Support special binary methods ("__add__" etc.) as Python does

[malinoff]

I'm not sure if this is a bug report or a feature request, but as of cython 0.27.3 __radd__, __rtruediv__ and other right side magic methods do not work:

In [1]: %load_ext Cython

In [2]: %%cython
   ...: cdef class Example:
   ...:   def __radd__(self, other):
   ...:     print('radd', self, other)
   ...:   def __rtruediv__(self, other):
   ...:     print('rtruediv', self, other)
   ...:

In [3]: 3 + Example()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-b54d8dd2451f> in <module>()
----> 1 3 + Example()

TypeError: unsupported operand type(s) for +: 'int' and '_cython_magic_c1fde2d95cc29b87ed98f696ccfef975.Example'

In [4]: 'foo' / Example()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-1e29a35d9b28> in <module>()
----> 1 'foo' / Example()

TypeError: unsupported operand type(s) for /: 'str' and '_cython_magic_c1fde2d95cc29b87ed98f696ccfef975.Example'

In [5]: import Cython

In [6]: Cython.__version__
Out[6]: '0.27.3'

I'm using the following workaround, but it looks ugly:

In [7]: %%cython
   ...: cdef class Example:
   ...:   def __add__(other, self):
   ...:     print('add', self, other)
   ...:   def __truediv__(other, self):
   ...:     print('truediv', self, other)
   ...:

In [8]: 3 + Example()
add <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a0f0> 3

In [9]: 'foo' / Example()
truediv <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a110> foo

and doesn't really work if we switch operands:

In [10]: Example() + 3
add 3 <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a110>

In [11]: Example() / 'foo'
truediv foo <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109468e60>

I'm not familiar with cython codebase, but a quick search through the source, issues and PRs doesn't show anything related.

[robertwb]

Cython's special methods still mostly reflect the C API rather than the Python API. See http://cython.readthedocs.io/en/latest/src/userguide/special_methods.html#arithmetic-methods . Here either the first or second parameter may be self, rather than having __rxxx__ versions of the operators. (It would be nice to support this however, similar to how the comparison methods were recently added.)

…

On Thu, Dec 28, 2017 at 3:14 AM, Dmitry Malinovsky ***@***.*** > wrote: I'm not sure if this is a bug report or a feature request, but as of cython 0.27.3 __radd__, __rtruediv__ and other right side magic methods do not work: In [1]: %load_ext Cython In [2]: %%cython ...: cdef class Example: ...: def __radd__(self, other): ...: print('radd', self, other) ...: def __rtruediv__(self, other): ...: print('rtruediv', self, other) ...: In [3]: 3 + Example()---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-3-b54d8dd2451f> in <module>()----> 1 3 + Example() TypeError: unsupported operand type(s) for +: 'int' and '_cython_magic_c1fde2d95cc29b87ed98f696ccfef975.Example' In [4]: 'foo' / Example()---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-4-1e29a35d9b28> in <module>()----> 1 'foo' / Example() TypeError: unsupported operand type(s) for /: 'str' and '_cython_magic_c1fde2d95cc29b87ed98f696ccfef975.Example' In [5]: import Cython In [6]: Cython.__version__ Out[6]: '0.27.3' I'm using the following workaround, but it looks ugly: In [7]: %%cython ...: cdef class Example: ...: def __add__(other, self): ...: print('add', self, other) ...: def __truediv__(other, self): ...: print('truediv', self, other) ...: In [8]: 3 + Example() add <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a0f0> 3 In [9]: 'foo' / Example() truediv <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a110> foo and doesn't really work if we switch operands: In [10]: Example() + 3 add 3 <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109d7a110> In [11]: Example() / 'foo' truediv foo <_cython_magic_45c6eacd188f8181199b3c76f32cba5d.Example object at 0x109468e60> I'm not familiar with cython codebase, but a quick search through the source, issues and PRs doesn't show anything related. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#2056>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAdqgcvFpRoV1V5VQmmG1gnKEIwYNzK5ks5tE3gWgaJpZM4ROO6o> .

[robertwb]

We should consider whether we want to move away from the "first or second argument is self" for the arithmetic methods for 3.0.

[scoder]

I think we should switch to normal Python semantics. As @robertwb said, in the same way as we support the comparison methods now.

[scoder]

See generate_richcmp_function() in ModuleNode.py together with the RichcmpSlot in TypeSlots.py.

The idea is to define the number slots with a new slot type BinopSlot that has two special Python methods as implementation, and then generate the nb_add etc. slots by dispatching to these methods (more precisely, their C functions) according to CPython's own logic in typeobject.c.

[scoder]

Marking as Cython 3.0 release blocker, in case we need backwards incompatible changes for this.