When is the `==` operator not equivalent to the `is` operator? (Python)

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In Python, the == operator is implemented in terms of the magic method __eq__, which by default implements it by identity comparison. You can, however, override the method in order to provide your own concept of object equality. Note, that if you do so, you will usually also override at least __ne__ (which implements the != operator) and __hash__, which computes a hash code for the instance.

I found it very helpful, even in Python, to make my __eq__ implementations comply with the rules set out in the Java language for implementations of the equals method, namely:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

the last one should probably replace null with None, but the rules are not as easy here in Python as in Java.

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