SCR103: unused-variable

Local variable is assigned but never read

Code	SCR103
Name	unused-variable
Category	Scripts (SCR)
Default severity	warning
Enabled by default	yes
Fix available	sometimes
Fix safety	unsafe
ign-lint equivalent	none

What it does

Reports local variables of functions (including Perspective scripts, which are function bodies) whose name is assigned but never read, neither in the function nor in a nested function or lambda. Assignments, with ... as x and except E, x count; parameters, loop variables, comprehension variables and imports (see SCR102) do not.

Not reported:

• names that are part of a tuple or list target (a, b = f(), where only one may be needed);
• names matching the dummy pattern: _, starting with _, dummy, ignored_..., unused_... (option dummy-pattern);
• functions that read their locals dynamically (locals(), vars(), eval, exec), for example "%(name)s" % locals(), and every function of a module with code that reads or writes a frame's or a module's names by itself (sys._getframe(), .f_locals, .tb_frame, e = eval, import __builtin__; the table is in docs/internal/fix-safety.md, "Frame routes: findings and fixes"). Reflection or introspection that is only a first hop (a Java member that leads to reflection such as .type or getColumnType(i), getattr with a computed name, sys.exc_info(), line numbers) does not silence the rule;
• the exception class in except ValueError, TypeError:, which SCR115 reports.

Why is this bad?

An unused variable is dead code at best; often it is a typo (the value was meant to be used under that name) or a forgotten step (result = system.db.runPrepQuery(...) whose result is never written anywhere).

Example

def runAction(self, event):
    rows = system.db.runNamedQuery("Orders/Open")
    count = len(rows)
    self.props.text = "%d open orders" % len(rows)

Use instead:

def runAction(self, event):
    rows = system.db.runNamedQuery("Orders/Open")
    self.props.text = "%d open orders" % len(rows)

Options

• dummy-pattern (regex, default ^(_.*|dummy.*|ignored_.*|unused_.*)$): names that may be unused.

Fix

For x = value with a single target: removes the statement when value calls nothing and binds nothing (literals, names, attribute and subscript reads, operators, comparisons), otherwise removes only x = and keeps the expression, so that its calls still run and the variables of a list comprehension stay bound (Python 2 leaks them: x = [i for i in y] followed by return i). Backticks call repr. What is kept is never a pointless statement for SCR114. Unsafe: the value may still be wanted under another name, and a removed read or operator can have effects in Python (value["pages"] raises KeyError when the key is missing; a property, __getattr__, a Java getter, __eq__, __contains__ or the __str__ that % formatting calls runs code).

pylint

Corresponds to W0612 unused-variable. pylint also reports unused names of tuple targets (a, b = f()); iglint does not.

Option reference

Set in [rules.SCR103] of iglint.toml (or [tool.iglint.rules.SCR103] in pyproject.toml).

Option	Type	Default	Description
dummy-pattern	string	"^(_.*|dummy.*|ignored_.*|unused_.*)$"	Names matching this regex may be unused.

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Generated from the rule metadata by iglint generate-docs; iglint rule SCR103 prints the same page in the terminal. Source: docs/rules/SCR103.md.