Confusion about UnboundLocalError: local variable 'number_sum' referenced before assignment when manually applying decorator - python-3.x

Question

Was experimenting with applying memoization decorator to recursive functions using standard python #decorator notation, which worked beautifully. According to much of the documentation I've read, these code examples are supposed to be equivalent:
# letting python decorator mechanism take care of wrapping your function with the decorator function
#decorator
def func(): . . .
print(func(arg))
AND
# manually wrapping the decorator function around the function
func = decorator(func)
print(func(arg))
When I do this, however, I get an UnboundLocalError on func.
If I change the code to
new_name = decorator(func)
print(new_name(func))
The code runs, but the decorator is only applied to the first call and not to any of the recursive calls (this did NOT surprise me), but I don't get any error messages either.
What seems weird to me, however, is the original error message itself.
If I experiment further and try the following code:
new_name = decorator(func)
func = new_name
print(func(arg))
I get the same error ON THE SAME LINE AS BEFORE (????)
In fact, if I form a chain of these assignments, with func = several_names_later
I still get the same error on the ORIGINAL line
Can anyone explain what is going on, and why I'm getting the error, and why it seems that the error is disconnected from the location of the variable in question?
As requested, here is most of the actual code (with just one of the recursive functions), hope it's not too much . . .
import functools
def memoize(fn):
cache = dict()
#functools.wraps(fn)
def memoizer(*args):
print(args)
if args not in cache:
cache[args] = fn(*args)
return cache[args]
return memoizer
##memoize
def number_sum(n):
'''Returns the sum of the first n numbers'''
assert(n >= 0), 'n must be >= 0'
if n == 0:
return 0
else:
return n + number_sum(n-1)
def main():
# Book I'm reading claims this can be done but I get error instead:
number_sum = memoize(number_sum) # this is the flagged line
print(number_sum(300))
#UnboundLocalError: local variable 'number_sum' referenced before assignment
# When I do this instead, only applies decorator to first call, as I suspected
# but works otherwise: no errors and correct - but slow - results
# No complaints about number_sum
wrapped_number_sum = memoize(number_sum)
print(wrapped_number_sum(300))
# This is what is odd:
# When I do any version of this, I get the same error as above, but always on
# the original line, flagging number_sum as the problem
wrapped_number_sum = memoize(number_sum) # the flagged line, no matter what
number_sum = wrapped_number_sum
print(number_sum(300))
OR even:
wrapped_number_sum = memoize(number_sum) # still the flagged line
another_variable = wrapped_number_sum
number_sum = another_variable
print(number_sum(300))
if __name__ == '__main__':
main()
I am more than a little mystified at this.