Develop Reference

Python Importing modules in a package

I currently have a module I created that has a number of functions.
It's getting quite large so I figured I should make it into a package and split the functions up to make it more manageable.
I'm just testing out how this all works before I do this for real so apologies if it seems a bit tenuous.
I've created a folder called pack_test and in it I have:
__init__.py
foo.py
bar.py
__init__.py contains:
__all__ = ['foo', 'bar']
from . import *
import subprocess
from os import environ
In the console I can write import pack_test as pt and this is fine, no errors.
pt. and two tabs shows me that I can see pt.bar, pt.environ, pt.foo and pt.subprocess in there.
All good so far.
If I want to reference subprocess or environ in foo.py or bar.py how do I do it in there?
If in bar.py I have a function which just does return subprocess.call('ls') it errors saying NameError: name 'subprocess' is not defined. There must be something I'm missing which enables me to reference subprocess from the level above? Presumably, once I can get the syntax from that I can also just call environ in a similar way?
The alternative as I could see it would be to have import subprocess in both foo.py and bar.py but then this seems a bit odd to me to have it appear across multiple files when I could have it the once at a higher level, particularly if I went on to have a large number of files rather than just 2 in this example.

TL;DR:
__init__.py :
import foo
import bar
__all__ = ["foo", "bar"]
foo.py:
import subprocess
from os import environ
# your code here
bar.py
import subprocess
from os import environ
# your code here
There must be something I'm missing which enables me to reference subprocess from the level above?
Nope, this is the expected behaviour.
import loads a module (if it isn't already), caches it in sys.modules (idem), and bind the imported names in the current namespace. Each Python module has (or "is") it's own namespace (there's no real "global" namespace). IOW, you have to import what you need in each module, ie if foo.py needs subprocess, it must explicitely import it.
This can seem a bit tedious at first but in the long run it really helps wrt/ maintainability - you just have to read the imports at the top of your module (pep 08: always put all imports at the beginning of the module) to know where a name comes from.
Also you should not use star imports (aka wild card imports aka from xxx import *) anywhere else than in your python shell (and even then...) - it's a maintainance time bomb. Not only because you don't know where each name comes from, but also because it's a sure way to rebind an already import name. Imagine that your foo module defines function "func". Somewhere you have "from foo import *; from bar import *", then later in the code a call to func. Now someone edits bar.py and adds a (distinct) "func" function, and suddenly you call fails, because you're not calling the expected "func". Now enjoy debugging this... And real-life examples are usually a bit more complex than this.
So if you fancy your mental sanity, don't be lazy, don't try to be smart either, just do the simple obvious thing: explicitely import the names you're interested in at the top of your modules.
(been here, done that etc)

You could create modules.py containing
import subprocess
import os
Then in foo.py or any of your files just have.
from modules import *
Your import statements in your files are then static and just update modules.py when you want to add an additional module accessible to them all.

Using modules imported from another import

I'm cleaning up a project that was refactored into smaller .py files. I noticed that a lot of modules are being imported again and again in various files. Some statements are in files that import another which has the same import statement used by the importing file. For example:
main.py
import alt
print (os.getcwd())
alt.py
import os
The print(os.getcwd()) throws a NameError: name 'os' is not defined. Shouldn't os be part of sys.modules when the import statement is executed in alt.py?
Is it possible to use a module imported by another module that was imported first?

They are available in the following namespace:
import alt
print (alt.os.getcwd())

To answer your immediate question, it is possible but not recommended.
Imports adjust the namespace in which they are made. This means that alt has an attribute os that can be accessed in main as
print(alt.os.getcwd())
This is not the recommended way, however, since it makes it less clear which actual os module you are using. You should do import os directly in main. This will not create a separate module object, so do not worry about cluttering your memory with duplicate modules.
The first time an import is encountered during runtime, it is added to the dictionary sys.modules, keyed by the fully qualified name of the module. Future import statements will look in sys.modules for an existing reference before doing any actual work.
In your case, import alt will create a module referenced by sys.modules['alt'] as well as by the name alt in main. The statement import os in alt will be run next. It will create a module referenced by sys.modules['os'] and alt.os. If you were to add a line import os in main after import alt, it will not create and load another module object. Instead, the name os will be bound to the same object pointed to by sys.modules['os'].
The following three versions of main will all call the same getcwd function:
Direct import (recommended):
import alt
import os
print(os.getcwd())
Use the reference in alt (harder to read/trace):
import alt
print(alt.os.getcwd())
Using sys.modules (really not recommended for production unless you know what you are doing):
import alt # necessary to trigger the actual import
import sys
print(sys.modules['os'].getcwd())
All imports load a full module. This even applies to imports of the form from os import getcwd. The module sys.modules['os'] is still created. The only difference is that the importing namespace will only have access to the name getcwd, not os. In your case, if alt contained from os import getcwd instead of import os, the three access methods would change as follows:
Unchanged.
print(alt.getcwd()) since alt.os no longer exists.
Unchanged.

