Understanding the global Statement in Python

When a function needs to modify a variable defined outside its scope, Python requires the global statement to declare that variable as global; otherwise the interpreter treats assignments as creating a new local variable, leading to errors such as UnboundLocalError.

Example of the error:

count = 1

def cc():
    count = count + 1

cc()
# UnboundLocalError: local variable 'count' referenced before assignment

Correct usage with global:

def cc():
    global count
    count = count + 1
    print(count)

cc()
# 2

Multiple variables can be declared in one statement, separated by commas:

num = 0

def cc():
    global count, num
    count = count + 1
    num = num + 2
    print(count, num)

cc()
# 3 2

Alternatively, a class attribute can serve a similar purpose:

class C:
    count = 3

def cc():
    C.count = C.count + 1
    print(C.count)

cc()
# 4

The global statement therefore acts like passing a variable into a function, allowing the function to read and modify the variable defined in the outer (module) scope.