Documentation confusion - implicit vs explicit declaration #226

omry · 2019-12-25T16:29:45Z

The documentation equates two kinds of attribute declarations:

explicit:

 class Derp:
    attribute: int = 1

    @property
    def property(self) -> int: ...

and implicit:

  class Derp:
    def __init__(self):
       self.attribute: int = 1

Those two things are not about explicit vs implicit declaration, but about class membership (static field) vs member field.

I think it's worth changing the doc to avoid confusion.

Example of difference:

In [1]: class Derp:
   ...:     x : int = 10
   ...:

In [2]: Derp.x
Out[2]: 10

In [3]:   class Derp:
   ...:     def __init__(self):
   ...:        self.attribute: int = 1
   ...:

In [4]: Derp.x
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-d80ae71850d9> in <module>
----> 1 Derp.x

grievejia · 2019-12-27T17:19:04Z

According to PEP 526, the first form of attribute declaration should actually be treated as instance membership as opposed to class membership by the type checker. If you explicitly want class membership, you'll need to use ClassVar:

class Derp:
  instance_attribute: int = 1  # instance membership
  class_attribute: ClassVar[int] = 1  # class membership

I don' t think Pyre currently fully enforce this distinction, but we might eventually go there in the future as I feel there's value in explicitly distinguishing between instance members and class members.

The fact that Derp.instance_attribute exists as class member at runtime is rather unfortunate -- but IMO that's more of a compromise to the legacy Python runtime when PEP 484/526 was introduced, not an intended behavior.

omry · 2019-12-28T16:41:47Z

Yeah, it does seem unfortunate.
I think a better recommendation for explicit class member typing is:

class Foo:
  x: int
  def __init__():
    self.x = 10

As opposed to the implicit one:

class Foo:
  def __init__():
    self.x : int = 10

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Documentation confusion - implicit vs explicit declaration #226

Documentation confusion - implicit vs explicit declaration #226

omry commented Dec 25, 2019 •

edited

grievejia commented Dec 27, 2019 •

edited

omry commented Dec 28, 2019

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Documentation confusion - implicit vs explicit declaration #226

Documentation confusion - implicit vs explicit declaration #226

Comments

omry commented Dec 25, 2019 • edited

grievejia commented Dec 27, 2019 • edited

omry commented Dec 28, 2019

omry commented Dec 25, 2019 •

edited

grievejia commented Dec 27, 2019 •

edited