Pydantic nestled list type with sub-list minimum length

I want to create a Pydantic class wrapping a list with string sub-lists that have to be at least of length two.

For example, the following are valid:

• [] empty list with no sublists is valid
• [["a", "b"], ["c", "d", "e"] is valid because each sub-list is of at least length 2

The following are not valid:

• [[]] an empty sub-list is not valid
• [["a"], ["b", "c"] one of the sub-lists has a length less than two which is invalid

How do I do this in Pydantic? I don't think I can nest conlist? Can I use typing.Annotated to accomplish this?

You can use the annotated_types package to accomplish this, as discussed in the Pydantic Documentation.

from typing import Annotated

from annotated_types import MinLen
import pydantic


class SubListModel(pydantic.BaseModel):
    my_list: list[Annotated[list[str], MinLen(2)]]


# Valid cases
valid_empty = SubListModel(my_list=[])  # Empty list
valid_lists = SubListModel(my_list=[["a", "b"], ["c", "d", "e"]])

# Invalid cases - these will raise ValidationError
try:
    invalid_empty_sublist = SubListModel(my_list=[[]])  # Will fail
except pydantic.ValidationError as e:
    print("Empty sublist error:", e)

try:
    invalid_short_sublist = SubListModel(my_list=[["a"], ["b", "c"]])  # Will fail
except pydantic.ValidationError as e:
    print("Short sublist error:", e)