Better error message while getting an invalid dictionary

[ZhenyiQ]

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Environment details

• Programming language: Python
• OS: Linux
• Language runtime version: Python 3.11
• Package version: 1.23.0

Steps to reproduce

1. minimal python example

from collections.abc import Sequence

from absl import app
import proto

class Composer(proto.Message):
given_name: str = proto.Field(proto.STRING, number=1)
family_name: str = proto.Field(proto.STRING, number=2)

class Song(proto.Message):
composer: Composer = proto.Field(Composer, number=1)


def main(argv: Sequence[str]) -> None:
if len(argv) > 1:
  raise app.UsageError("Too many command-line arguments.")
song = Song({"composer": "john scott"})
print(song)


if __name__ == "__main__":
app.run(main)

2. run the script with python file_name.py, one gets the following error message

Traceback (most recent call last):
  File "/usr/local/google/home/zhenyiqi/vertexai/python/proto-plus-issues/proto-plus-example.py", line 27, in <module>
    app.run(main)
  File "/usr/local/google/home/zhenyiqi/venv_vertexai/lib/python3.11/site-packages/absl/app.py", line 308, in run
    _run_main(main, args)
  File "/usr/local/google/home/zhenyiqi/venv_vertexai/lib/python3.11/site-packages/absl/app.py", line 254, in _run_main
    sys.exit(main(argv))
             ^^^^^^^^^^
  File "/usr/local/google/home/zhenyiqi/vertexai/python/proto-plus-issues/proto-plus-example.py", line 22, in main
    song = Song({"composer": "john scott"})
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/google/home/zhenyiqi/venv_vertexai/lib/python3.11/site-packages/proto/message.py", line 615, in __init__
    super().__setattr__("_pb", self._meta.pb(**params))
                               ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Message must be initialized with a dict: _default_package.Song

It is true that it's the code itself that's problematic (users should use {"given_name": "john", "family_name": "scott"}) but from the error message, it's really hard to realize what was wrong especially when a proto object has many nested proto objects. It would be nice to be clear about 1) what is the field that's causing this issue (in this case, it's the Composer that's problematic, not the Song) 2) what the user is using to initialize that field ("john scott" is the user input, but we expect a dict here).

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!