Question

I have the following decorator with parameters:

from functools import wraps
def pdecor(p):
    def decorator(fn):
        @wraps(fn)
        def wrapper(*args, **kwargs):
            p -= 1
            return fn(*args, **wargs)
        return wrapper
    return decorator

Trying to use the decorator results in :

>>> @pdecor(1)
... def run(): pass
...
>>> run()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in wrapper
UnboundLocalError: local variable 'p' referenced before assignment
>>>

Why can’t I change the p?

Answer 1

Because you assign to p inside wrapper, Python treats the p inside wrapper as local to wrapper. In Python 3 you can use nonlocal p to mark p as referenced from an outer scope. In Python 2 there is no way to assign to the intermediate p, although you can get a reference to the same value by passing it into the nested functions as a keyword argument (e.g., def decorator(fn, p=p)).

However, it’s not clear what you’re getting at with this anyway. p is already only local to pdecor. No code outside pdecor can access p, so decrementing it won’t have any effect on any code elsewhere. So whether you can decrement p or not, it won’t really accomplish anything.