Message.is_attachment() throws the exception below:

 File "/usr/lib/python3.5/email/message.py", line 956, in is_attachment
    return False if c_d is None else c_d.content_disposition == 'attachment'
AttributeError: 'str' object has no attribute 'content_disposition'

It seems to work fine for me with:

return c_d == 'attachment'

Can you provide a minimum reproducer?  It looks like you are somehow managing to use EmailMessage with the compat34 policy, which shouldn't happen unless you work at it, or I overlooked something.

Perhaps EmailMessage should disallow setting policy to compat32.

I mean compat32.

Ok I think the bug is not where I thought. First, my workaround does not work. Second, I use the following to generate my Message:

message_from_string(s, _class=EmailMessage)

Maybe the bug is in the parser?

Yes, I will make a small reproducer.

Ah, yes, that would do it.  EmailMessage should definitely throw an error if it gets compat32 as a policy, that would have told you what you were doing "wrong".

What you want to be doing is:

  msg = message_from_string(mystring, policy=default)

That will get you an EmailMessage object.  Note, however, that you almost never want message_from_string, because real email messages are bytes objects, not strings.  string only works if your message is ASCII only. (There's an open issue about documenting this properly...and a big doc revision in the offing.)