[Python-ideas] PEP 484 (Type Hints) -- first draft round

Eugene Toder eltoder at gmail.com
Wed Jan 21 01:20:17 CET 2015

On Tue, Jan 20, 2015 at 6:23 PM, Andrew Barnert <
abarnert at yahoo.com.dmarc.invalid> wrote:

> As I understand it (and please correct me if I'm wrong), Greg is making a
> very simple point:
> If type annotations are only for static type checking, as done by
> something like MyPy, they, by definition, have no use at runtime. But
> annotations are about storing information with functions at runtime. So the
> proposal is inherently storing useless information at runtime. (And it's
> also preventing anyone else from storing useful information there, but
> that's not the main issue.)
I believe the argument is slightly different. The main issue is not with
storing the annotation, but with evaluating it. This evaluation does not
help a static type checker in any way, but introduces problems with forward
references, and limits the syntax. For example, a type checker could
support the common and natural "T1|T2" syntax for the Union type if the
interpreter did not attempt to evaluate it. The current solution is to
abandon some syntax, and put other things in quotes, which is a bit ugly.

The second issue is that the annotations are only supported on the function
arguments and return value. There are other things that are useful to
annotate, most obviously class attributes and local variables. The current
solution is to put these annotations into comments, which is not pretty too.

I think these are both good points. They can be addressed by introducing an
entirely new syntax, like suggested above, or by changing and extending how
the current syntax works. E.g. one can imagine that annotations are not
evaluated. They are always stored as strings for the benefit of
introspection, but otherwise are not touched by the interpreter. Also,
syntax is added to put annotations on other constructs. At least for the 2
cases above it should not be not hard, and we can store these new
annotations on the class object and on the function object respectively.
Obviously not evaluating annotations is a breaking change, so it can be a
non-starter, but it will make things cleaner.

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