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Inconsistent equality and hashing — CodeQL query help documentation

ID: py/equals-hash-mismatch
Kind: problem
Security severity: 
Severity: warning
Precision: very-high
Tags:
   - quality
   - reliability
   - correctness
   - external/cwe/cwe-581
Query suites:
   - python-code-quality.qls
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

A hashable class has an __eq__ method, and a __hash__ method that agrees with equality. When a hash method is defined, an equality method should also be defined; otherwise object identity is used for equality comparisons which may not be intended.

Note that defining an __eq__ method without defining a __hash__ method automatically makes the class unhashable in Python 3. (even if a superclass defines a hash method).

Recommendation

If a __hash__ method is defined, ensure a compatible __eq__ method is also defined.

To explicitly declare a class as unhashable, set __hash__ = None, rather than defining a __hash__ method that always raises an exception. Otherwise, the class would be incorrectly identified as hashable by an isinstance(obj, collections.abc.Hashable) call.

Example

In the following example, the A class defines an hash method but no equality method. Equality will be determined by object identity, which may not be the expected behaviour.

class A:
    def __init__(self, a, b):
        self.a = a 
        self.b = b

    # No equality method is defined
    def __hash__(self):
        return hash((self.a, self.b))

References