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Debugging decorators in Python

  • Difficulty Level : Easy
  • Last Updated : 21 Aug, 2020

Decorators in Python are really a very powerful feature. If you are a web developer and you have used the Django framework or even some other development frameworks you would have already come across decorators.

For an overview decorators are wrapper functions that wrap an existing function or a method and modify its features. Let’s take a short example. Consider that you have a speak function that returns a neutral message

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Python3




def speak():
    """Returns a neutral message"""
    return "Hi, Geeks!"
  
# printing the output
print(speak())

Output:



Hi, Geeks!

Suppose that you need to modify the function to return a message in a happy tone. So let’s create a decorator for this.

Python3




# decorator
def make_geek_happy(func):
    def wrapper():
        neutral_message = func()
        happy_message = neutral_message + " You are happy!"
        return happy_message
    return wrapper
  
#using the decorator 
@make_geek_happy
def speak():
    """Returns a neutral message"""
    return "Hi, Geeks!"
  
print(speak())

Output:

Hi, Geeks! You are happy!

Debugging a decorator

In this way, the decorators can also be used to modify different functions and make them more useful. However, there are some drawbacks to this process. When we wrap the original function in a decorator the metadata of the original function gets lost. Consider the below program but this time we use the decorator in another way just to make you understand.

If you try to access any of the metadata of the positive_message function it actually returns the metadata of the wrapper inside the decorator. 

Python3




# decorator
def make_geek_happy(func):
    def wrapper():
        neutral_message = func()
        happy_message = neutral_message + " You are happy!"
        return happy_message
    return wrapper
  
def speak():
    """Returns a neutral message"""
    return "Hi!"
  
  
# wrapping the function in the decorator
# and assigning it to positive_message
positive_message = make_geek_happy(speak)
  
print(positive_message())
  
print(speak.__name__) 
print(speak.__doc__) 
print(positive_message.__name__)
print(positive_message.__doc__)

Output:

Hi! You are happy!
speak
Returns a neutral message
wrapper
None

These results make it really very difficult for debugging. But thanks to Python it also has a solution to fix this problem without much effort. We just need to use the functools.wraps() decorator included in the Python standard library.

Here’s an example:

Python3




# importing the module
import functools
  
# decorator
def make_geek_happy(func):
    @functools.wraps(func)
    def wrapper():
        neutral_message = func()
        happy_message = neutral_message + " You are happy!"
        return happy_message
    return wrapper
  
def speak():
    """Returns a neutral message"""
    return "Hi!"
  
positive_message = make_geek_happy(speak)
print(positive_message())
  
print(speak.__name__) 
print(speak.__doc__) 
print(positive_message.__name__)
print(positive_message.__doc__)

Output:

Hi! You are happy!
speak
Returns a neutral message
speak
Returns a neutral message



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