support postponed annotations and ForwardRef in python 3.7

[samuelcolvin]

Change Summary

support postponed annotations in python 3.7

Related issue number

fix #234 partial solution use of ForwardRef for the model itself isn't yet supported. Since almost all of the field setup logic assumes the model already exists this will be much more complicated

ForwardRef should now be fully supported. If the ForwardRef can't evaluate when creating the Model you'll need to call Model.update_forward_refs() after it's been created.

Also note that the Model will need to be defined in the root of the module so it's included in globals, this is a limitation of get_type_hints it could be fixed in future by implementing our own version of get_type_hints but that's beyond the scope of this PR I think.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

Merging #348 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #348   +/-   ##
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  Coverage     100%   100%           
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  Files          14     14           
  Lines        1824   1857   +33     
  Branches      356    364    +8     
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+ Hits         1824   1857   +33

[samuelcolvin]

Humm, so 5 people ask for this or similar, but no one is willing to take 20 minutes and test it or give any feedback. Not a great reflection on the open source community.

I guess I'll wait a couple more days and merge it.

[StephenBrown2]

Just ran into this issue, and found it works for me!

Pydantic 0.18
Python 3.7
Example:

from __future__ import annotations
from pydantic import BaseModel

class CalendarRule(BaseModel):
    id: int
    calendar: int
    children: t.List[CalendarRule]

the above no longer crashes when I import annotations from __future__.

Thanks for your work on this!

[samuelcolvin]

Thanks, great to hear.

[theenglishway]

Works for me as well, although with a slight catch compared to @StephenBrown2

(Python 3.7.3rc1, Pydantic 0.23)

I have this model :

class Entity(BaseModel):
    name: str
    relatives: List[Entity] = []

If I only add from __future__ import annotations, pydantic yells with the (very informative) message : pydantic.errors.ConfigError: field "relatives" not yet prepared so type is still a ForwardRef, you might need to call Entity.update_forward_refs().

Obviously adding that piece of code just works 👍

I've had some little issues with the documentation regarding this feature though, for which I'm making an issue and a PR.

[samuelcolvin]

great to hear it works, PR for docs very welcome.