Python 'UnboundLocalError: local variable referenced before assignment' (newer versions: 'cannot access local variable ... where it is not associated with a value'): why does it happen?

Cause: Python decides at compile time whether a name in a function is local. If you assign to a name anywhere in the function body (including x += 1, a for target, an import, or a with ... as), that name is local for the entire function. Reading it before the assignment runs then fails, even if a global or enclosing variable with the same name exists. Code can start failing after an innocent edit that adds an assignment further down. Typical case: reading a module-level counter, then doing counter += 1 inside the function. Another case: assigning a variable only inside an if or try branch and using it afterwards. Fix (best first): 1. If you meant a local, give it a value before use on every code path (initialise it above the if/try, or add an else branch). 2. If you meant the module-level variable, declare global name at the top of the function before using or assigning it. Better still, avoid mutable global state: pass the value in as a parameter and return the new value. 3. If it belongs to an enclosing function (a closure), declare nonlocal name in the inner function. 4. If you only need to read the outer value, do not assign to that name in the function; use a different local name for the result. 5. Beware of a name that is local because a later line assigns it, such as shadowing a builtin like input or sum. Verify: call the function, including the branch that previously failed, and confirm no UnboundLocalError; a linter such as pyflakes or a type checker will also flag possibly-unbound variables. Sources: https://docs.python.org/3/faq/programming.html https://docs.python.org/3/reference/executionmodel.html