Skip to content

🔒 Field Constraints

The --field-constraints option converts all con* annotations (like conint, constr) to Field constraint options.

🤔 Why use this?

Mypy may show errors for con* annotations on fields. The --field-constraints option resolves this problem by using standard Field() constraints instead.

📝 Example

Convert simple JSON Schema model.json to pydantic model model.py:

model.json 

{
  "type":  "object",
  "properties": {
    "name": {
      "type": "string",
      "maxLength": 64
    }
  },
  "required": ["name"]
}

❌ Without --field-constraints option

datamodel-codegen --input model.json --input-file-type jsonschema --output-model-type pydantic_v2.BaseModel > model.py

Generated model.py 

# generated by datamodel-codegen:
#   filename:  model.json
#   timestamp: 2020-07-20T15:37:56+00:00

from __future__ import annotations

from pydantic import BaseModel, constr


class  Model(BaseModel):
  name: constr(max_length=64)

🔴 Run mypy... 

$ mypy model.py
model.py:3: error: Invalid type comment or annotation
model.py:3: note: Suggestion: use constr[...] instead of constr(...)
Found 1 error in 1 file (checked 1 source file)

mypy shows errors! 😱

✅ With --field-constraints option

datamodel-codegen \
  --input model.json \
  --input-file-type jsonschema \
  --output-model-type pydantic_v2.BaseModel \
  --use-annotated \
  --field-constraints \
  > model.py

Generated model.py 

# generated by datamodel-codegen:
#   filename:  model.json
#   timestamp: 2020-07-20T15:47:21+00:00

from __future__ import annotations

from typing import Annotated

from pydantic import BaseModel, Field


class Model(BaseModel):
    name: Annotated[str, Field(max_length=64)]

🟢 Run mypy... 

$ mypy model.py
Success: no issues found in 1 source file

No errors! 🎉

📖 See Also