[BUG] [Python] OpenAPI code generator ignores pattern constraint on array items

[claudiadpp]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Python OpenAPI code generator does not generate field_validator methods for pattern constraints defined at the items level of array properties. It only generates pattern validators for top-level string properties.

When an OpenAPI spec defined an array property like this:

tags:
  type: array
  maxItems: 50
  items:
    type: string
    maxLength: 128
    pattern: "^[A-Z0-9_\\- ]+$"

The generator correctly applies maxLength on each item via Annotated[str, Field(strict=True, max_length=128)] and maxItems on the array via Field(max_length=50), but silently ignores the pattern constraint on the items.

Generated output (current, incorrect)

tags: Optional[Annotated[List[Annotated[str, Field(strict=True, max_length=128)]], Field(max_length=50)]] = Field(default=None)

No field_validator is generated for the pattern on the items. There is no validation whatsoever that each string in the array matches ^[A-Z0-9_\\- ]+$.

Expected output (correct)

The generator should produce a field_validator (or equivalent Annotated constraint) that validates each item in the array against the pattern regex. For example:

tags: Optional[Annotated[List[Annotated[str, Field(strict=True, max_length=128)]], Field(max_length=50)]] = Field(default=None)

@field_validator('tags')
@classmethod
def tags_validate_items_pattern(cls, value):
    """Validates the regular expression on each item"""
    if value is None:
        return value
    pattern = r"^[A-Z0-9_\- ]+$"
    for i, item in enumerate(value):
        if not re.match(pattern, item):
            raise ValueError(
                f"Item at index {i} ('{item}') must validate the regular expression {pattern}"
            )
    return value

Contrast with top-level pattern (works correctly)

For a non-array string property with pattern, the generator does produce a validator. For example, name with pattern: "^.*$" correctly generates:

@field_validator('name')
def name_validate_regular_expression(cls, value):
    """Validates the regular expression"""
    if not re.match(r"^.*$", value):
        raise ValueError(r"must validate the regular expression /^.*$/")
    return value

This confirms the generator has the capability to produce pattern validators, but the logic is only triggered for top-level string properties, not for items within arrays.

Defining it with $ref

I also tried modifying API and include this as a $ref

Tags:
  type: string
  maxLength: 128
  pattern: "^[A-Z0-9_\\- ]+$"

....

tags:
  type: array
  maxItems: 50
  items:
    $ref: "#/components/schemas/Tags"

but this still does not generate a field_validator for the regex pattern.

openapi-generator version

openapi-generator version 7.19.0
generator: python-fastapi (pydantic v2)