Scope and closures: variables that are not the variable you meant
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
What is scope and closure bug?
The name you wrote resolves to a different variable than the one you meant — a shadowed copy, a shared binding, or one that does not exist yet at the moment the line runs.
Why it happens
Closures capture variables, not values, so callbacks created in a loop can share one binding and all see the final value. Assigning to a name anywhere in a Python function makes it local for the whole function, even before the assignment. Re-declaring a name inside a callback shadows the outer one, so updates land on the copy. And in JavaScript `this` is decided by how a function is called, not where it was written.
How to recognise it
- Every item in a loop behaves like the last one.
- A counter is visibly incremented but stays at its initial value.
- Python reports a variable as local when it is clearly defined above.
- A method works when called normally and breaks when passed as a callback.
Errors and symptoms this causes
- UnboundLocalError: cannot access local variable
- ReferenceError: Cannot access before initialization
- all callbacks use the last value
- counter stays at zero
- this is undefined
How to fix it
Do not re-declare a name you meant to update from an enclosing scope. Capture per-iteration values explicitly — a default argument in Python, let rather than var in JavaScript. Keep a method attached to its object when passing it somewhere. Declare before use, always.
Practise scope and closure bugs
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