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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

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CAST recommendations

References

https://www.quantifiedcode.com/knowledge-base/performance/Use%20%60extend%28%29%60%20for%20list%20concatenation/3kr7yXet [nz_btn text="See features" link="http://casthighlight.wpengine.com/outputs-analytics/" target="_self" icon="icon-stats" animate="false" animation_type="ghost" color="black" size="small" shape="rounded" type="normal" hover_normal="fill" hover_ghost="fill"][nz_btn text="How it works" link="http://casthighlight.wpengine.com/how-it-works/" target="_self" icon="icon-cog" animate="false" animation_type="ghost" color="black" size="small" shape="rounded" type="ghost" hover_normal="fill" hover_ghost="fill"]