Why import only at module level?

[François Pinard]

[Peter Hansen]
> Paul Rubin wrote:

> > That's what the Python style guides advise.  They don't seem to like

> >    def frob(x):
> >       import re
> >       if re.search('sdf[234]xyz', x): ...

> > instead preferring that you pollute your module's global namespace
> > with the names of all your imports.  What's the point of that?

> Maintainability.  It's also well understood that there are potential
> benefits to the approach you show above, late-loading of code being
> one of them (which can improve startup time for certain apps), but
> as maintainability should almost always be the primary consideration,
> the standard advice is to put stuff together at the top where it's
> clear to maintainers which modules are used in the code (in other
> words, what a given module is coupled to).

I never understood this "standard advice", nor how it is related to
maintainability.  What makes a module significantly more maintainable
by merely grouping `import' statements at the beginning?  What is it so
crucial to know that the `re' module is used, or not, in a program?  It
looks like a tiny detail to me.

If for some strange reason I urgently needed to know everything that a
program imports, I guess I would then `grep' the source for the word
`import'i, or just search with an editor! :-) This is surely not a need
I often have, and for from enough for justifying the convention.

If within a `def', I will often use a module which I do not use
elsewhere in a program, there is no reason to make it global.  Global
variables should be avoided on the average, and moreover, Python is
faster at accessing a local than a global.

I'm not really crusading for either method, but maybe a bit against
the mere existence of the "standard advice", unless it acquires some
better justification.  Of course, "maintainability" is a virtue, but the
relation between maintainability and the "standard advice" is asserted
as if it were to be evident, without explanation.

-- 
François Pinard   http://www.iro.umontreal.ca/~pinard