SCR118: used-before-assignment

Variable is read where no assignment can have happened

Code	SCR118
Name	used-before-assignment
Category	Scripts (SCR)
Default severity	error
Enabled by default	yes
Fix available	no
ign-lint equivalent	none

What it does

Reports reads of a function's local variable (or a module-level name in module code) that no assignment reaches on any path: it is read before its first assignment, after del, or only assigned in a branch that cannot run. The analysis follows if/else, loops (a later iteration sees earlier assignments), try/except/else/finally, with, break, continue, return and raise. A variable that is assigned on at least one path is never reported: the rule only reports reads that fail every time they run.

A name is local to a function when the function assigns it anywhere, even after the read; then a module-level variable of the same name is not visible in the function (Python raises UnboundLocalError).

Unreachable reads, scopes with exec or from x import *, and reads in a try that catches NameError are not reported. Module-level reads fall back to builtins and to names that functions bind with global.

Why is this bad?

The read raises UnboundLocalError (functions) or NameError (module code) every time it runs.

Example

COUNTER = 0

def onMessageReceived(self, payload):
    COUNTER += 1
    self.custom.count = COUNTER

Use instead:

def onMessageReceived(self, payload):
    self.custom.count = (self.custom.count or 0) + 1

pylint

Corresponds to E0601 used-before-assignment (definite cases only; pylint's possibly-used-before-assignment is not reported). pylint misses the augmented assignment case (N += 1 in a function where N is a module variable), which always raises UnboundLocalError.

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Generated from the rule metadata by iglint generate-docs; iglint rule SCR118 prints the same page in the terminal. Source: docs/rules/SCR118.md.