Re: Advice on the style to use in imports

[Marco Bizzarri]

On Sat, Aug 30, 2008 at 2:20 PM, Fredrik Lundh <fredrik@python ware.comwrote:

>
importing objects instead of the module (namespace) they live in can cause
all sorts of aliasing and dependency issues. avoid unless you know exactly
what you're doing.
>
</F>
>

Thanks Fredrik; I understand that is the underlying message of your article.

I'm just confused because PEP8 seems to suggest that the from module
import Class style is acceptable; is there a big "if you know what are
doing" before, which I'm unable to see?

Regards
Marco


--
Marco Bizzarri

http://iliveinpisa.blogspot.com/

http://notenotturne.blogspot.com/

[bearophileHUGS@lycos.com]

Re: Advice on the style to use in imports

Marco Bizzarri:

I'm just confused because PEP8 seems to suggest that the from module
import Class style is acceptable; is there a big "if you know what are
doing" before, which I'm unable to see?

from somemodule import somename

is often acceptable IHMO, but there are some things to consider:
- you and the person that reads your code have to remember where
somename comes from. So you can do it for well known names like izip
or imap, but if you import lots of names from lots of modules, things
may become too much complex. So often it can be better to import just
the modules, and use somemodule.some name.
- somemodule.some name is longer to write and to read, and if it's
repeated many times it may worsen the program readability, making
lines of code and expressions too much long and heavy. So you have to
use your brain (this means that you may have to avoid standard
solutions). Note that you can use a compromise, shortening the module
name like this:
import somemodule as sm
Then you can use:
sm.somename
- somemodule.some name requires an extra lookup, so in long tight loops
(if you don't use Psyco) it slows down the code. This can be solved
locally, assigning a local name into a function/method (or even in
their argument list, but that's a hack to be used only once in a
while):
localname = somemodule.some name

Bye,
bearophile

[Marco Bizzarri]

Re: Advice on the style to use in imports

Hi bearophile

On Sat, Aug 30, 2008 at 4:04 PM, <bearophileHUGS @lycos.comwrote :

>
from somemodule import somename
>
is often acceptable IHMO, but there are some things to consider:
- you and the person that reads your code have to remember where
somename comes from. So you can do it for well known names like izip
or imap, but if you import lots of names from lots of modules, things
may become too much complex. So often it can be better to import just
the modules, and use somemodule.some name.

Yes, that's true; but when I see that I need so many symbols from
another module, I take that as an hint that either my module is doing
too many things, and it should be splitted, or it is tightly coupled
to that module... actually, being forced to write all the imports in
that way was a tool into inspecting the dependencies in our project.

- somemodule.some name is longer to write and to read, and if it's
repeated many times it may worsen the program readability, making
lines of code and expressions too much long and heavy. So you have to
use your brain (this means that you may have to avoid standard
solutions). Note that you can use a compromise, shortening the module
name like this:
import somemodule as sm
Then you can use:
sm.somename

it is not a problem to have special cases, as long as they are
"special"; I'm looking for more or less accepted solutions; of course
any project has some area where it is better to follow readability
over standards; I'm just trying to understand the standards, then I
will deviate from them.

I feel like I'm learning to drive: first I learn the rules, then I
learn the exceptions ;)

- somemodule.some name requires an extra lookup, so in long tight loops

The slowdown was what in the first place made me import all the names
directly; but I'm not afraid too much from that, right now.

(if you don't use Psyco) it slows down the code. This can be solved
locally, assigning a local name into a function/method (or even in
their argument list, but that's a hack to be used only once in a
while):
localname = somemodule.some name
>
Bye,
bearophile
--

http://mail.python.org/mailman/listinfo/python-list

>

Thanks again for sharing your thoughts with me, bearophile.


--
Marco Bizzarri

http://iliveinpisa.blogspot.com/

http://notenotturne.blogspot.com/