[Python-ideas] mimetypes.guess_type should not return deprecated mimetype application/x-javascript

[mil...@gmail.com]

mimetypes are parsed from the file /etc/mime.types

cat /etc/mime.types | grep javascript
application/javascript js
application/x-javascript js

actual:
mimetypes.guess_type("x.js") == "application/x-javascript"
-> deprecated mimetype

expected:
mimetypes.guess_type("x.js") == "application/javascript"

spoiler: here, the "x-" part is deprecated.

mimetypes.guess_type returns the deprecated mimetype
because python returns the last entry in the /etc/mime.types file
which is sorted alphabetically

proposed solution:
use a smarter conflict-resolution when parsing /etc/mime.types.
when two entries are found for one file-extension,
avoid using a deprecated mimetype.
rfc4329 lists 16 items for "unregistered media type"
these could be hard-coded as set-of-strings, or as regex.

related bug report
https://bugs.python.org/issue46035

mimetypes.guess_type
https://docs.python.org/3/library/mimetypes.html#mimetypes.guess_type

unregistered media type
https://datatracker.ietf.org/doc/html/rfc4329#section-3
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[Paul Bryan]

+1

"x-" prefix indicates ad hoc (unofficial), not deprecated.

I agree, an official MIME type should be preferred over an unofficial one. 

[Cameron Simpson]

On 18Jan2022 08:43, Paul Bryan <pbr...@anode.ca> wrote:
>+1
>
>"x-" prefix indicates ad hoc (unofficial), not deprecated.
>
>I agree, an official MIME type should be preferred over an unofficial
>one. 

Maybe, but I disagree about the proposed solution. There are
circumstances where you want a specific MIME type guess, and the best
way to do that is the apply a specific ordering to your mime.types file
so that the desired type comes first, and is found first.

And looking at milahu's example, that's exactly what was there:

application/javascript        js
application/x-javascript      js

It would be reasonable and generally desireable to take the first one.

The problem with saying "oh, let's examine the types and exclude or
deprioritise the ones we don't like such as x-*" as that removes control
from the user (the caller of mimetypes.guess_type() and the author of
/etc/mime.types). It embeds fixed policy _inside_ mimetypes.guess_type()
where it can't be turned off without growing a heap of weird mode flags.

The better fix it to honour the order of the mime.types file as
_expressing_ policy, which is what the OP already has - they just don't
have guess_type() doing it that way.

Arguments for embedding _policy_ inside guess_type() will be met with my
standard example: the ancient Netscape proxy config had regexp based
redirect rules, not uncommon. But Netscape prioritised these by the
length of the regexp instead of the config file ordering. Insanity
abounded with regexp wackiness purely to make some rules longer than
others.

Cheers,
Cameron Simpson <c...@cskk.id.au>

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[Jelle Zijlstra]

Perhaps there should be a guess_all_types() function in addition to guess_type() that returns all possible types, so that the user can select the type they want using any criterion. This would be nicely symmetrical with the existing guess_extension and guess_all_extensions functions.

[Cameron Simpson]

Indeed. +1

That would allow applying policy beyond the mime.types file ordering.

Cheers,
Cameron Simpson <c...@cskk.id.au>
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