Python Decorators
Decorators let you add extra behavior to a function, without changing the function's code.
A decorator is a function that takes another function as input and returns a new function.
Basic Decorator
Define the decorator first, then apply it with @decorator_name above the function.
Example
A basic decorator that uppercases the return value of the decorated function.
def changecase(func):
def myinner():
return func().upper()
return myinner
@changecase
def myfunction():
return "Hello Sally"
print(myfunction())
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By placing @changecase directly above the function definition,
the function myfunction is being "decorated" with the changecase
function.
The function changecase is the decorator.
The function myfunction is the function that gets decorated.
Multiple Decorator Calls
A decorator can be called multiple times. Just place the decorator above the function you want to decorate.
Example
Using the @changecase decorator on two functions:
def changecase(func):
def myinner():
return func().upper()
return myinner
@changecase
def myfunction():
return "Hello Sally"
@changecase
def otherfunction():
return "I am speed!"
print(myfunction())
print(otherfunction())
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Arguments in the Decorated Function
Functions that requires arguments can also be decorated, just make sure you pass the arguments to the wrapper function:
Example
Functions with arguments can also be decorated:
def changecase(func):
def myinner(x):
return func(x).upper()
return myinner
@changecase
def myfunction(nam):
return "Hello " + nam
print(myfunction("John"))
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*args and **kwargs
Sometimes the decorator function has no control over the arguments passed from decorated function, to solve this problem,
add (*args, **kwargs)
to the wrapper function, this way the wrapper function can accept any number, and any type of arguments, and pass them to the decorated function.
Example
Secure the function with *args and **kwargs arguments:
def changecase(func):
def myinner(*args, **kwargs):
return func(*args, **kwargs).upper()
return myinner
@changecase
def myfunction(nam):
return "Hello " + nam
print(myfunction("John"))
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Decorator With Arguments
Decorators can accept their own arguments by adding another wrapper level.
Example
A decorator factory that takes an argument and transforms the casing based on the argument value.
def changecase(n):
def changecase(func):
def myinner():
if n == 1:
a = func().lower()
else:
a = func().upper()
return a
return myinner
return changecase
@changecase(1)
def myfunction():
return "Hello Linus"
print(myfunction())
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Multiple Decorators
You can use multiple decorators on one function.
This is done by placing the decorator calls on top of each other.
The decorators are called in the order they are specified.
Example
One decorator for upper case, and one for adding a greeting:
def changecase(func):
def myinner():
return func().upper()
return myinner
def addgreeting(func):
def myinner():
return "Hello " + func() + " Have a good day!"
return myinner
@changecase
@addgreeting
def myfunction():
return "Tobias"
print(myfunction())
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Preserving Function Metadata
Functions in Python has metadata that can be accessed using the __name__ and __doc__ attributes.
Example
Normally, a function's name can be returned with the __name__ attribute:
def myfunction():
return "Have a great day!"
print(myfunction.__name__)
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But, when a function is decorated, the metadata of the original function is lost.
Example
Try returning the name from a decorated function and you will not get the same result:
def changecase(func):
def myinner():
return func().upper()
return myinner
@changecase
def myfunction():
return "Have a great day!"
print(myfunction.__name__)
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To fix this, Python has a built-in function called functools.wraps that can be used to preserve the original function's name and docstring.
Example
Import functools.wraps to preserve the original function name and docstring.
import functools
def changecase(func):
@functools.wraps(func)
def myinner():
return func().upper()
return myinner
@changecase
def myfunction():
return "Have a great day!"
print(myfunction.__name__)
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