[BUG] TypeError: Object of type '_GenericAlias' is not JSON serializable

Describe the bug

I am unable to debug the issue with some of my model schemas (the traceback for this issue is not helpful). For context, I have a Base model that contains the relevant json_encoders for the non standard fields (datetime, ObjectId, etc), yet there is some pydantic field that isn’t being translated.

The schemas uses the following EXTRA fields (from pydantic):

AnyUrl
Dict
SecretStr
Field
Union

Environment

OS: Linux
FastAPI Version 0.5.2
Python version: 3.7.4

Traceback

...
File "/usr/local/lib/python3.7/site-packages/pydantic/schema.py", line 503, in model_type_schema
f, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix, known_models=known_models
File "/usr/local/lib/python3.7/site-packages/pydantic/schema.py", line 186, in field_schema
s['default'] = encode_default(field.default)
File "/usr/local/lib/python3.7/site-packages/pydantic/schema.py", line 725, in encode_default
return pydantic_encoder(dft)
File "/usr/local/lib/python3.7/site-packages/pydantic/json.py", line 59, in pydantic_encoder
raise TypeError(f"Object of type '{obj.__class__.__name__}' is not JSON serializable")
TypeError: Object of type '_GenericAlias' is not JSON serializable

Issue Analytics

State:
Created 3 years ago
Comments:10 (4 by maintainers)

Top GitHub Comments

12reactions

dbantycommented, Jun 4, 2020

In looking at the error again, I did a search for _GenericAlias and it looks like that used in the internals of the typing module. So my guess is that somewhere you’re assigning a type to a field in Pydantic instead of using a type annotation (e.g. field = List instead of field: List).

That doesn’t help you narrow down where the error is just what it might be.

When does this exception happen? When you start up the app or when you call a certain endpoint?

If on startup you could try commenting out some end points (or even better, whole routers if you use them) to narrow down the issue.

You could also navigate to the Pyndantic source in your environment and put in a breakpoint (if you’re using an IDE) or print statement.

File “/usr/local/lib/python3.7/site-packages/pydantic/schema.py”, line 186 maybe?

5reactions

dbantycommented, Aug 18, 2020

The issue is using a generic from the typing module as a value instead of just a type hint. Specifically invitees_in: List[InviteePatch] = List[InviteePatch] is saying “assign this generic type to my variable if a value is not provided”.

There are multiple ways the fix it but I assume you want an empty list as a default so you would do something like invitees_in: List[InviteePatch] = Body([]).

If you don’t want the default and can make it a required param then just do invitees_in: List[InviteePatch]

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