3.8. Closures¶

3.8.1. Rationale¶

def f(x):
    def g(y):
        return x + y
    return g

3.8.2. Nested Function¶

Function inside the function
Nested functions can access the variables of the enclosing scope

def run():
    def hello():
        print('hello')
    return hello


hello()
# Traceback (most recent call last):
#     ...
# NameError: name 'hello' is not defined

run()
# <function run.<locals>.hello at 0x104e13700>

run()()
# hello

result = run()
result()
# hello

3.8.3. What is closure?¶

technique by which the data is attached to some code even after end of those other original functions is called as closures
Closures can avoid use of global variables and provides some form of data hiding
When the interpreter detects the dependency of inner nested function on the outer function, it stores or makes sure that the variables in which inner function depends on are available even if the outer function goes away
Method of binding data to a function without actually passing them as parameters is called closure
It is a function object obj that remembers values in enclosing scopes even if they are not present in memory
Closures provide some sort of data hiding as they are used as callback functions
This helps us to reduce the use of global variables
Useful for replacing hard-coded constants
Closures prove to be efficient way when we have few functions in our code

firstname = 'Mark'
lastname = 'Watney'

def hello():
    print(firstname, lastname)


hello()
# Mark Watney

firstname = 'Mark'

def run():
    lastname = 'Watney'

    def hello():
        print(firstname, lastname)

    return hello


result = run()
result()
# Mark Watney

firstname = 'Mark'

def run():
    lastname = 'Watney'

    def hello():
        print(firstname, lastname)

    return hello


result = run()
del run
result()
# Mark Watney

3.8.4. Assignments¶

3.8.4.1. Function Closure Define¶

Assignment name: Function Closure Define
Last update: 2020-10-13
Complexity level: easy
Lines of code to write: 4 lines
Estimated time of completion: 5 min
Solution: solution/function_closure_define.py

English

Define function check which takes func: Callable as an argument
Define closure function wrapper inside check
Function wrapper takes *args and **kwargs as arguments
Function wrapper returns None
Function check must return wrapper: Callable

Polish

Zdefiniuj funkcję check, która przyjmuje func: Callable jako argument
Zdefiniuj funkcję closure wrapper wewnątrz check
Funkcja wrapper przyjmuje *args i **kwargs jako argumenty
Funkcja wrapper zwraca None
Funkcja check ma zwracać wrapper: Callable

Output

>>> assert callable(check)
>>> assert callable(check(lambda x: x))
>>> result = check(lambda x: x).__call__()
>>> result is None
True

3.8.4.2. Function Closure Call¶

Assignment name: Function Closure Call
Last update: 2020-10-13
Complexity level: easy
Lines of code to write: 9 lines
Estimated time of completion: 5 min
Solution: solution/function_closure_call.py

English

Define function check with parameter func: Callable
Define closure function wrapper inside check
Function wrapper takes arbitrary number of positional and keyword arguments
Function wrapper prints hello from wrapper on the screen
Function check must return wrapper: Callable
Define function hello() which prints hello from function
Define result with result of calling check(hello)
Delete check using del keyword
Call result
Compare result with "Output" section (see below)

Polish

Zdefiniuj funkcję check z parametrem func: Callable
Zdefiniuj funkcję closure wrapper wewnątrz check
Funkcja wrapper przyjmuje dowolną ilość argumentów pozycyjnych i nazwanych
Funkcja wrapper wypisuje hello from wrapper
Funkcja check ma zwracać wrapper: Callable
Zdefiniuj funkcję hello(), która wypisuje hello from function
Zdefiniuj zmienną result, która jest wynikiem wywołania check(hello)
Skasuj check za pomocą słowa kluczowego del
Wywołaj result
Porównaj wyniki z sekcją "Output" (patrz poniżej)

Output

>>> from inspect import isfunction
>>> assert isfunction(hello)
>>> assert isfunction(result)
>>> assert not hasattr(__name__, 'check')

>>> hello()
hello from function

>>> result()
hello from wrapper

>>> check()
Traceback (most recent call last):
    ...
NameError: name 'check' is not defined