Comparison using is when operands support__eq__¶
ID: py/comparison-using-isKind: problemSecurity severity: Severity: warningPrecision: highTags: - quality - reliability - correctnessQuery suites: - python-security-and-quality.qls
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When you compare two values using theis orisnot operator, it is the object identities of the two values that is tested rather than their equality. If the class of either of the values in the comparison redefines equality then theis operator may returnFalse even though the objects compare as equal. Equality is defined by the__eq__ or, in Python2,__cmp__ method. To compare two objects for equality, use the== or!= operator instead.
Recommendation¶
When you want to compare the value of two literals, use the comparison operator== or!= in place ofis orisnot.
If the uniqueness property or performance are important then use an object that does not redefine equality.
Example¶
In the first line of the following example the programmer tests the value ofvalue againstDEFAULT using theis operator. Unfortunately, this may fail when the function is called with the string"default".
To function correctly, change the expressionvalueisDEFAULT tovalue==DEFAULT. Alternatively, if the uniqueness property is desirable, then change the definition ofDEFAULT to either of the alternatives below.
DEFAULT="default"defget_color(name,fallback):ifnameinCOLORS:returnCOLORS[name]eliffallbackisDEFAULT:returnDEFAULT_COLORelse:returnfallback#This worksprint(get_color("spam","def"+"ault"))#But this does notprint(get_color("spam","default-spam"[:7]))#To fix the above code change to objectDEFAULT=object()#Or if you want better repr() output:classDefault(object):def__repr__(self):return"DEFAULT"DEFAULT=Default()
References¶
Python Standard Library:Comparisons.