Python closures

last modified January 29, 2024

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.

#!/usr/bin/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:

• it is a nested function
• it has access to a free variable in outer scope
• it is returned from the enclosing function

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.

#!/usr/bin/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

Python closure with nonlocal keyword

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

#!/usr/bin/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

Python closures vs classes

Python closures can be an alternate solution to small classes.

#!/usr/bin/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.

#!/usr/bin/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.

Source

The Python Language Reference

In this article we have worked with Python closures.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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