Here is my problem, i'm working with an API, precisely with a high-order function that only accepts functions with N arguments. (I cannot monkey-patch this API).
#this is an example of a high order function i may encounter
#there are many more of such functions in the API that require N ammount of arguments
#this example fct required 3 arg, but a valid solution should adapt to any required args count
def high_order_function(f):
"""high order function expecting a function with 3 arguments!"""
print(f"\nprocessing function {f.__name__}")
if f.__code__.co_argcount!=3:
raise Exception(f"Error Expecting a function with 3 arguments, the passed function got {f.__code__.co_argcount}")
print("Function is Ok")
#...
return None
And my problem is that I simply cannot use any wrapper because of this check. what am I supposed to do ?
def my_wrapper(func):
import functools
@functools.wraps(func)
def inner(*args, **kwargs):
print("wrapped1!")
r = func(*args,**kwargs)
print("wrapped2!")
return r
return inner
def original(a, b, c):
return None
wrapped = my_wrapper(original)
high_order_function(original)
#ok!
high_order_function(wrapped)
#will cause error
#because wrapped.__code__.co_argcount == 0 and is readonly!
CodePudding user response：
After a lot of tinkering, I found a pretty procedural way that might work for you.
The trick was to use __code__.update(). There are some caveats, probably more than I know.
def high_order_function(f):
"""high order function expecting a function with 3 arguments!"""
print(f"\nprocessing function {f.__name__}")
if f.__code__.co_argcount!=3:
raise Exception(f"Error Expecting a function with 3 arguments, the passed function got {f.__code__.co_argcount}")
print("Function is Ok")
#...
return None
def my_wrapper(func):
import functools
@functools.wraps(func)
def inner(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z):
kwargs = locals().copy()
del kwargs["func"]
print("wrapped1!")
r = func(**kwargs) # func(*kwargs.values()) would work too
print("wrapped2!")
return r
func_args = func.__code__.co_varnames
inner.__code__ = inner.__code__.replace(co_varnames=func_args, co_argcount=len(func_args))
return inner
def original(a, b, c):
return None
wrapped = my_wrapper(original)
high_order_function(original)
high_order_function(wrapped)
Result
processing function original
Function is Ok
processing function original
Function is Ok
functools.wraps changes the name of inner to original
Caveats
__code__.replace()raisedValueError: code: varnames is too smallwheninner's parameters were*argsor**kwargs- If
innerinstead had no parameters thenlocals()inside it would not get the supplied values, therefore you got the whole alphabet instead - Inside
inneryou can access the parameters by the letter like normal if you're sure it´s supplied otherwise you'll getIndexError: tuple index out of range - I recommend to use e.g.
kwargs.get("d")instead __code__.updatemay only be for 3.8 , it hassys.version_info >= (3, 8)in the source code
CodePudding user response：
Why not define 2 version of the inner function in your wrapper based on whether you need to pass the wrapped function to higher_order_function or not.
Something like this:
def high_order_function(f):
"""high order function expecting a function with 3 arguments!"""
print(f"\nprocessing function {f.__name__}")
if f.__code__.co_argcount != 3:
raise Exception(
f"Error Expecting a function with 3 arguments, the passed function got {f.__code__.co_argcount}")
print("Function is Ok")
# ...
return None
def my_wrapper(func, higher_order_compatible=True): # switch to control whether should be compatible with the higher_order_function or not
import functools
if higher_order_compatible:
@functools.wraps(func)
def inner(a, b, c, *args, **kwargs): # extra args to satisfy the condition
print("wrapped1!")
r = func(a, b, c, *args, **kwargs)
print("wrapped2!")
return r
else:
@functools.wraps(func)
def inner(*args, **kwargs): # normal wrapper
print("wrapped1!")
r = func(*args, **kwargs)
print("wrapped2!")
return r
return inner
def original(a, b, c):
return None
wrapped_compat = my_wrapper(original, higher_order_compatible=True)
wrapped_nocompat = my_wrapper(original, higher_order_compatible=False)
print("Original")
high_order_function(original)
print("Compatible")
high_order_function(wrapped_compat)
print("Not compatible")
try:
high_order_function(wrapped_nocompat)
except:
print("no not working")
Results in:
Original
processing function original
Function is Ok
Compatible
processing function original
Function is Ok
Not compatible
processing function original
no not working
CodePudding user response：
co_argcount: number of arguments (not including keyword only arguments, * or ** args)
Hence the goal is to bypass such definition. Make a fake signature with 3 fake parameters, these are taken into consideration by the code attribute co_argcount. Then the parameters of the original function must by passed as keys.
def wrapper(f):
def extended_signature(fake1=None, fake2=None, fake3=None, **kwargs):
return f(**kwargs)
return extended_signature
def a(q, w): print(q, w)
a_wrapped = wrapper(a)
high_order_function(a_wrapped)(q=1, w=2)
#processing function true_signature
#Function is Ok
CodePudding user response：
This is an attempt to solve the problem
i feel like it's almost a potential solution
However it is not working as expected, strange. f()for a function object class is not as f.__call__() hmm
def my_wrapper(func):
import copy
def inner(*args, **kwargs):
print("wrapped front")
r = func(*args, **kwargs)
print("wrapped end")
return r
newfunc = copy.deepcopy(func)
newfunc.__name__ = func.__name__ "_wrapped"
newfunc.__call__ = inner
return newfunc
def original(a, b, c=6):
print("original",a,b,c)
return None
###testing if the original function work
high_order_function(original)
#will pass requirement
###testing if the wrap works?
high_order_function(my_wrapper(original))
#will pass requirement, however the wrap did not work
CodePudding user response：
well, here's a solution, couldn't find a procedural way to generate the functions... it scales up to 5 forced arguments
def my_wrapper(func):
"""see https://stackoverflow.com/questions/73601340/how-to-wrap-a-function-with-accurate-code-argcount?noredirect=1#comment129973896_73601340
yes this is a shit show, did not found a procedural way to generate functions. tried exec() code generation & was also a mess"""
#find back the expected arguments so func.__code__.co_argcount will be accurate
if (func.__defaults__ is not None):
force_arg = func.__code__.co_argcount - len(func.__defaults__)
else: force_arg = func.__code__.co_argcount
import functools
if (force_arg==0):
@functools.wraps(func)
def inner(**kwargs):
print("wrapped1!")
r = func(**kwargs)
print("wrapped2!")
return r
elif (force_arg==1):
@functools.wraps(func)
def inner(a,**kwargs):
print("wrapped1!")
r = func(a,**kwargs)
print("wrapped2!")
return r
elif (force_arg==2):
@functools.wraps(func)
def inner(a,b,**kwargs):
print("wrapped1!")
r = func(a,b,**kwargs)
print("wrapped2!")
return r
elif (force_arg==3):
@functools.wraps(func)
def inner(a,b,c,**kwargs):
print("wrapped1!")
r = func(a,b,c,**kwargs)
print("wrapped2!")
return r
elif (force_arg==4):
@functools.wraps(func)
def inner(a,b,c,d,**kwargs):
print("wrapped1!")
r = func(a,b,c,d,**kwargs)
print("wrapped2!")
return r
elif (force_arg==5):
@functools.wraps(func)
def inner(a,b,c,d,e,**kwargs):
print("wrapped1!")
r = func(a,b,c,d,e,**kwargs)
print("wrapped2!")
return r
else: raise Exception("my_wrapper() do not support more than 5 forced argument")
return inner