Python Basics — Scope & Closures

Scope & Closures

Understanding scope — where variables live and how long they exist — is essential for writing correct, bug-free Python code. This chapter covers how Python resolves variable names and introduces closures, a powerful pattern that combines functions with their surrounding state.

What You’ll Learn

#	Topic	What You’ll Master
1	Local & Global Scope	Variables inside vs outside functions
2	LEGB Rule	Python’s name resolution order
3	global & nonlocal	Modifying outer variables from inner scopes
4	Closures	Functions that remember their environment
5	Scope Best Practices	Writing clean, maintainable scoped code
6	Practice Exercises	Test your understanding

Quick Preview

# A closure — a function that remembers its enclosing scope
def make_multiplier(factor):
    def multiply(number):
        return number * factor  # factor is "closed over"
    return multiply

double = make_multiplier(2)
triple = make_multiplier(3)

print(double(5))   # 10
print(triple(5))   # 15

Why Scope Matters

Problem	Caused By	Solution
UnboundLocalError	Assigning to a variable that shadows a global	Use global keyword or pass as argument
Unexpected value changes	Modifying a global from inside a function	Avoid mutable globals; pass data explicitly
Hard-to-debug code	Relying on global state	Keep functions pure — input in, output out
Stale data in closures	Late binding in loops	Use default arguments or functools.partial

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Start Learning: Local & Global Scope →