Fix BaseModel.__repr__ with extra fields

[povilasb]

Change Summary

Consider this example:

from pydantic import BaseModel


class Person(BaseModel):
    name: str

    class Config:
        extra = "allow"


me = Person(name="povilas")
me.age = 30
print(me)

Latest pydantic master branch would crash with

Traceback (most recent call last):
  File "/private/tmp/main.py", line 13, in <module>
    print(me)
  File "/private/tmp/pydantic/pydantic/utils.py", line 361, in __str__
    return self.__repr_str__(' ')
  File "/private/tmp/pydantic/pydantic/utils.py", line 343, in __repr_str__
    return join_str.join(repr(v) if a is None else f'{a}={v!r}' for a, v in self.__repr_args__())
  File "/private/tmp/pydantic/pydantic/main.py", line 856, in __repr_args__
    return [(k, v) for k, v in self.__dict__.items() if self.__fields__[k].field_info.repr]
  File "/private/tmp/pydantic/pydantic/main.py", line 856, in <listcomp>
    return [(k, v) for k, v in self.__dict__.items() if self.__fields__[k].field_info.repr]
KeyError: 'age'

Pydantic 1.8.2 works fine and this is a regression after 362f4a5 .

This commit makes sure we only try to access defined fields.
As a side effect this means extra fields do not get into model representation.

I'm not sure if that's the expected behavior though.

Related issue number

#3234

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable - no public interface changes
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review.

[povilasb]

please review

[christianbundy]

This looks good to me, and resolves an error that I was able to reproduce locally.

[samuelcolvin]

This is a duplicate of #3241