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
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