Python local functions (contains the nonlocal keyword)

In Python, local functions (also known as nested functions) are functions defined within other functions. They can be used to create closures and for organizing your code. The nonlocal keyword in Python allows you to access and modify variables from the enclosing scope within a nested function. Here's a tutorial on local functions and the nonlocal keyword in Python:

Defining local functions

Local functions are defined within the scope of another function, using the def keyword:

def outer_function():
    def inner_function():
        # function body

Using local functions

Local functions can be called within the scope of the enclosing function:

def outer_function():
    def inner_function():
        print("Hello from the inner function!")

    inner_function()

outer_function()  # Output: Hello from the inner function!

Accessing variables from the enclosing scope

Local functions can access variables from the enclosing scope, but can't modify them without using the nonlocal keyword:

def outer_function():
    outer_var = "I'm an outer variable"

    def inner_function():
        print("Inside inner_function:", outer_var)

    inner_function()

outer_function()  # Output: Inside inner_function: I'm an outer variable

Modifying variables from the enclosing scope using nonlocal

The nonlocal keyword allows you to modify variables from the enclosing scope within a nested function:

def outer_function():
    outer_var = "I'm an outer variable"

    def inner_function():
        nonlocal outer_var  # Use the nonlocal keyword to modify the outer_var
        outer_var = "I'm modified by the inner function"
        print("Inside inner_function:", outer_var)

    inner_function()
    print("Inside outer_function:", outer_var)

outer_function()

Output:

Inside inner_function: I'm modified by the inner function
Inside outer_function: I'm modified by the inner function

In this example, the inner_function modifies the outer_var using the nonlocal keyword. This allows the outer_var to be updated within the outer_function scope as well.

Closures

A closure is a nested function that captures and remembers the values of the variables in the enclosing scope, even after the outer function has completed execution:

def make_multiplier(factor):
    def multiplier(n):
        return n * factor

    return multiplier

times_two = make_multiplier(2)
times_three = make_multiplier(3)

print(times_two(4))  # Output: 8
print(times_three(4))  # Output: 12

In this example, the make_multiplier function returns a nested function multiplier, which captures the value of the factor variable. The times_two and times_three functions are created as closures, each with their own captured value of factor.

In summary, local functions in Python are functions defined within other functions. They can access and modify variables from the enclosing scope using the nonlocal keyword, and can be used to create closures. Local functions are useful for organizing your code and for creating function factories or decorators.

How to define local functions in Python:
- Description: Create functions within the scope of another function, known as local functions.
- Code:
```
def outer_function():
    def local_function():
        print("This is a local function.")
    local_function()
```
Scope and lifetime of local functions in Python:
- Description: Local functions have a limited scope, and their lifetime is confined to the scope of the enclosing function.
- Code:
```
def outer_function():
    def local_function():
        print("Local function inside outer function.")
    local_function()
```
Accessing variables with the nonlocal keyword in Python:
- Description: Use the nonlocal keyword to access and modify variables in the enclosing (non-global) scope.
- Code:
```
def outer_function():
    x = 10
    def local_function():
        nonlocal x
        x += 5
        print(f"Modified x: {x}")
    local_function()
```

Nested functions and nonlocal variables in Python:

Description: Combine nested functions with nonlocal variables for more complex functionality.

Code:

def outer_function():
    x = 10
    def inner_function():
        nonlocal x
        x *= 2
        print(f"Modified x: {x}")
    inner_function()

Using nonlocal to modify variables in outer scopes in Python:

Description: Dynamically modify variables in outer scopes using the nonlocal keyword.

Code:

def outer_function():
    x = 10
    def local_function():
        nonlocal x
        x += 5
        print(f"Modified x: {x}")
    return local_function

Dynamic behavior of local functions with nonlocal:
- Description: Explore how local functions with nonlocal provide dynamic behavior in different contexts.
- Code:
```
def outer_function(base):
    def local_function(x):
        nonlocal base
        base += x
        return base
    return local_function
```
Type hints and local functions in Python:
- Description: Apply type hints to local functions for improved code readability and understanding.
- Code:
```
def outer_function() -> None:
    def local_function() -> int:
        return 42
    local_function()
```
Scope resolution in Python functions with nonlocal:
- Description: Understand how scope resolution works in Python functions, especially with the use of nonlocal.
- Code:
```
def outer_function():
    x = 10
    def local_function():
        nonlocal x
        print(x)
    local_function()
```