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Clarify that annotation-only doesn't work w/ decorators (#486) 

Fixes #466
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Hynek Schlawack 2019-02-09 13:55:48 +01:00 committed by GitHub
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2 changed files with 14 additions and 3 deletions
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3
docs/init.rst
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@ -97,6 +97,7 @@ This is when default values come into play:
C(a=42, b=[], c=[], d={})
It's important that the decorated method -- or any other method or property! -- doesn't have the same name as the attribute, otherwise it would overwrite the attribute definition.
You also cannot use type annotations to elide the :func:`attr.ib` call for ``d`` as explained in :doc:`types`.
Please note that as with function and method signatures, ``default=[]`` will *not* do what you may think it might do:
@ -161,7 +162,7 @@ If the value does not pass the validator's standards, it just raises an appropri
...
ValueError: x must be smaller or equal to 42
Again, it's important that the decorated method doesn't have the same name as the attribute.
Again, it's important that the decorated method doesn't have the same name as the attribute and that you can't elide the call to :func:`attr.ib`.
Callables

14
docs/types.rst
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@ -23,9 +23,19 @@ That means that on modern Python versions, the declaration part of the example f
>>> attr.fields(SomeClass).a_number.type
<class 'int'>
You can still use :func:`attr.ib` for advanced features, but you don't have to.
You will still need :func:`attr.ib` for advanced features, but not for the common cases.
Please note that these types are *only metadata* that can be queried from the class and they aren't used for anything out of the box!
One of those features are the decorator-based features like defaults.
It's important to remember that ``attrs`` doesn't do any magic behind your back.
All the decorators are implemented using an object that is returned by the call to :func:`attr.ib`.
Attributes that only carry a class annotation do not have that object so trying to call a method on it will inevitably fail.
*****
Please note that types -- however added -- are *only metadata* that can be queried from the class and they aren't used for anything out of the box!
In practice, their biggest usefulness shows in combination with mypy.
mypy