load accepts Sequence rather than Iterable (rejects generators)

[sloria]

addresses #1898

from marshmallow import Schema, fields, validates_schema


class MySchema(Schema):
    fld = fields.String()

    @validates_schema
    def _validate(self, data, **kwargs):
        print("schema validator called")


MySchema().load(({"fld": "abc"}, {"fld": "def"}), many=True)
# schema validator called
# schema validator called


def gen():
    yield from ({"fld": "abc"}, {"fld": "def"})


MySchema().load(gen(), many=True)
# Before this change, silently fails to call schema validator
# After this change, an error is raised:
# marshmallow.exceptions.ValidationError: {"_schema": ["Invalid input type."]}

Caveat: for some reason, mypy doesn't error when passing a generator. it seems that generators are covariant with Sequence and are therefore equivalent for type-checking purposes. not sure if there is a better way to handle this.

def gen():
    yield from ({"fld": "abc"}, {"fld": "def"})


assert isinstance(gen(), Sequence) is False
MySchema().load(gen(), many=True)  # mypy accepts this

[sloria]

ah, looks like this breaks handling of sets. will investigate later

update: fixed

[sloria]

this is ready for review. i thought this was a breaking change since it changes what we're accepting as valid input. but one could argue this is bugfix, since non-sequence inputs are silently failing to call schema validators. should this be backported to v3?