Kursinhalt

Mastering Python: Closures and Decorators

1. Scopes

What are scopes?Built-In and Global Scopes Local Scope Non-local Scope Challenge: Device Boots Count

2. Closure

What is Closure?Implementation of Closure Closure with Arguments Challenge: Text Cache

3. Decorators

What are Decorators?Decorator Usage Flexible Decorators Challenge: Type Validation Decorator Decorator with Parameters Challenge: Flexible Type Validation Decorator

Implementation of Closure

Let's consider the example of closure:


              1234567891011121314151617
            
def outer(argument):
    def inner():
        nonlocal argument
        argument += 5

        return argument

    return inner

variable1 = outer(11)
variable2 = outer(9)

print("First variable1 returns", variable1())
print("Second variable1 returns", variable1())

print("First variable2 returns", variable2())
print("Second variable2 returns", variable2())

We can operate with the enclosed data: the nonlocal keyword provides the ability to change argument (similar to the global keyword).

Steps to create closure

Step 1:
Create the outer function with its local scope.

def outer():
    a = 4

Step 2:
Create an inner function with local scope defined inside the outer function.

def outer():
    a = 4
    
    def inner():

Step 3:
Access the non-local (outer scope) data from within the inner local scope.

def outer():
    a = 4
    
    def inner():
        nonlocal a

Step 4:
Return the inner function and assign it to the variable.

def outer():
    a = 4
    
    def inner():
        nonlocal a

    return inner

variable = outer()

Step 5:
The local scope of the outer function is removed once it has finished executing.

#def outer():
#    a = 4
#    
    def inner():
        nonlocal a
#
#    return inner

variable = outer()

Step 6:
The inner function returned by the outer function is assigned to the variable and has an enclosing scope with data.

#def outer():
#    a = 4
#    
    def inner():
        nonlocal a
#
#    return inner

variable = inner
variable()  # is equal to inner()

The enclosing scope occupies less memory than the non-local scope because it contains only the dependent values. Enclosed variables/objects do not have names and are represented as memory addresses.

The closure and enclosing scope are commonly used in decorators.

Note

Every time the outer() function is called, a new inner() function is defined.

The returned inner() functions are distinct and independent objects.

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Abschnitt 2. Kapitel 2

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