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Python closures

Python closures tutorial shows how to use closure functions in Python.

Python functions are first-class citizens. This means that functions have equal status with other objects in Python. Functions can be assigned to variables, stored in collections, created and deleted dynamically, or passed as arguments.

A nested function, also called an inner function, is a function defined inside another function.

nested_fun.py
#!/usr/bin/env python


def main():

    def build_message(name):

        msg = f'Hello {name}'
        return msg

    name = input("Enter your name: ")
    msg = build_message(name)

    print(msg)


if __name__ == "__main__":
    main()

The build_message() is a nested function. It is defined and invoked inside its outer main() function.

Python closures

A closure is a nested function which has access to a free variable from an enclosing function that has finished its execution. Three characteristics of a Python closure are:

A free variable is a variable that is not bound in the local scope. In order for closures to work with immutable variables such as numbers and strings, we have to use the nonlocal keyword.

Python closures help avoiding the usage of global values and provide some form of data hiding. They are used in Python decorators.

Python simple closure example

The following is a simple example of a Python closure.

simple_closure.py
#!/usr/bin/env python

def make_printer(msg):

    msg = "hi there"

    def printer():
        print(msg)

    return printer


myprinter = make_printer("Hello there")
myprinter()
myprinter()
myprinter()

In the example, we have a make_printer() function, which creates and returns a function. The nested printer() function is the closure.

myprinter = make_printer("Hello there")

The make_printer() function returns a printer() function and assigns it to the myprinter variable. At this moment, it has finished its execution. However, the printer() closure still has access to the msg variable.

$ ./simple_closure.py
hi there
hi there
hi there

This is the output.

Python closure with nonlocal keyword

The nonlocal keyword allows us to modify a variable with immutable type in the outer function scope.

counter.py
#!/usr/bin/env python

def make_counter():

    count = 0
    def inner():

        nonlocal count
        count += 1
        return count

    return inner


counter = make_counter()

c = counter()
print(c)

c = counter()
print(c)

c = counter()
print(c)

The example creates a counter function.

def make_counter():

    count = 0
    def inner():

        nonlocal count
        count += 1
        return count

    return inner

By using the nonlocal keyword, the count variable becomes a free variable. Now we can modify it.

$ ./counter.py
1
2
3

This is the output.

Python closures vs classes

Python closures can be an alternate solution to small classes.

summer.py
#!/usr/bin/env python

class Summer():

    def __init__(self):
        self.data = []

    def __call__(self, val):

        self.data.append(val)
        _sum = sum(self.data)

        return _sum

summer = Summer()

s = summer(1)
print(s)

s = summer(2)
print(s)

s = summer(3)
print(s)

s = summer(4)
print(s)

We have a Summer class, which sums values passed to the object.

def __init__(self):
    self.data = []

The data is kept in the object attribute and is created in the constructor.

def __call__(self, val):

    self.data.append(val)
    _sum = sum(self.data)

    return _sum

Each time the instance is called, the value is appended and the sum is calculated and returned.

The following is an alternate solution with Python closure.

summer2.py
#!/usr/bin/env python


def make_summer():

    data = []

    def summer(val):

        data.append(val)
        _sum = sum(data)

        return _sum

    return summer

summer = make_summer()

s = summer(1)
print(s)

s = summer(2)
print(s)

s = summer(3)
print(s)

s = summer(4)
print(s)

We have the same functionality with a Python closure.

def make_summer():

    data = []

    def summer(val):

        data.append(val)
        _sum = sum(data)

        return _sum

    return summer

Because the data is a list which is mutable, we do not have to use the nonlocal keyword.

In this tutorial, we have worked with Python closures.

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