[Python-Dev] Why does PEP 8 advise against explicit relative imports?

[Thomas Wouters]

On Tue, Jul 16, 2013 at 1:40 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> PEP 8 advises developers to use absolute imports rather than explicit
> relative imports.
>
> Why? Using absolute imports couple the internal implementation of a
> package to its public name - you can't just change the top level
> directory name any more, you have to go through and change all the
> absolute imports as well. You also can't easily vendor a package that
> uses absolute imports inside another project either, since all the
> absolute imports will break.
>

The problem with relative imports (both explicit and implicit) is that it
makes it less obvious when you are importing a module under the wrong name.
If a package ends up in sys.path directly (for example, by executing
something that lives inside it directly) then an explicit relative import
directly in the package will fail, but an explicit relative import in a
sub-package won't, and you can end up with the subtly confusing mess of
multiple imports of the same .py file under different names.


> What's the *concrete* benefit of using absolute imports that overcomes
> these significant shortcomings in maintainability and composability?
>

In my experience changing the name of a package is problematic for more
reasons than just 'import'. We used to do the put-in-a-package thing for
third-party packages at Google, and we quit doing it because it was just
too painful: it's hard to find all the places that reference the package by
name (think of __import__, or mucking in sys.modules), and it's extra
problematic with packages that contain extension modules. You can't rename
a package, even for 'vendoring a package', without carefully checking
whether it'll work -- unless you don't care if it'll work, of course :) The
alternative is to include a package *without* changing its name (by adding
the right directory, without __init__.py, to sys.path) and that has worked
out a lot better for us.

Even so, there's no reason for *the standard library* to use explicit
relative imports, and that's what PEP 8 is supposed to cover, right? :)

-- 
Thomas Wouters <thomas at python.org>

Hi! I'm an email virus! Think twice before sending your email to help me
spread!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20130716/aaad9e4a/attachment.html>