Why does calling Python's 'magic method' not do type conversion like it would for the corresponding operator?

When I subtract a float from an integer (e.g. 1-2.0), Python does implicit type conversion (I think). But when I call what I thought was the same operation using the magic method __sub__, it suddenly does not anymore.

What am I missing here? When I overload operators for my own classes, is there a way around this other than explicitly casting input to whatever type I need?

a=1

a.__sub__(2.)

# returns NotImplemented

a.__rsub__(2.)

# returns NotImplemented

# yet, of course:

a-2.

# returns -1.0

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

add a comment |

What am I missing here? When I overload operators for my own classes, is there a way around this other than explicitly casting input to whatever type I need?

a=1

a.__sub__(2.)

# returns NotImplemented

a.__rsub__(2.)

# returns NotImplemented

# yet, of course:

a-2.

# returns -1.0

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

add a comment |

What am I missing here? When I overload operators for my own classes, is there a way around this other than explicitly casting input to whatever type I need?

a=1

a.__sub__(2.)

# returns NotImplemented

a.__rsub__(2.)

# returns NotImplemented

# yet, of course:

a-2.

# returns -1.0

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

What am I missing here? When I overload operators for my own classes, is there a way around this other than explicitly casting input to whatever type I need?

a=1

a.__sub__(2.)

# returns NotImplemented

a.__rsub__(2.)

# returns NotImplemented

# yet, of course:

a-2.

# returns -1.0

python type-conversion implicit-conversion magic-methods

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

edited Feb 20 at 4:30

Solomon Ucko

7742822

edited Feb 20 at 4:30

Solomon Ucko

7742822

edited Feb 20 at 4:30

Solomon Ucko

7742822

asked Feb 19 at 22:31

doppler

34819

asked Feb 19 at 22:31

doppler

34819

asked Feb 19 at 22:31

doppler

34819

add a comment |

2 Answers
2

active

oldest

votes

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)

-1.0

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

There is no implicit type conversion built into the subtraction operator. float.__rsub__ has to handle ints manually. If you want type conversion in your own operator implementations, you'll have to handle that manually too.

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

9

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

5

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

4

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

|
show 1 more comment

@user2357112 already said it well but there's nothing like an example.

class A:

   def __sub__(self, other):

       print('A.__sub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



   def __rsub__(self, other):

       print('A.__rsub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



class B:

   def __sub__(self, other):

       print('B.__sub__')

       if not isinstance(other, B):

           return NotImplemented

       return 0

a1 = A()

a2 = A()

b = B()



a1 - a2

A.__sub__

# 0

Objects a1 and a2 are compatible (both type A), a valid result is returned.

Next, consider,

b - a1

B.__sub__

A.__rsub__

# TypeError: unsupported operand type(s) for -: 'B' and 'A'

Objects b and a1 are not compatible. First, b.__sub__ is tried, which returns NotImplemented, so a1.__rsub__ is tried, which also returns NotImplemented. So a TypeError is raised.

Finally,

a1 - b

A.__sub__

# TypeError: unsupported operand type(s) for -: 'A' and 'B'

This time, a1.__sub__ is tried first, which returns NotImplemented. Now, since b.__rsub__ is not defined, a TypeError is raised.

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)

-1.0

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

9

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

5

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

4

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

|
show 1 more comment

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)

-1.0

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

9

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

5

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

4

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

|
show 1 more comment

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)

-1.0

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)

-1.0

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

edited Feb 19 at 22:38

answered Feb 19 at 22:35

user2357112

157k12172266

answered Feb 19 at 22:35

user2357112

157k12172266

answered Feb 19 at 22:35

user2357112

157k12172266

9

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

5

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

4

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

|
show 1 more comment

9

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

5

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

4

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

It's worth noting that the NotImplemented result that is returned by the calls in the question are the signal to try the reverse method.

– Blckknght
Feb 19 at 22:37

Thanks! I was aware it would try __rsub__ but didn't know it would reverse the argument order.

– doppler
Feb 19 at 22:41

@doppler: It'd be pretty pointless to have the left operand handle both __sub__ and __rsub__. That'd just be two methods with the exact same job, and the right operand would have no opportunity to supply an implementation.

– user2357112
Feb 19 at 22:42

@user2357112 so self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?

– doppler
Feb 19 at 22:44

@doppler: No. self.__rsub__(other) is called for other - self if other can't handle it. Calling other.__sub__(self) would be pointless. We already know other can't handle it.

