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
python casting
asked 5 hours ago
dopplerdoppler
1467
add a comment |
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
python casting
asked 5 hours ago
dopplerdoppler
1467
add a comment |
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
python casting
asked 5 hours ago
dopplerdoppler
1467
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
python casting
python casting
asked 5 hours ago
dopplerdoppler
1467
asked 5 hours ago
dopplerdoppler
1467
asked 5 hours ago
dopplerdoppler
1467
asked 5 hours ago
dopplerdoppler
1467
asked 5 hours ago
dopplerdoppler
1467
1467
add a comment |
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.
answered 5 hours ago
user2357112user2357112
155k12163258
6
It's worth noting that theNotImplementedresult that is returned by the calls in the question are the signal to try the reverse method.
– Blckknght
5 hours ago
Thanks! I was aware it would try__rsub__but didn't know it would reverse the argument order.
– doppler
5 hours ago
2
@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
4 hours ago
@user2357112 soself.__rsub__(other)really just callsother.__sub__(self), if that makes any sense?
– doppler
4 hours ago
2
@doppler: No.self.__rsub__(other)is called forother - selfifothercan't handle it. Callingother.__sub__(self)would be pointless. We already knowothercan't handle it.
– user2357112
4 hours ago
|
show 1 more comment
@user2357112 already said it well but 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, so a TypeError is raised.
answered 38 mins ago
coldspeedcoldspeed
132k23139223
add a comment |
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2 Answers
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2 Answers
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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.
answered 5 hours ago
user2357112user2357112
155k12163258
6
It's worth noting that theNotImplementedresult that is returned by the calls in the question are the signal to try the reverse method.
– Blckknght
5 hours ago
Thanks! I was aware it would try__rsub__but didn't know it would reverse the argument order.
– doppler
5 hours ago
2
@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
4 hours ago
@user2357112 soself.__rsub__(other)really just callsother.__sub__(self), if that makes any sense?
– doppler
4 hours ago
2
@doppler: No.self.__rsub__(other)is called forother - selfifothercan't handle it. Callingother.__sub__(self)would be pointless. We already knowothercan't handle it.
– user2357112
4 hours ago
|
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.
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.
answered 5 hours ago
user2357112user2357112
155k12163258
6
It's worth noting that theNotImplementedresult that is returned by the calls in the question are the signal to try the reverse method.
– Blckknght
5 hours ago
Thanks! I was aware it would try__rsub__but didn't know it would reverse the argument order.
– doppler
5 hours ago
2
@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
4 hours ago
@user2357112 soself.__rsub__(other)really just callsother.__sub__(self), if that makes any sense?
– doppler
4 hours ago
2
@doppler: No.self.__rsub__(other)is called forother - selfifothercan't handle it. Callingother.__sub__(self)would be pointless. We already knowothercan't handle it.
– user2357112
4 hours ago
|
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.
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.
answered 5 hours ago
user2357112user2357112
155k12163258
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.
answered 5 hours ago
user2357112user2357112
155k12163258
edited 5 hours ago
answered 5 hours ago
user2357112user2357112
155k12163258
answered 5 hours ago
user2357112user2357112
155k12163258
answered 5 hours ago
user2357112user2357112
155k12163258
155k12163258
6
It's worth noting that theNotImplementedresult that is returned by the calls in the question are the signal to try the reverse method.
– Blckknght
5 hours ago
Thanks! I was aware it would try__rsub__but didn't know it would reverse the argument order.
– doppler
5 hours ago
2
@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
4 hours ago
@user2357112 soself.__rsub__(other)really just callsother.__sub__(self), if that makes any sense?
– doppler
4 hours ago
2
@doppler: No.self.__rsub__(other)is called forother - selfifothercan't handle it. Callingother.__sub__(self)would be pointless. We already knowothercan't handle it.
– user2357112
4 hours ago
|
show 1 more comment
6
It's worth noting that theNotImplementedresult that is returned by the calls in the question are the signal to try the reverse method.
– Blckknght
5 hours ago
Thanks! I was aware it would try__rsub__but didn't know it would reverse the argument order.
– doppler
5 hours ago
2
@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
4 hours ago
@user2357112 soself.__rsub__(other)really just callsother.__sub__(self), if that makes any sense?
– doppler
4 hours ago
2
@doppler: No.self.__rsub__(other)is called forother - selfifothercan't handle it. Callingother.__sub__(self)would be pointless. We already knowothercan't handle it.
– user2357112
4 hours ago
6
6
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
5 hours ago
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
5 hours ago
Thanks! I was aware it would try
__rsub__ but didn't know it would reverse the argument order.– doppler
5 hours ago
Thanks! I was aware it would try
__rsub__ but didn't know it would reverse the argument order.– doppler
5 hours ago
2
2
@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
4 hours ago
@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
4 hours ago
@user2357112 so
self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?– doppler
4 hours ago
@user2357112 so
self.__rsub__(other) really just calls other.__sub__(self), if that makes any sense?– doppler
4 hours ago
2
2
@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
4 hours ago
@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
4 hours ago
|
show 1 more comment
@user2357112 already said it well but 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, so a TypeError is raised.
answered 38 mins ago
coldspeedcoldspeed
132k23139223
add a comment |
@user2357112 already said it well but 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, so a TypeError is raised.
answered 38 mins ago
coldspeedcoldspeed
132k23139223
add a comment |
@user2357112 already said it well but 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, so a TypeError is raised.
answered 38 mins ago
coldspeedcoldspeed
132k23139223
@user2357112 already said it well but 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, so a TypeError is raised.
answered 38 mins ago
coldspeedcoldspeed
132k23139223
answered 38 mins ago
coldspeedcoldspeed
132k23139223
answered 38 mins ago
coldspeedcoldspeed
132k23139223
answered 38 mins ago
coldspeedcoldspeed
132k23139223
132k23139223
add a comment |
add a comment |
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