What will happen when a closure captures a loop variable, and why do all closures created in the loop end up referencing the same final value?

Difficulty: 8/10

Closures, Late Binding, Loop Variables, Debugging

Closures capture variables by reference, so they see the final loop value

In Python, a closure captures the variable itself, not the value it holds at the time the closure is created. When you define a function inside a loop that references the loop variable, all the functions share the same cell for that variable. After the loop finishes, the variable holds its last value, so every closure sees that final value. This is called late binding. To capture the current value, you must bind it explicitly, for example by using a default argument (lambda x=i: x) or by wrapping the closure in a factory function that takes the value as a parameter. This is a common source of bugs in code that creates callbacks, lambdas, or functions inside loops.

1
Closures capture variables, not values. The variable is a cell shared by all closures created in the same scope.
2
Using a default argument (def f(x=i): ...) binds the current value at definition time.
3
A factory function (def make(i): def f(): return i; return f) creates a new scope per iteration.
4
functools.partial can also bind values at creation time.
5
Common mistake: assuming each lambda in a loop captures a different value. They all capture the same variable.
6
Common mistake: using a loop variable in a callback and being surprised it uses the final value when called later.
7
Version note: this behavior is consistent across Python 3 and is a direct consequence of how closures and cells work.

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Scenario Questions

0-2 years experience

1You create three lambdas in a loop and they all print the same number. Why?
2How do you make each lambda remember a different value?

2-5 years experience

1You register callbacks in a loop and they all use the last item. How do you fix it?
2You use a list comprehension with lambdas and get unexpected results. What is the cause?

5-8 years experience

1You are building an event system where handlers are created in a loop. How do you ensure each handler captures its own data?
2A closure inside a loop causes a memory leak because it captures a large object. How do you avoid it?

8+ years experience

1Design a callback registration API that avoids late-binding pitfalls for users.
2Explain how closures, cells, and the LEGB rule interact in nested functions and loops, and how to inspect captured variables at runtime.

Follow-up Questions

How does the cell object work in CPython for closures?
What is the difference between late binding in closures and default arguments?

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