Velocity Reviews

Velocity Reviews (
-   Python (
-   -   mimetypes.guess_type broken in windows on py2.7 and python 3.X (

Gelonida N 09-26-2012 08:54 AM

mimetypes.guess_type broken in windows on py2.7 and python 3.X

I'm still experiencing the pleasure of migrating legacy code from Python
2.6. to 2.7 which I expected to be much less painful.
(In fact migration on Linux is rather smooth, but Windows is another story)

Let's take the simple command

import mimetypes
print mimetypes.guess_type('a.jpg')

The result an old pythons ( 2.7)
is ('image/jpeg', None)

Ther result on non windows platform is
for python 2.7 / 3.X is the same

However. The result for 2.7 / 3.x on windows is now
('image/pjpeg', None) # pjpeg instead of jpeg

On Windows many file suffixes will report wrong mime types.

The problem is know for about two years.

The main reason is, that under wWindows default values are
fetched from Python and then 'updated' mime-types are
fetched from the Windows registry.
The major problem is, that almost ALL windows PCs have BROKEN mime
types. so the good predefined mime types are just replaced with broken
MS mime types.

I wonder how many applications, that will try to migrate to 2.7 / 3.0
will fail due to this incompatibility in the mimetypes library

There is a workaround (but first people have to detect the problem and
to find it):
Add these two lines somewhere in your code BEFORE any other imported
library might have called a mimetypes function

import mimetypes

I still wonder if it wouldn't be better to have the default behaviour of
2.7 / 3.0 on windows such, that all the users who're not aware of this
issue will not have their code broken.

My suggestion for windows would be to have following default behaviour:

- !st read the mimetypes from the registry if possible
- 2nd read the Python default mimetypes and override the
'broken' MS definitions

Only if a user explicitely calls mimetypes.init() they would have
differente behaviour.

The new behaviour breaks portability of Python code between Windows and
Linux and I think the attempt should be to be as cross platform as
possible. and not to be. At least one of the reasons why I use pythin
is, that it allows to be rather cross-platform

An alternative suggestion could be to never read the registry or
/etc/mimetypes by default.

What would definitely be rather important is add a big warning in the
documentation and a recommendation of how to write cross platform
compatible code.

Somebody developing on Linux might not even know, that the code will not
work on windows jsst because of this tiny issue.

The unfortunate fact, that this issue was not fixed two years ago means,
that perhaps meanwhile some code is out, that relies on the current
behaviour. However I'm not sure, that anybody relies on the fact, that
code will not work the same way on windows and on Linux.

Any thoughts?

All times are GMT. The time now is 03:10 AM.

Powered by vBulletin®. Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO ©2010, Crawlability, Inc.