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Inconsistent equality and hashing

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

Click to see the query in the CodeQL repository

In order to conform to the object model, classes that define their own equality method should also define their own hash method, or be unhashable. If the hash method is not defined then the hash of the super class is used. This is unlikely to result in the expected behavior.

A class can be made unhashable by setting its __hash__ attribute to None.

In Python 3, if you define a class-level equality method and omit a __hash__ method then the class is automatically marked as unhashable.

Recommendation

When you define an __eq__ method for a class, remember to implement a __hash__ method or set __hash__ = None.

Example

In the following example the Point class defines an equality method but no hash method. If hash is called on this class then the hash method defined for object is used. This is unlikely to give the required behavior. The PointUpdated class is better as it defines both an equality and a hash method. If Point was not to be used in dicts or sets, then it could be defined as UnhashablePoint below.

To comply fully with the object model this class should also define an inequality method (identified by a separate rule).

# Incorrect: equality method defined but class contains no hash method
class Point(object):

    def __init__(self, x, y):
        self._x = x
        self._y = y

    def __repr__(self):
        return 'Point(%r, %r)' % (self._x, self._y)

    def __eq__(self, other):
        if not isinstance(other, Point):
            return False
        return self._x == other._x and self._y == other._y


# Improved: equality and hash method defined (inequality method still missing)
class PointUpdated(object):

    def __init__(self, x, y):
        self._x = x
        self._y = y

    def __repr__(self):
        return 'Point(%r, %r)' % (self._x, self._y)

    def __eq__(self, other):
        if not isinstance(other, Point):
            return False
        return self._x == other._x and self._y == other._y

    def __hash__(self):  
        return hash(self._x) ^ hash(self._y)

# Improved: equality method defined and class instances made unhashable
class UnhashablePoint(object):

    def __init__(self, x, y):
        self._x = x
        self._y = y

    def __repr__(self):
        return 'Point(%r, %r)' % (self._x, self._y)

    def __eq__(self, other):
        if not isinstance(other, Point):
            return False
        return self._x == other._x and self._y == other._y

    #Tell the interpreter that instances of this class cannot be hashed
    __hash__ = None

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