CodeQL documentation

Nested loops with same variable reused after inner loop body

ID: py/nested-loops-with-same-variable-reused
Kind: problem
Security severity: 
Severity: error
Precision: very-high
   - maintainability
   - correctness
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

In Python variables have function-wide scope which means that if two variables have the same name in the same scope, they are in fact one variable. Consequently, nested loops in which the target variables have the same name in fact share a single variable. Such loops are difficult to understand as the inner loop will modify the target variable of the outer loop. This may lead to unexpected behavior if the loop variable is used after the inner loop has terminated.


Rename the inner loop variable.


This example shows a function that processes a sequence of lists of numbers. It prints out the largest element from each of the lists. In the first version, the variable x gets overwritten by the inner loop, resulting in the wrong output. In the second function, the error has been fixed by renaming the inner loop variable to stop it overwriting the outer loop variable.

def largest_elements(l):
    for x in l:
        maxnum = 0
        for x in x:
            maxnum = max(x, maxnum)
        # The outer loop variable x has now been overwritten by the inner loop.
        print "The largest element in the list", x, "is", maxnum

def largest_elements_correct(l):
    for x in l:
        maxnum = 0
        for y in x:
            maxnum = max(y, maxnum)
        print "The largest element in the list", x, "is", maxnum


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