How to Create a Counter Closure in Python

Build a closure in Python that remembers and increments a counter across calls without using global variables.

Medium Python 3.0+ Aug 9, 2026 Functions & basics 13 views 0 copies

closures nonlocal state functions

Python code

13 lines

Python 3.0+

def create_counter(start=0):
    count = start
    def increment():
        nonlocal count
        count += 1
        return count
    return increment

if __name__ == "__main__":
    counter = create_counter(10)
    print(counter())
    print(counter())
    print(counter())

Output

stdout

11
12
13

How it works

This example uses a closure — an inner function that captures variables from its enclosing scope. The nonlocal keyword inside increment tells Python that count refers to the variable in create_counter, not a new local one. Each call to increment looks up and modifies that same count, so the state persists between calls. Since the inner function holds a reference to its closure environment, the counter retains its value even after create_counter finishes executing.

Common mistakes

Forgetting `nonlocal`, which causes a `UnboundLocalError` when assigning to `count`
Using `global` instead of `nonlocal`, which would only work for module-level variables
Returning `count` before incrementing, so the first call returns the initial value instead of start+1

Variations

Use a mutable default argument like `def increment(count=[start])` for older Python versions without `nonlocal`
Implement the same counter as a class with a `__call__` method for an object-oriented alternative

Real-world use cases

Generate sequential IDs for logging or tracing events in a multi-request web application.
Track retry attempts for a network call within a single function invocation.
Create per-user request counters in an API middleware without global mutable state.

How to Create a Counter Closure in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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