Scope

Scope decides which variables a piece of code can see. Python has local, global, and nonlocal scopes.

Local scope

A variable created inside a function is local to it:

python
def myfunc():
    x = 300

### local
    print(x)

myfunc()

### 300
print(x)

### NameError: x is not defined

Function inside function

A variable in the outer function is visible to the inner one:

python
def outer():
    x = 300
    def inner():
        print(x)

### can see the outer x
    inner()

outer()

### 300

Global scope

A variable created outside any function is global; everyone can read it:

python
x = 300

### global

def myfunc():
    print(x)

myfunc()

### 300
print(x)

### 300

The global keyword

To modify a global from inside a function, declare it with global:

python
def myfunc():
    global x
    x = 500

myfunc()
print(x)

### 500

Without global, the function would create a new local x.

The nonlocal keyword

nonlocal changes a variable in the enclosing (outer) function:

python
def outer():
    x = 300
    def inner():
        nonlocal x
        x = 200
    inner()
    print(x)

### 200

outer()

Reading order

If a name isn't local, Python looks outward: local, then enclosing, then global, then built-in.

TL;DR

• Local variables exist only in their function.
• Global variables are readable everywhere.
• global modifies a global; nonlocal modifies the enclosing scope.
• Python searches scopes from inside out.