– user2357112
Feb 19 at 22:52

|
show 1 more comment

@user2357112 already said it well but there's nothing like an example.

class A:

   def __sub__(self, other):

       print('A.__sub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



   def __rsub__(self, other):

       print('A.__rsub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



class B:

   def __sub__(self, other):

       print('B.__sub__')

       if not isinstance(other, B):

           return NotImplemented

       return 0

a1 = A()

a2 = A()

b = B()



a1 - a2

A.__sub__

# 0

Objects a1 and a2 are compatible (both type A), a valid result is returned.

Next, consider,

b - a1

B.__sub__

A.__rsub__

# TypeError: unsupported operand type(s) for -: 'B' and 'A'

Objects b and a1 are not compatible. First, b.__sub__ is tried, which returns NotImplemented, so a1.__rsub__ is tried, which also returns NotImplemented. So a TypeError is raised.

Finally,

a1 - b

A.__sub__

# TypeError: unsupported operand type(s) for -: 'A' and 'B'

This time, a1.__sub__ is tried first, which returns NotImplemented. Now, since b.__rsub__ is not defined, a TypeError is raised.

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

add a comment |

@user2357112 already said it well but there's nothing like an example.

class A:

   def __sub__(self, other):

       print('A.__sub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



   def __rsub__(self, other):

       print('A.__rsub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



class B:

   def __sub__(self, other):

       print('B.__sub__')

       if not isinstance(other, B):

           return NotImplemented

       return 0

a1 = A()

a2 = A()

b = B()



a1 - a2

A.__sub__

# 0

Objects a1 and a2 are compatible (both type A), a valid result is returned.

Next, consider,

b - a1

B.__sub__

A.__rsub__

# TypeError: unsupported operand type(s) for -: 'B' and 'A'

Objects b and a1 are not compatible. First, b.__sub__ is tried, which returns NotImplemented, so a1.__rsub__ is tried, which also returns NotImplemented. So a TypeError is raised.

Finally,

a1 - b

A.__sub__

# TypeError: unsupported operand type(s) for -: 'A' and 'B'

This time, a1.__sub__ is tried first, which returns NotImplemented. Now, since b.__rsub__ is not defined, a TypeError is raised.

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

add a comment |

@user2357112 already said it well but there's nothing like an example.

class A:

   def __sub__(self, other):

       print('A.__sub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



   def __rsub__(self, other):

       print('A.__rsub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



class B:

   def __sub__(self, other):

       print('B.__sub__')

       if not isinstance(other, B):

           return NotImplemented

       return 0

a1 = A()

a2 = A()

b = B()



a1 - a2

A.__sub__

# 0

Objects a1 and a2 are compatible (both type A), a valid result is returned.

Next, consider,

b - a1

B.__sub__

A.__rsub__

# TypeError: unsupported operand type(s) for -: 'B' and 'A'

Objects b and a1 are not compatible. First, b.__sub__ is tried, which returns NotImplemented, so a1.__rsub__ is tried, which also returns NotImplemented. So a TypeError is raised.

Finally,

a1 - b

A.__sub__

# TypeError: unsupported operand type(s) for -: 'A' and 'B'

This time, a1.__sub__ is tried first, which returns NotImplemented. Now, since b.__rsub__ is not defined, a TypeError is raised.

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

@user2357112 already said it well but there's nothing like an example.

class A:

   def __sub__(self, other):

       print('A.__sub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



   def __rsub__(self, other):

       print('A.__rsub__')

       if not isinstance(other, A):

           return NotImplemented

       return 0



class B:

   def __sub__(self, other):

       print('B.__sub__')

       if not isinstance(other, B):

           return NotImplemented

       return 0

a1 = A()

a2 = A()

b = B()



a1 - a2

A.__sub__

# 0

Objects a1 and a2 are compatible (both type A), a valid result is returned.

Next, consider,

b - a1

B.__sub__

A.__rsub__

# TypeError: unsupported operand type(s) for -: 'B' and 'A'

Objects b and a1 are not compatible. First, b.__sub__ is tried, which returns NotImplemented, so a1.__rsub__ is tried, which also returns NotImplemented. So a TypeError is raised.

Finally,

a1 - b

A.__sub__

# TypeError: unsupported operand type(s) for -: 'A' and 'B'

This time, a1.__sub__ is tried first, which returns NotImplemented. Now, since b.__rsub__ is not defined, a TypeError is raised.

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

edited Feb 20 at 8:08

answered Feb 20 at 3:03

coldspeed

137k24151237

answered Feb 20 at 3:03

coldspeed

137k24151237

answered Feb 20 at 3:03

coldspeed

137k24151237

add a comment |

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