In Python, special methods corresponding to numeric operators, rich comparison operators and the __length_hint__ method should return
NotImplemented when the operation is not supported. These methods should not raise NotImplementedError as callers don’t
expect it and won’t catch this exception.
For example A + B is equivalent to calling A.__add__(B). If this binary operation is not supported by class A,
A.__add__(B) should return NotImplemented. The interpreter will then try the reverse operation, i.e.
B.__radd__(A). This enables adding new operations by changing only one class instead of two.
This rule raises an issue when one of the following methods raises NotImplementedError instead of returning
NotImplemented:
- __lt__(self, other)
- __le__(self, other)
- __eq__(self, other)
- __ne__(self, other)
- __gt__(self, other)
- __ge__(self, other)
- __add__(self, other)
- __sub__(self, other)
- __mul__(self, other)
- __matmul__(self, other)
- __truediv__(self, other)
- __floordiv__(self, other)
- __mod__(self, other)
- __divmod__(self, other)
- __pow__(self, other[, modulo])
- __lshift__(self, other)
- __rshift__(self, other)
- __and__(self, other)
- __xor__(self, other)
- __or__(self, other)
- __radd__(self, other)
- __rsub__(self, other)
- __rmul__(self, other)
- __rmatmul__(self, other)
- __rtruediv__(self, other)
- __rfloordiv__(self, other)
- __rmod__(self, other)
- __rdivmod__(self, other)
- __rpow__(self, other[, modulo])
- __rlshift__(self, other)
- __rrshift__(self, other)
- __rand__(self, other)
- __rxor__(self, other)
- __ror__(self, other)
- __iadd__(self, other)
- __isub__(self, other)
- __imul__(self, other)
- __imatmul__(self, other)
- __itruediv__(self, other)
- __ifloordiv__(self, other)
- __imod__(self, other)
- __ipow__(self, other[, modulo])
- __ilshift__(self, other)
- __irshift__(self, other)
- __iand__(self, other)
- __ixor__(self, other)
- __ior__(self, other)
- __length_hint__(self)
Noncompliant Code Example
class MyClass:
def __add__(self, other):
raise NotImplementedError() # Noncompliant
def __radd__(self, other):
raise NotImplementedError() # Noncompliant
class MyOtherClass:
def __add__(self, other):
return 42
def __radd__(self, other):
return 42
MyClass() + MyOtherClass() # This will raise NotImplementedError
Compliant Solution
class MyClass:
def __add__(self, other):
return NotImplemented
def __radd__(self, other):
return NotImplemented
class MyOtherClass:
def __add__(self, other):
return 42
def __radd__(self, other):
return 42
MyClass() + MyOtherClass() # This returns 42
See
