Having already use flat packages, I was not expecting the issue I encountered with nested packages. Here is…
Directory layout
dir
|
+-- test.py
|
+-- package
|
+-- __init__.py
|
+-- subpackage
|
+-- __init__.py
|
+-- module.py
Content of init.py
Both package/__init__.py
and package/subpackage/__init__.py
are empty.
Content of module.py
# file `package/subpackage/module.py`
attribute1 = "value 1"
attribute2 = "value 2"
attribute3 = "value 3"
# and as many more as you want...
Content of test.py
(3 versions)
Version 1
# file test.py
from package.subpackage.module import *
print attribute1 # OK
That's the bad and unsafe way of importing things (import all in a bulk), but it works.
Version 2
# file test.py
import package.subpackage.module
from package.subpackage import module # Alternative
from module import attribute1
A safer way to import, item by item, but it fails, Python don't want this: fails with the message: "No module named module". However …
# file test.py
import package.subpackage.module
from package.subpackage import module # Alternative
print module # Surprise here
… says <module 'package.subpackage.module' from '...'>
. So that's a module, but that's not a module /-P 😯 … uh
Version 3
# file test.py v3
from package.subpackage.module import attribute1
print attribute1 # OK
This one works. So you are either forced to use the overkill prefix all the time or use the unsafe way as in version #1 and disallowed by Python to use the safe handy way? The better way, which is safe and avoid unecessary long prefix is the only one which Python reject? Is this because it loves import *
or because it loves overlong prefixes (which does not help to enforce this practice)?.
Sorry for the hard words, but that's two days I trying to work around this stupid‑like behavior. Unless I was totally wrong somewhere, this will leave me with a feeling something is really broken in Python's model of package and sub‑packages.
Notes
- I don't want to rely on
sys.path
, to avoid global side effects, nor on*.pth
files, which are just another way to play withsys.path
with the same global effets. For the solution to be clean, it has to be local only. Either Python is able to handle subpackage, either it's not, but it should not require to play with global configuration to be able to handle local stuff. - I also tried use imports in
package/subpackage/__init__.py
, but it solved nothing, it do the same, and complainssubpackage
is not a known module, whileprint subpackage
says it's a module (weird behavior, again).
May be I'm entirely wrong tough (the option I would prefer), but this make me feel a lot disappointed about Python.
Any other known way beside of the three I tried? Something I don't know about?
(sigh)
—– %< —– edit —– >% —–
Conclusion so far (after people's comments)
There is nothing like real sub‑package in Python, as all package references goes to a global dictionnary, only, which means there's no local dictionary, which implies there's is no way to manage local package reference.
You have to either use full prefix or short prefix or alias. As in:
Full prefix version
from package.subpackage.module import attribute1
# An repeat it again an again
# But after that, you can simply:
use_of (attribute1)
Short prefix version (but repeated prefix)
from package.subpackage import module
# Short but then you have to do:
use_of (module.attribute1)
# and repeat the prefix at every use place
Or else, a variation of the above.
from package.subpackage import module as m
use_of (m.attribute1)
# `m` is a shorter prefix, but you could as well
# define a more meaningful name after the context
Factorized version
If you don't mind about importing multiple entity all at once in a batch, you can:
from package.subpackage.module import attribute1, attribute2
# and etc.
Not in my first favorite taste (I prefer to have one import statement per imported entity), but may be the one I will personally favor.
Update (2012-09-14):
Finally appears to be OK in practice, except with a comment about the layout. Instead of the above, I used:
from package.subpackage.module import (
attribute1,
attribute2,
attribute3,
...) # and etc.
Best Answer
You seem to be misunderstanding how
import
searches for modules. When you use an import statement it always searches the actual module path (and/orsys.modules
); it doesn't make use of module objects in the local namespace that exist because of previous imports. When you do:The second line looks for a package called
package.subpackage
and importsmodule
from that package. This line has no effect on the third line. The third line just looks for a module calledmodule
and doesn't find one. It doesn't "re-use" the object calledmodule
that you got from the line above.In other words
from someModule import ...
doesn't mean "from the module called someModule that I imported earlier..." it means "from the module named someModule that you find on sys.path...". There is no way to "incrementally" build up a module's path by importing the packages that lead to it. You always have to refer to the entire module name when importing.It's not clear what you're trying to achieve. If you only want to import the particular object attribute1, just do
from package.subpackage.module import attribute1
and be done with it. You need never worry about the longpackage.subpackage.module
once you've imported the name you want from it.If you do want to have access to the module to access other names later, then you can do
from package.subpackage import module
and, as you've seen you can then domodule.attribute1
and so on as much as you like.If you want both --- that is, if you want
attribute1
directly accessible and you wantmodule
accessible, just do both of the above:If you don't like typing
package.subpackage
even twice, you can just manually create a local reference to attribute1: