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Python Closure

Last Updated on September 22, 2023 by Mayank Dham

Python closures are a powerful and elegant construct that combines the worlds of functions and data encapsulation. Closures allow a function to "remember" variables from its containing (enclosing) scope after the scope has completed executing. This feature enables programmers to create flexible and dynamic functions that keep state and context consistent across multiple invocations.

Python closures provide a versatile solution to problems requiring the maintenance of specific values between function calls without the use of global variables. They allow developers to write cleaner, more modular code, allowing them to create functions that are not only self-contained but also adaptable to a variety of scenarios.

As we explore the world of Python closures, we will look at their mechanics, use cases, and how they help to create more expressive and efficient code.

Nested Function in Python :

Nested functions can access and modify variables that are defined in their parent functions, which is the main distinction between them and other types of functions.
Some Points about nested functions are :

  1. Variables that are not explicitly passed as input arguments can be used by nested functions.
  2. You may build a handle to a nested function in a parent function that holds the information required to execute the nested function.

It is a function inside a function for example:

def external():
    say = 'Hello'
    def internal():
    return internal

The closure is made up of the "internal" function and the "say" variable.

Python Closure:

A nested function called a "python closure" enables us to access variables from an outer function even after it has been closed.

A closure is a type of function object that retains values from scopes around it even when they are not in memory.

In Python, closure refers to an inner function object that operates like an object, remembers the local variables it was created with, and retains access to them even after the outer function has done executing.

It can also be defined as a technique for binding data to a function without actually passing it as a parameter.

Let’s understand this concept using an example given below:

Code implementation:

def external(text):  
    #inner function  
    def internal():  
external('Learn at prepbytes')


Learn at prepbytes

We will discuss a few things that you may describe as anomalies in this piece of code in this article. As an illustration, consider a nested function, a variable that was declared in an external function but was used by an internal function, and an external function that returned a function instead of a variable.

Now let’s understand what we did in this program-

  1. A function “external” that takes the parameter text has been developed, and inside of it, another function that prints the value of text has also been made.
  2. As we can clearly see, the program above calls ‘internal’ function and we then send a string value to external function.
  3. When this program is executed, the expected result is shown.

Based on the above learning we have the following observations-

  1. We will invoke functions that can be outside the scope in python, with the assistance of closure.
  2. Closure, as defined here, is a function object that keeps track of the values in the contained scope.
  3. Each variable in a function is mapped to its value or a reference to its name in a record called a closure in Python.
  4. It helps us to access the variables using closure copies.

These are some of the observations we got so far from our learning.

Non-local Keyword:

To specify that a variable is not local, use the phrase nonlocal, Below are some of the points related to it-

  • The nonlocal keyword is used when working with variables inside nested functions when the variable should not be a member of the inner function.
  • With the nonlocal keyword, we can change an immutable type variable in the outside function scope.
  • Each variable has a defined scope. The scope is essentially the section of the program’s code where the variable can be identified and reached if necessary.

To create global variables, use the keyword ‘global’.

Uses of Python Closures:

Below are the features and uses of python closures where it should be used-

  1. Closures offer some sort of data hiding because they are used as callback functions. As a result, we use fewer global variables.
  2. Closures can be implemented as an alternative for global scope.
  3. An essential application of closures is data hiding.
  4. Closures appear to be an effective method when we only have a few functions in our code. However, if we require a lot of functions, we should use classes (OOP).
  5. Closures can be used to prevent using classes in unnecessary situations.
  6. Closures are widely used by Python decorators as well.

When we can use a Closure:

There are certain conditions when we can use python closures which are as follow-

  1. There must be nested functions in program.
  2. The enclosed function’s value must be referred by the function.
  3. The nested function must be returned by the enclosing function.

Here is the conclusion what we have learnt in this article so far-

  • In general, a Python closure is a nested function with some data associated with it.
  • Learnt from the above example how to call python closures.
  • It has availability to a free variable inside the outer scope.
  • Functions outside of their scope can be called with the aid of closures.
  • Learn about Free variable -a variable which is not bound in the local space. The nonlocal keyword is necessary for closures to function with immutable variables such as strings and integers.

Frequently Asked Questions (FAQs)

Here are some of the frequently asked questions on Python Closure.

Q1. What is a closure in Python?
A closure in Python refers to a nested function that "remembers" and has access to variables from its containing (enclosing) function’s scope, even after the enclosing function has finished executing. This allows the closure to maintain state between function calls.

Q2. How is a closure created in Python?
A closure is created by defining a nested function within another function and returning the nested function from the outer function. The nested function captures and retains variables from its enclosing scope.

Q3. What’s the advantage of using closures?
Closures are particularly useful for creating functions that require some form of persistent state between multiple invocations. They provide a cleaner alternative to using global variables and offer improved modularity and encapsulation.

Q4. Can closures be used to implement data hiding or private variables?
While closures can somewhat emulate encapsulation, they don’t provide the same level of data hiding as classes and objects in object-oriented programming. Python’s convention of using underscores to indicate "private" variables is more effective for encapsulation.

Q5. What are common use cases for closures?
Closures are often used for implementing decorators, managing resources (like file handles or network connections), and creating factory functions. They are also helpful for creating functions with customizable behavior, such as callback functions used in event-driven programming.

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