Python decorator basics, part II
By Miguel Alatorre
February 21, 2014
This is a continuation of my previous post: Python decorator basics. Here I’ll talk about a decorator with optional arguments. Let’s say we want to pass an optional argument to the same debug decorator:
def debug(msg=None):
def actual_decorator(f):
def wrapper(*args):
if msg:
print msg
return f(*args)
return wrapper
return actual_decorator
@debug("Let's multiply!")
def mul(x, y):
return x*y
Calling mul:
mul(5, 2)
Let's multiply!
10
Excellent. Now let’s decorate without a msg and call mul:
@debug
def mul(x, y):
return x*y
mul(5, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: actual_decorator() takes exactly 1 argument (2 given)
</module></stdin>
Oh oh. Let’s see what happens at time of decoration:
mul = debug(mul)
Hmmm, mul gets passed to debug as it’s argument and then the arguments (5, 2) are passed to actual_decorator, since debug returns actual_decorator. To resolve this we need to always call the decorator as a function:
@debug()
def mul(x, y):
return x*y
mul(5, 2)
10
Assuming that we always expect the msg parameter to be a non-callable, another option would be to check the type of argument passed to the debug decorator:
def debug(msg=None):
def actual_decorator(f):
def wrapper(*args):
if msg:
print msg
return f(*args)
return wrapper
if callable(msg):
# debug decorator called without an argument so
# msg is the function being decorated
return debug()(msg)
return actual_decorator
@debug
def mul(x, y):
return x*y
mul(5, 2)
10
@debug("Let's multiply!")
def mul(x, y):
return x*y
mul(5, 2)
Let's multiply!
10
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