from email.message import EmailMessage

inner = EmailMessage()
inner.add_attachment("some data", "text/plain", filename="*"*35)

outer1 = EmailMessage()
outer1.set_type("multipart/signed")  # affected by generator.py/_handle_multipart_signed
outer1.attach(inner)

outer2 = EmailMessage()
outer2.set_type("multipart/signeX")  # not affected by generator.py/_handle_multipart_signed
outer2.attach(inner)

# When accessing given submessage, nothing weird happens
outer1.get_payload()[0].as_string() == outer2.get_payload()[0].as_string()  # True

# However, when accessing whole message at once, headers folding change for the outer1 `multipart/signed` message
inner.as_string() in outer1.as_string()  # !False!
inner.as_string() in outer2.as_string()  # True

    def _handle_multipart_signed(self, msg):
        # The contents of signed parts has to stay unmodified in order to keep
        # the signature intact per RFC1847 2.1, so we disable header wrapping.
        # RDM: This isn't enough to completely preserve the part, but it helps.
        p = self.policy
        self.policy = p.clone(max_line_length=0)
        try:
            self._handle_multipart(msg)
        finally:
            self.policy = p

from email import policy
pol = policy.default.clone(max_line_length=0)
inner = EmailMessage(policy=pol)
inner.add_attachment("some data", "text/plain", filename="*"*35)  # ValueError: maxlinelen must be at least 4