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__eq__ not overridden when adding attributes

ID: py/missing-equals
Kind: problem
Security severity: 
Severity: warning
Precision: high
Tags:
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

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A class that defines attributes that are not present in its superclasses may need to override the __eq__() method (__ne__() should also be defined).

Adding additional attributes without overriding __eq__() means that the additional attributes will not be accounted for in equality tests.

Recommendation

Override the __eq__ method.

Example

In the following example the ColorPoint class subclasses the Point class and adds a new attribute, but does not override the __eq__ 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

class ColorPoint(Point):

    def __init__(self, x, y, color):
        Point.__init__(self, x, y)
        self._color = color

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

#ColorPoint(0, 0, Red) == ColorPoint(0, 0, Green) should be False, but is True.

#Fixed version
class ColorPoint(Point):

    def __init__(self, x, y, color):
        Point.__init__(self, x, y)
        self._color = color

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

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

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