PEP 572: Python assignment expressions has been accepted

Re:PEP 572: Python assignment expressions has been accepted by chriswarbo on Tuesday July 03, 2018 @09:02AM (#17449776)

Hmm, good catch. Interestingly, I considered using `a + b` as sugar for `a.__add__(b)`, but rejected that because they're not quite equivalent. For example if `__add__` is defined using a magic method:

    >>> class A(object):
    ...   def __getattribute__(self, attr):
    ...     if attr == "__add__":
    ...       return lambda *_: "hello world"
    ...     return None
    ...
    >>> a = A()
    >>> a.__add__(A())
    'hello world'
    >>> a + A()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: unsupported operand type(s) for +: 'A' and 'A'

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Re:PEP 572: Python assignment expressions has been accepted by bilboa on Tuesday July 03, 2018 @02:25PM (#17452560)

This is because the actual translation of `a + b` is a little more complicated. It's something like this:

  if '__add__' in a.__dict__:
      try:
          return a.__add__(b)
      except NotImplemented:
          if '__radd__' in b.__dict__:
            return b.__radd__(a)
  elif '__radd__' in b.__dict__:
      return b.__radd__(a)
  
  raise TypeError('unsupported operand type(s) for +: '{}' and '{}'.format(type(a), type(b))

In particular, the runtime seems to directly look for '__add__' in the object's __dict__, rather than just trying to invoke `__add__`, so your `__getattribute__` method isn't enough to make it work. If you add an actual `__add__` method to A your example will work.

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Re:PEP 572: Python assignment expressions has been accepted by chriswarbo on Tuesday July 03, 2018 @03:23PM (#17452949)

Yeah, I had a feeling the real 'expansion' would be more involved. I didn't know about the lookup in __dict__, that's good to know.

I'm reminded of PHP, where (at least in version 5.*) we could write:

    $myObject->foo = function() { return "hello world"; };

    $x = $myObject->foo;
    $x(); // Hello world

    $myObject->foo();  // Error: no such method 'foo'

(Taken from an old comment https://news.ycombinator.com/item?id=8119419 )

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Re:PEP 572: Python assignment expressions has been accepted by bilboa on Tuesday July 03, 2018 @05:28PM (#17453736)

I found the official docs on how python special methods are looked up.

https://docs.python.org/3/reference/datamodel.html#special-l...

Apparently the runtime is even more picky than I showed. The method has to be defined on the object's type, not in the object's instance dictionary. So, really the lookup is something like:

  if hasattr(type(a), '__add__'):

The link I provided explains the rationale for bypassing the instance dictionary and `__getattribute__` method.

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