Python Decorators Tutorial

Assigning Functions to Variables

To kick us off we create a function that will add one to a number whenever it is called. We'll then assign the function to a variable and use this variable to call the function.

def plus_one(number):
    return number + 1

add_one = plus_one
add_one(5)

6

Defining Functions Inside Other Functions 

Next, we'll illustrate how you can define a function inside another function in Python. Stay with me, we'll soon find out how all this is relevant in creating and understanding decorators in Python.

def plus_one(number):
    def add_one(number):
        return number + 1


    result = add_one(number)
    return result
plus_one(4)

5

Passing Functions as Arguments to Other Functions

Functions can also be passed as parameters to other functions. Let's illustrate that below.

def plus_one(number):
    return number + 1

def function_call(function):
    number_to_add = 5
    return function(number_to_add)

function_call(plus_one)

6

Functions Returning Other Functions

A function can also generate another function. We'll show that below using an example.

def hello_function():
    def say_hi():
        return "Hi"
    return say_hi
hello = hello_function()
hello()

'Hi'

Understanding Closures

Python allows a nested function to access the outer scope of the enclosing function. This is a critical concept in decorators, known as a closure.

A closure in Python is a function that remembers the environment in which it was created, even after that environment is no longer active. This means a nested function can "close over" variables from its enclosing scope and continue to use them.

Closures are essential for understanding decorators because decorators rely on the ability of a nested wrapper function to access and modify the state of the enclosing decorator function.

Example of a closure:

def outer_function(message):
    def inner_function():
        print(f"Message from closure: {message}")
    return inner_function

closure_function = outer_function("Hello, closures!")
closure_function()
# Output: Message from closure: Hello, closures!

In this example:

• inner_function is a closure because it accesses message, a variable from its enclosing scope (outer_function).
• Even though outer_function has finished executing, inner_function retains access to message.

When you create a decorator, the wrapper function (inside the decorator) is a closure. It retains access to the function being decorated and any additional state or arguments defined in the decorator function. For example:

def simple_decorator(func):
    def wrapper():
        print("Before the function call")
        func()
        print("After the function call")
    return wrapper

@simple_decorator
def greet():
    print("Hello!")

greet()
# Output:
# Before the function call
# Hello!
# After the function call

Here, wrapper is a closure that remembers the function greet and adds behavior before and after its execution.

Creating Decorators

With these prerequisites out of the way, let's go ahead and create a simple decorator that will convert a sentence to uppercase. We do this by defining a wrapper inside an enclosed function. As you can see it very similar to the function inside another function that we created earlier.

def uppercase_decorator(function):
    def wrapper():
        func = function()
        make_uppercase = func.upper()
        return make_uppercase

    return wrapper

Our decorator function takes a function as an argument, and we shall, therefore, define a function and pass it to our decorator. We learned earlier that we could assign a function to a variable. We'll use that trick to call our decorator function.

def say_hi():
    return 'hello there'

decorate = uppercase_decorator(say_hi)
decorate()

'HELLO THERE'

However, Python provides a much easier way for us to apply decorators. We simply use the @ symbol before the function we'd like to decorate. Let's show that in practice below.

@uppercase_decorator
def say_hi():
    return 'hello there'

say_hi()

'HELLO THERE'

Applying Multiple Decorators to a Single Function

We can use multiple decorators to a single function. However, the decorators will be applied in the order that we've called them. Below we'll define another decorator that splits the sentence into a list. We'll then apply the uppercase_decorator and split_string decorator to a single function.

import functools
def split_string(function):
    @functools.wraps(function)
    def wrapper():
        func = function()
        splitted_string = func.split()
        return splitted_string

    return wrapper 

@split_string
@uppercase_decorator
def say_hi():
    return 'hello there'
say_hi()

['HELLO', 'THERE']

From the above output, we notice that the application of decorators is from the bottom up. Had we interchanged the order, we'd have seen an error since lists don't have an upper attribute. The sentence has first been converted to uppercase and then split into a list.

