0UT1S's

_TYPE_FIELDS_SEQUENCE = typing.Sequence[
    typing.Union[tuple[str, _TYPE_FIELD_VALUE_TUPLE], RequestField]
]
_TYPE_FIELDS = typing.Union[
    _TYPE_FIELDS_SEQUENCE,
    typing.Mapping[str, _TYPE_FIELD_VALUE_TUPLE],
]

def choose_boundary() -> str:
    """
    Our embarrassingly-simple replacement for mimetools.choose_boundary.
    """
    return binascii.hexlify(os.urandom(16)).decode()

def iter_field_objects(fields: _TYPE_FIELDS) -> typing.Iterable[RequestField]:
    """
    Iterate over fields.

Supports list of (k, v) tuples and dicts, and lists of
    :class:`~urllib3.fields.RequestField`.

"""
    iterable: typing.Iterable[RequestField | tuple[str, _TYPE_FIELD_VALUE_TUPLE]]

if isinstance(fields, typing.Mapping):
        iterable = fields.items()
    else:
        iterable = fields

for field in iterable:
        if isinstance(field, RequestField):
            yield field
        else:
            yield RequestField.from_tuples(*field)

def encode_multipart_formdata(
    fields: _TYPE_FIELDS, boundary: str | None = None
) -> tuple[bytes, str]:
    """
    Encode a dictionary of ``fields`` using the multipart/form-data MIME format.

:param fields:
        Dictionary of fields or list of (key, :class:`~urllib3.fields.RequestField`).
        Values are processed by :func:`urllib3.fields.RequestField.from_tuples`.

:param boundary:
        If not specified, then a random boundary will be generated using
        :func:`urllib3.filepost.choose_boundary`.
    """
    body = BytesIO()
    if boundary is None:
        boundary = choose_boundary()

for field in iter_field_objects(fields):
        body.write(f"--{boundary}\r\n".encode("latin-1"))

writer(body).write(field.render_headers())
        data = field.data

if isinstance(data, int):
            data = str(data)  # Backwards compatibility

if isinstance(data, str):
            writer(body).write(data)
        else:
            body.write(data)

body.write(b"\r\n")

body.write(f"--{boundary}--\r\n".encode("latin-1"))

content_type = f"multipart/form-data; boundary={boundary}"

return body.getvalue(), content_type