String concatenation in loop causes production risks

This code insight counts one violation each time an operator + or += is used with a string parameter inside a loop.

Note : the rule is restricted to expression with litteral string. Variable string need semantic to be detected.

bad

employee_table = '<table>'
    for last_name, first_name in employee_list:
        employee_table += '<tr><td>%s, %s</td></tr>' % (last_name, first_name)
        employee_table += '</table>'

good

items = ['<table>']
    for last_name, first_name in employee_list:
        items.append('<tr><td>%s, %s</td></tr>' % (last_name, first_name))
        items.append('</table>')
    employee_table = ''.join(items)

Why you should care

Avoid using the + and += operators to accumulate a string within a loop. Since strings are immutable, this creates unnecessary temporary objects and results in quadratic rather than linear running time. Instead, add each substring to a list and ”.join the list after the loop terminates (or, write each substring to a io.BytesIO buffer).

Business Impacts

Production Risk

CAST recommendations

References

https://www.quantifiedcode.com/knowledge-base/performance/Use%20%60extend%28%29%60%20for%20list%20concatenation/3kr7yXet

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