Note: When stacking decorators, it's a common practice to use functools.wraps to ensure that the metadata of the original function is preserved throughout the stacking process. This helps maintain clarity and consistency in debugging and understanding the properties of the decorated function.

Accepting Arguments in Decorator Functions

Sometimes we might need to define a decorator that accepts arguments. We achieve this by passing the arguments to the wrapper function. The arguments will then be passed to the function that is being decorated at call time.

def decorator_with_arguments(function):
    def wrapper_accepting_arguments(arg1, arg2):
        print("My arguments are: {0}, {1}".format(arg1,arg2))
        function(arg1, arg2)
    return wrapper_accepting_arguments


@decorator_with_arguments
def cities(city_one, city_two):
    print("Cities I love are {0} and {1}".format(city_one, city_two))

cities("Nairobi", "Accra")

My arguments are: Nairobi, Accra Cities I love are Nairobi and Accra

Defining General Purpose Decorators

To define a general purpose decorator that can be applied to any function we use args and **kwargs. args and **kwargs collect all positional and keyword arguments and stores them in the args and kwargs variables. args and kwargs allow us to pass as many arguments as we would like during function calls.

def a_decorator_passing_arbitrary_arguments(function_to_decorate):
    def a_wrapper_accepting_arbitrary_arguments(*args,**kwargs):
        print('The positional arguments are', args)
        print('The keyword arguments are', kwargs)
        function_to_decorate(*args)
    return a_wrapper_accepting_arbitrary_arguments

@a_decorator_passing_arbitrary_arguments
def function_with_no_argument():
    print("No arguments here.")

function_with_no_argument()

The positional arguments are ()
The keyword arguments are {}
No arguments here.

Let's see how we'd use the decorator using positional arguments.

@a_decorator_passing_arbitrary_arguments
def function_with_arguments(a, b, c):
    print(a, b, c)

function_with_arguments(1,2,3)

The positional arguments are (1, 2, 3)
The keyword arguments are {}
1 2 3

Keyword arguments are passed using keywords. An illustration of this is shown below.

@a_decorator_passing_arbitrary_arguments
def function_with_keyword_arguments():
    print("This has shown keyword arguments")

function_with_keyword_arguments(first_name="Derrick", last_name="Mwiti")

The positional arguments are ()
The keyword arguments are {'first_name': 'Derrick', 'last_name': 'Mwiti'}
This has shown keyword arguments

Note: The use of **kwargs in the decorator allows it to handle keyword arguments. This makes the general-purpose decorator versatile and capable of handling a variety of argument types during function calls.

Passing Arguments to the Decorator

Now let's see how we'd pass arguments to the decorator itself. In order to achieve this, we define a decorator maker that accepts arguments then define a decorator inside it. We then define a wrapper function inside the decorator as we did earlier.

def decorator_maker_with_arguments(decorator_arg1, decorator_arg2, decorator_arg3):
    def decorator(func):
        def wrapper(function_arg1, function_arg2, function_arg3) :
            "This is the wrapper function"
            print("The wrapper can access all the variables\n"
                  "\t- from the decorator maker: {0} {1} {2}\n"
                  "\t- from the function call: {3} {4} {5}\n"
                  "and pass them to the decorated function"
                  .format(decorator_arg1, decorator_arg2,decorator_arg3,
                          function_arg1, function_arg2,function_arg3))
            return func(function_arg1, function_arg2,function_arg3)

        return wrapper

    return decorator

pandas = "Pandas"
@decorator_maker_with_arguments(pandas, "Numpy","Scikit-learn")
def decorated_function_with_arguments(function_arg1, function_arg2,function_arg3):
    print("This is the decorated function and it only knows about its arguments: {0}"
           " {1}" " {2}".format(function_arg1, function_arg2,function_arg3))

decorated_function_with_arguments(pandas, "Science", "Tools")

