I have MIME message and parse content. Later I walk on message headers by call for and receive follow traceback:

Traceback (most recent call last):
...
    for m in msg:  (msg is email.message.Message instance)
  File "C:\Python27\lib\email\message.py", line 294, in __getitem__
    return self.get(name)
  File "C:\Python27\lib\email\message.py", line 360, in get
    name = name.lower()
AttributeError: 'int' object has no attribute 'lower'

But with items() all work properly.

It looks like your 'name' variable holds an integer, not a string giving a header name.

Oh, I think I misread your traceback.

Can you provide the actual code you are running, please?  And probably the message you are parsing, as well.

You just need a messsage object with one or more header fields:

>>> msg = message_from_string("Name: value\r\n\r\n")
>>> for m in msg:
...     pass
... 
AttributeError: 'int' object has no attribute 'lower'

Python 2 does not implement __iter__(), so it is the default sequence iteration protocol causing this behaviour. The documentation does not mention iteration either: <https://docs.python.org/2.7/library/email.message.html#email.message.Message.__len__>. It mentions a “mapping-like interface”, but since it explicitly lists __len__(), __contains__(), etc, and not __iter__(), I would say you should not assume iteration is supported.

Python 3 does implement Message.__iter__(), but it does not seem to be documented. The method seems to be added as a side effect of Guido removing and restoring the email package:

https://github.com/python/cpython/commit/1058618#diff-92a78c52ebc0a8908cbd06c8155f8bdb (removed without __iter__)
https://hg.python.org/cpython/file/d5f3c2f416f2/Lib/email/message.py (added back including __iter__)

The __iter__() method was added by Barry in <http://svn.python.org/view/sandbox/trunk/emailpkg/5_0-exp/email/message.py?r1=57344&r2=57343&pathrev=57344>:
“Added an __iter__() to email.message.Message for iterating over the message’s headers.”

On the other hand, earlier he said it was ambiguous what the behaviour should be: Issue 1017329.

I had no memory that __iter__ wasn't implemented in python2, thanks Martin.  So, this isn't really a bug, though one could argue it should be mentioned explicitly in the 2.7 docs, since it is definitely counter-intuitive to a python programmer.

Python 2-only issue.