You are importing os only in the in the submodul level of alt. os is to say so only available by access through alt.os. A way around this would be to import all from alt as from alt import *, but this is not what you should do...
As a general rule of thumb, you should re-import your directly called-to modules on the top level of the module that you currently process.
thus at main.py:
import os
and everything is fine

You write it this way
__author__ = 'kerberos'
__date__ = '2017/10/25 20:48 '
import alt
print(alt.os.getcwd())
This is the result:
C:\Users\Administrator\Desktop\11

Python imported module dependency

Let's say I have file main.py :
import math
import mymodule.py
print(math.ceil(5/3))
and then mymodule.py :
print(math.ceil(10/3))
mymodule.py gives an error that math is not defined, even though its parent module has it imported.
Considering both main.py and mymodule.py need to use the math lib, do I need to import it twice? It just seems non-optimal. What's the most pythonic way to solve this issue?
I know it's a dumb example, but I'm trying to fragment a code I made into several modules for organization, and this issue appeared multiple times in several levels

mymodule.py is parent for main.py since you are importing mymodule within main.
You need to import math within mymodule so that it gets inherited in main.
Then there won't be a need to import within main.
mymodule.py
import math
main.py
import mymodule
print mymodule.math.pow(10,2)
Result:
>>>
100.0
>>>

This is really very basic. If you have something in a separate file, like mymodule.py, then you can import that function in any python file easily in the same directory.
two files:
mymodule.py:
import math
def aFunc():
return math.ceil(10/3)
# We could also just use this file as a standalone
if __name__ == "__main__":
print(aFunc())
main.py:
import mymodule
print(mymodule.aFunc())
You could also specifically call out the function you want to import.
main.py (alternative):
from mymodule import aFunc
print(aFunc())

How to import appropriately

I have been wondering for a long time how to appropriately import in Python, in relation to the way I'm going to explain now:
Imagine I've got two modules built by myself, modulea and moduleb, and have a last one which uses both the modulea and b, so modulec.
Furthermore, modulea uses moduleb.
The modules are all one-file, which form part of a big module which has got its own __init__.py file.
+ My project
-
-> modulea.py
-> moduleb.py
-> modulec.py
-> __init__.py # Here I import modulea, b, c.
-> main_module.py # This is were I do all the hardcore.
I've created a file for importing all the modules I need, instead of importing them one by one in each script, and to know if I have already imported it, I would have to check file for file if I have already imported that or not.
I could simply declare, as you have been saying below in the comment box, the required modules in each module, but I think that's not practical and it's easier to import them all only in one file.
Tell me if this is a good practice or not.
If I import
import modulea
import moduleb
import modulec
It won't work (ImportError) because modulea does not implicitly import moduleb inside the code, and it is imported before than the module it requires (b)
If I did
import moduleb
import modulea
import modulec
I guess they will work because since the runnning script already imports moduleb before a, it should also be accessible to a and not only to the running script; and the same I guess it happens with modulec.
Supposing this is true, which I think indeed, will it work if the import were done in only one line? Are the modules imported and included into the script one by one or are they rather imported altogether and them enabled for use, after the statement finishes (the one-line import call)?
import moduleb, modulea, modulec
And if it does, does it shorten too much the processing time of the code, or is it basically the same, despite the aesthetic matter?

You are overthinking way too much.
Each module must be "self contained" in the sense that itself satisfies its dependencies. If modulea needs moduleb, you do
# modulea.py
import moduleb
Then moduleb has to import its dependencies, if any:
# moduleb.py
import json # for instance
And finally modulec needs modulea and moduleb, you just import them:
# modulec.py
import modulea
import moduleb # <-- actually this import does not reload the moduleb, since it was already loaded in modulea, and it was cached.
There is nothing more about this.
And if you just need a function from a module, you can just import it into the namespace like this.
from moduleb import my_cool_function
Just as style convention, you should do each import in a new line:
import json
from moduleb import my_cool_function
from moduleb import another_not_so_cool_function

According to the python style guide you should use an import statement for each individual module. It will also tell you everything else you need to know about the conventions surrounding importing modules.
import moduleb
import modulea
import modulec
Also, as some of the commenters have indicated, you don't seem to be importing these modules properly within each script, thus creating errors here in this script.

You can put all of your imports into one place if you'd like.
myproject/internal.py
from moduleb import *
from modulea import *
from modulec import *
Now, your other modules can just import it
myproject/modulec.py
from internal import *
Although its usually preferable to import everything in the module that uses it, there are conditions where that is not desirable. I have a project with many small scripts that test product functionality. It would be a bother to have lots of imports in each of them.

How do I make these relative imports work in Python 3?

I have a directory structure that looks like this:
project/
__init__.py
foo/
__init.py__
first.py
second.py
third.py
plum.py
In project/foo/__init__.py I import classes from first.py, second.py and third.py and put them in __all__.
There's a class in first.py named WonderfulThing which I'd like to use in second.py, and want to import by importing * from foo. (It's outside of the scope of this question why I'd like to do so, assume I have a good reason.)
In second.py I've tried from .foo import *, from foo import * and from . import * and in none of these cases is WonderfulThing imported. I also tried from ..foo import *, which raises an error "Attempted relative import beyond toplevel package".
I've read the docs and the PEP, and I can't work out how to make this work. Any assistance would be appreciated.
Clarification/Edit: It seems like I may have been misunderstanding the way __all__ works in packages. I was using it the same as in modules,
from .first import WonderfulThing
__all__ = [ "WonderfulThing" ]
but looking at the docs again it seems to suggest that __all__ may only be used in packages to specify the names of modules to be imported by default; there doesn't seem to be any way to include anything that's not a module.
Is this correct?
A non-wildcard import failed (cannot import name WonderfulThing). Trying from . import foo failed, but import foo works. Unfortunately, dir(foo) shows nothing.

Edit: I did misunderstand the question: No __all__ is not restricted to just modules.
One question is why you want to do a relative import. There is nothing wrong with doing from project.foo import *, here. Secondly, the __all__ restriction on foo won't prevent you from doing from project.foo.first import WonderfulThing, or just from .first import WonderfulThing, which still will be the best way.
And if you really want to import a a lot of things, it's probably best to do from project import foo, and then use the things with foo.WonderfulThing instead for doing an import * and then using WonderfulThing directly.
However to answer your direct question, to import from the __init__ file in second.py you do this:
from . import WonderfulThing
or
from . import *