The wrapper can access all the variables
    - from the decorator maker: Pandas Numpy Scikit-learn
    - from the function call: Pandas Science Tools
and pass them to the decorated function
This is the decorated function, and it only knows about its arguments: Pandas Science Tools

Debugging Decorators

As we have noticed, decorators wrap functions. The original function name, its docstring, and parameter list are all hidden by the wrapper closure: For example, when we try to access the decorated_function_with_arguments metadata, we'll see the wrapper closure's metadata. This presents a challenge when debugging.

decorated_function_with_arguments.__name__

'wrapper'

decorated_function_with_arguments.__doc__

'This is the wrapper function'

In order to solve this challenge Python provides a functools.wraps decorator. This decorator copies the lost metadata from the undecorated function to the decorated closure. Let's show how we'd do that.

import functools

def uppercase_decorator(func):
    @functools.wraps(func)
    def wrapper():
        return func().upper()
    return wrapper

@uppercase_decorator
def say_hi():
    "This will say hi"
    return 'hello there'

say_hi()

'HELLO THERE'

When we check the say_hi metadata, we notice that it is now referring to the function's metadata and not the wrapper's metadata.

say_hi.__name__

'say_hi'

say_hi.__doc__

'This will say hi'

It is advisable and good practice to always use functools.wraps when defining decorators. It will save you a lot of headache in debugging.

Class-Based Decorators

While function-based decorators are common, Python also allows you to create class-based decorators, which provide greater flexibility and maintainability, especially for complex use cases. A class-based decorator is a class with a __call__ method that allows it to behave like a function.

class UppercaseDecorator:
    def __init__(self, function):
        self.function = function

    def __call__(self, *args, **kwargs):
        result = self.function(*args, **kwargs)
        return result.upper()

@UppercaseDecorator
def greet():
    return "hello there"

print(greet())
# Output: HELLO THERE

How it works:

1. The __init__ method initializes the decorator with the function to be decorated.
2. The __call__ method is invoked when the decorated function is called, allowing the decorator to modify its behavior.

Advantages of class-based decorators:

• Stateful decorators: Class-based decorators can maintain state using instance variables, unlike function-based decorators which require closures or global variables.
• Readability: For complex decorators, encapsulating logic in a class can make the code more organized and easier to understand.

Example of a stateful decorator:

class CallCounter:
    def __init__(self, function):
        self.function = function
        self.count = 0

    def __call__(self, *args, **kwargs):
        self.count += 1
        print(f"Function {self.function.__name__} has been called {self.count} times.")
        return self.function(*args, **kwargs)

@CallCounter
def say_hello():
    print("Hello!")

say_hello()
say_hello()
# Output:
# Function say_hello has been called 1 times.
# Hello!
# Function say_hello has been called 2 times.
# Hello!

Real-World Decorator Use Case: Caching

The lru_cache decorator is a built-in tool in Python that caches the results of expensive function calls. This improves performance by avoiding redundant calculations for repeated inputs.

Example:

from functools import lru_cache

@lru_cache(maxsize=128)
def fibonacci(n):
    if n < 2:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)

print(fibonacci(50))  # Subsequent calls with the same argument are much faster

Other common uses for decorators:

• Logging: Track function calls, arguments, and return values for debugging or auditing.

• Authentication: Enforce access control in web applications like Flask or Django.

• Execution timing: Measure and optimize function execution time for performance-critical tasks.

• Retry mechanism: Automatically retry failed function calls, useful in network operations.

• Input validation: Validate function arguments before execution.

Python Decorators Summary

Decorators dynamically alter the functionality of a function, method, or class without having to directly use subclasses or change the source code of the function being decorated. Using decorators in Python also ensures that your code is DRY(Don't Repeat Yourself). Decorators have several use cases such as:

• Authorization in Python frameworks such as Flask and Django
• Logging
• Measuring execution time
• Synchronization

To learn more about Python decorators check out Python's Decorator Library.