SCR115: except-tuple-without-parentheses

except A, B: binds B instead of catching it

Code	SCR115
Name	except-tuple-without-parentheses
Category	Scripts (SCR)
Default severity	error
Enabled by default	yes
Fix available	always
Fix safety	unsafe
ign-lint equivalent	none

What it does

Reports except ValueError, TypeError: where the name after the comma is an exception class: a builtin exception, a class the script defines, or a capitalized name it imports with from m import Name. The name is looked up like Python does (the handler's function, enclosing functions, the module, the builtins), ignoring the handler itself. A module (except ValueError, sys: after import sys), any other variable, and a name the handler body reads are variables, as written.

Why is this bad?

In Python 2, except A, B: means "catch A and store the exception in the variable B". TypeError is not caught, and worse, the name TypeError is rebound to the caught exception for the rest of the function.

Example

try:
    value = int(text)
except ValueError, TypeError:
    value = 0

Use instead:

try:
    value = int(text)
except (ValueError, TypeError):
    value = 0

SCR103 unused-variable and SCR117 redefined-builtin do not report the name of such a handler: this rule explains the mistake.

Fix

Adds the parentheses. Unsafe: the fixed handler catches a second exception class and no longer rebinds the name, so the program behaves differently (that is the point, but iglint cannot prove the author meant the class). No fix when a comment sits between the two names.

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Generated from the rule metadata by iglint generate-docs; iglint rule SCR115 prints the same page in the terminal. Source: docs/rules/SCR115.md.