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fastapipydantic

Is it possible to impose the length for a list attribute of the request body with fastapi?

Is it possible to specify the length of a list in the schema of the request body (or response)? There is support for validating the length of a string passed in the url with the Query function but I see nothing for lists.. Possible use case would be sending list of floats of fixed size to feed to a ML model.

Solution

  • You can either use the Field function with min_length and max_length:

    from pydantic import Field

class Foo(BaseModel):
    fixed_size_list_parameter: float = Field(..., min_length=4, max_length=4)

    .. or you can use the conlist (constrained list) type from pydantic:

    from pydantic import conlist

class Foo(BaseModel):
    # these were named min_length and max_length in Pydantic v1.10
    fixed_size_list_parameter: conlist(float, min_length=4, max_length=4)

    This limits the list to exactly four entries of the float type.