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Unused local variable

ID: py/unused-local-variable
Kind: problem
Security severity: 
Severity: recommendation
Precision: very-high
Tags:
   - maintainability
   - useless-code
   - external/cwe/cwe-563
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

A local variable is defined (by an assignment) but never used.

It is sometimes necessary to have a variable which is not used. These unused variables should have distinctive names, to make it clear to readers of the code that they are deliberately not used. The most common conventions for indicating this are to name the variable _ or to start the name of the variable with unused or _unused.

The query accepts the following names for variables that are intended to be unused:

  • Any name consisting entirely of underscores.

  • Any name containing unused.

  • The names dummy or empty.

  • Any “special” name of the form __xxx__. Variables that are defined in a group, for example x, y = func() are handled collectively. If they are all unused, then this is reported. Otherwise they are all treated as used.

Recommendation

If the variable is included for documentation purposes or is otherwise intentionally unused, then change its name to indicate that it is unused, otherwise delete the assignment (taking care not to delete right hand side if it has side effects).

Example

In this example, the random_no variable is never read but its assignment has a side effect. Because of this it is important to remove only the left hand side of the assignment in line 10.

import random

def write_random_to_file():
    no = random.randint(1, 10)
    with open("random.txt", "w") as file:
        file.write(str(no))
    return no

def write_random():
    random_no = write_random_to_file()
    print "A random number was written to random.txt"

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