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Title: email: msg.items() returns different values before and after msg.as_string()
Type: behavior Stage: resolved
Components: Documentation Versions: Python 3.1, Python 3.2, Python 2.7, Python 2.6
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: r.david.murray Nosy List: barry, georg.brandl, jnelson, r.david.murray
Priority: normal Keywords: patch

Created on 2009-10-13 18:58 by jnelson, last changed 2022-04-11 14:56 by admin. This issue is now closed.

File name Uploaded Description Edit
email_generate_mutates_message_docfix.patch r.david.murray, 2009-12-14 18:12
Messages (6)
msg93935 - (view) Author: Jon Nelson (jnelson) Date: 2009-10-13 18:58
import os
  from email.mime.multipart import MIMEMultipart
except ImportError:
  from email.MIMEMultipart import MIMEMultipart

m = MIMEMultipart('form-data')

print m.items()
print m.items()

print out:

[('Content-Type', 'multipart/form-data'), ('MIME-Version', '1.0')]
[('Content-Type', 'multipart/form-data;
boundary="===============0836597002796039051=="'), ('MIME-Version', '1.0')]

The latter is correct, the former is not - it is missing the boundary!
items() should behave the same regardless of whether or not as_string()
has been called.

Confirmed in 2.4 and 2.6
msg94264 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2009-10-20 02:44
The problem here is that when you are constructing a message the
boundary is calculated "as needed", and returning the list of items is
not one of the times when the boundary is needed.  I agree that it is
not good design that as_string can mutate the object it is called on;
however the alternative (throwing away the calculated boundary after
message Generation) means that stringifying the same message twice would
produce two strings that are not equal...which also seems like bad
design.  But perhaps that would be less bad design.

The underlying problem here is that the things being returned by items
are strings, when they should be Header objects.  That problem isn't
going to be resolved until the email package rewrite happens.
msg94304 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2009-10-20 22:46
Thinking about this further, it seems to me that it is in fact more
important that turning a message into a string produces the same result
no matter how many times it is called on the same (otherwise unmutated)
object than that it avoid mutating the object tree itself.  (This is
called idempotence and it is an important interface concept in certain
circumstances, and I think this is one of them).

The documentation should make the expectations clear, however.
msg96387 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2009-12-14 18:12
Here's a doc patch.
msg96631 - (view) Author: Georg Brandl (georg.brandl) * (Python committer) Date: 2009-12-19 18:08
Patch looks good. (I'm a bit unsure about the syntax "ie: blah" and "eg:
blah" though.)
msg97528 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2010-01-10 17:43
Applied in r77412, with ie and eg changed to 'for example'.
Date User Action Args
2022-04-11 14:56:53adminsetgithub: 51368
2010-01-10 17:43:50r.david.murraysetstatus: open -> closed
resolution: fixed
messages: + msg97528

stage: patch review -> resolved
2009-12-19 18:08:08georg.brandlsetmessages: + msg96631
2009-12-14 18:12:33r.david.murraysetfiles: + email_generate_mutates_message_docfix.patch
keywords: + patch
messages: + msg96387

stage: needs patch -> patch review
2009-10-21 01:04:56r.david.murraysetassignee: georg.brandl -> r.david.murray
2009-10-20 22:46:55r.david.murraysetnosy: + georg.brandl
messages: + msg94304

assignee: georg.brandl
components: + Documentation, - Library (Lib)
stage: test needed -> needs patch
2009-10-20 02:44:46r.david.murraysetpriority: normal
versions: + Python 3.1, Python 2.7, Python 3.2, - Python 2.5, Python 2.4
nosy: + barry, r.david.murray

messages: + msg94264

stage: test needed
2009-10-13 18:58:05jnelsoncreate