SCR107: mutable-default-argument

Mutable value as a parameter default

Code	SCR107
Name	mutable-default-argument
Category	Scripts (SCR)
Default severity	warning
Enabled by default	yes
Fix available	no
ign-lint equivalent	none

What it does

Reports parameter defaults that are mutable objects created once: list, dict and set literals and comprehensions, and calls to the builtins list(), dict(), set() and bytearray().

Why is this bad?

The default is evaluated once, when the def runs. Every call that does not pass the argument shares the same object, so changes made by one call are seen by the next. Library modules live for the whole gateway uptime: such a list keeps growing across every Perspective session.

Example

def addAlarm(alarm, seen=[]):
    seen.append(alarm)
    return len(seen)

Use instead:

def addAlarm(alarm, seen=None):
    if seen is None:
        seen = []
    seen.append(alarm)
    return len(seen)

pylint

Corresponds to W0102 dangerous-default-value.

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Generated from the rule metadata by iglint generate-docs; iglint rule SCR107 prints the same page in the terminal. Source: docs/rules/SCR107.md.