords, you can get them out of the kw dictionary (see below). If you need them as positional AND keyword arguments, then I think you should be able to use inspect.getargspec on the original function, and then process args and kw in func_wrapper.
Code below tested on Ubuntu 14.04 with Python 2.7, 3.4 (both Ubuntu-provided) and 3.5 (from Continuum).
from functools import wraps
def my_decorator(decorator_arg1=None, decorator_arg2=False):
# Inside the wrapper maker
def _decorator(func):
# Do Something 1
@wraps(func)
def func_wrapper(
*args,
**kwds):
# new_arg1, new_arg2 *CANNOT* be positional args with this technique
new_arg1 = kwds.pop('new_arg1',False)
new_arg2 = kwds.pop('new_arg2',None)
# Inside the wrapping function
# Calling the wrapped function
if new_arg1:
print("new_arg1 True branch; new_arg2 is {}".format(new_arg2))
return func(*args, **kwds)
else:
print("new_arg1 False branch; new_arg2 is {}".format(new_arg2))
# do something with new_arg2
return func(*args, **kwds)
def added_function():
# Do Something 2
print('added_function')
func_wrapper.added_function = added_function
return func_wrapper
return _decorator
@my_decorator(decorator_arg1=4, decorator_arg2=True)
def foo(a, b):
print("a={}, b={}".format(a,b))
def bar():
pass
#foo(1,2,True,7) # won't work
foo(1, 2, new_arg1=True, new_arg2=7)
foo(a=3, b=4, new_arg1=False, new_arg2=42)
foo(new_arg2=-1,b=100,a='AAA')
foo(b=100,new_arg1=True,a='AAA')
foo.added_function()
if __name__=='__main__':
import sys
sys.stdout.flush()
bar()
Output is
new_arg1 True branch; new_arg2 is 7
a=1, b=2
new_arg1 False branch; new_arg2 is 42
a=3, b=4
new_arg1 False branch; new_arg2 is -1
a=AAA, b=100
new_arg1 True branch; new_arg2 is None
a=AAA, b=100
added_function