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Use __init__.py to modify sys path is a good idea?

I want to ask you something that came to my mind doing some stuff.
I have the following structure:
src
- __init__.py
- class1.py
+ folder2
- __init__.py
- class2.py
I class2.py I want to import class1 to use it. Obviously, I cannot use
from src.class1 import Class1
cause it will produce an error. A workaround that works to me is to define the following in the __init__.py inside folder2:
import sys
sys.path.append('src')
My question is if this option is valid and a good idea to use or maybe there are better solutions.
Another question. Imagine that the project structure is:
src
- __init__.py
- class1.py
+ folder2
- __init__.py
- class2.py
+ errorsFolder
- __init__.py
- errors.py
In class1:
from errorsFolder.errors import Errors
this works fine. But if I try to do in class2 which is at the same level than errorsFolder:
from src.errorsFolder.errors import Errors
It fails (ImportError: No module named src.errorsFolder.errors)
Thank you in advance!

Despite the fact that it's slightly shocking to have to import a "parent" module in your package, your workaround depends on the current directory you're running your application, which is bad.
import sys
sys.path.append('src')
should be
import sys,os
sys.path.append(os.path.join(os.path.dirname(os.path.abspath(__file__)),os.pardir))
to add the parent directory of the directory of your current module, regardless of the current directory you're running your application from (your module may be imported by several applications, which don't all require to be run in the same directory)

One correct way to solve this is to set the environment variable PYTHONPATH to the path which contains src. Then import src.class1 will always work, regardless of which directory you start in.

from ..class1 import Class1 should work (at least it does here in a similar layout, using python 2.7.x).
As a general rule: messing with sys.path is usually a very bad idea, specially if this allows a same module to be imported from two different paths (which would be the case with your files layout).
Also, you may want to think twice about your current layout. Python is not Java and doesn't require (nor even encourage) a "one class per module" approach. If both classes need to work together, they might be better in a same module, or at least in modules at the same level in the package tree (note that you can use the top-level package's __init__ as a facade to provide direct access to objects defined in submodules / subpackages). NB : I'm not saying that your current layout is necessarily wrong, just that it might not be the simplest one.

There is also a solution that does not involve the use of the environment variable PYTHONPATH.
In src/ create setup.py with these contents:
from setuptools import setup
setup()
Now you can do pip install -e /path/to/src and import to your hearts content.
-e, --editable <path/url> Install a project in editable mode (i.e. setuptools "develop mode") from a local project path or a VCS url.

No, Its not good. Python takes modules in two ways:
Python looks for its modules and packages in $PYTHONPATH.
Refer: https://docs.python.org/2/using/cmdline.html#envvar-PYTHONPATH
To find out what is included in $PYTHONPATH, run the following code in python (3):
import sys
print(sys.path)
all folder containing init.py are marked as python package.(if they are subdirectories under PYTHONPATH)
By help of these two ways you can full-fill the need to create a python project.

import issues with maintaining 2 different python folders

dirFoo1\
Foo1.py
lib\
bar1.py
dirFoo3
Foo.cpp
dirFoo2
Foo2.py
lib\
bar2.py
Foo1 snippet
# Foo1.py
import lib.bar1
Class bar1_class:
....
Foo2 snippet
# Foo2.py
sys.path.append("../dirFoo1/lib")
sys.path.append("../lib") # tried removing this too
import Foo1
....
Error message in file Foo2.py:
File <path_for_Foo2>
import Foo1 File <path_for_Foo1>
from lib.bar1 import bar1_class
ImportError: No module named bar1
When trying to use the library from dirFoo1\Foo1.py in dirFoo2\Foo2.py. Both Foo1 and Foo2 has its own imports from its sub directory lib.
When importing Foo1 in Foo2 with the sys path set,
sys.path.append(dirFoo1)
import error shows up on code in Foo1
import lib.something_from_bar1
reports error saying something_from_bar1 doesnt exist.
seems like its referring to lib in dirFoo2.
I do have appropriate __init__.py in places under dirFoo1 and dirFoo2.
Is there a way to import the library from another folder that is not linked with __init__.py ?

If both of your dirFooX folders are on the Python module search path, you will have a conflict between the two lib folders, which each define a different lib package. Only one of them will be accessible to your code. There are a few different ways you can resolve the conflict.
The simplest would be to rename one or both of the lib folders, and update the import statements to match. The name lib is not very descriptive for a package that is accessible at the top level of the module namespace. Perhaps they should be Foo1Lib and Foo2Lib.
Another option is to make dirFoo1 and dirFoo2 into packages, so that the lib packages are subpackages in two different namespaces. This is probably the best approach, since it shouldn't require any messing around with sys.path to make the imports work. Just make sure you run your script from the top level folder (above the dirFooX folders, and use the -m flag to run python -m dirFoo2.Foo2 rather than python dirFoo2/Foo2.py. You'll need to update the imports everywhere to use either absolute names (from dirFoo1.lib.bar1 import bar_class) or explicit relative names (from .lib.bar1 import bar_class).
My final option is probably not really an appropriate choice for your situation, but it would work. Starting in Python 3.3, you can have "namespace" packages, which can load modules from multiple folders rather than just one. If lib was a namespace package, it would allow the bar1.py and bar2.py files to both show up as part of the lib package inside the interpreter. This is primarily intended for big projects like Django which can have many extra features added on to them by third-party modules. Using namespace packages allows all those add-on modules to be put into a shared namespace. You could make lib a namespace package if you really want to (though as I said above, lib is so generic that it's probably a bad idea to use that name at the top level). All you'd need to do is to delete the __init__.py files in each lib folder. Like I said though, this is mostly intended for large projects where many modules that should be part of a single package are being written and distributed by separate authors. Unless your two lib folders are very closely related, it's probably not an appropriate choice for your situation.

Python finding some, not all custom packages

I have a project with the following file structure:
root/
run.py
bot/
__init__.py
my_discord_bot.py
dice/
__init__.py
dice.py
# dice files
help/
__init__.py
help.py
# help files
parser/
__init__.py
parser.py
# other parser files
The program is run from within the root directory by calling python run.py. run.py imports bot.my_discord_bot and then makes use of a class defined there.
The file bot/my_discord_bot.py has the following import statements:
import dice.dice as d
import help.help as h
import parser.parser as p
On Linux, all three import statements execute correctly. On Windows, the first two seem to execute fine, but then on the third I'm told:
ImportError: No module named 'parser.parser'; 'parser' is not a package
Why does it break on the third import statement, and why does it only break on Windows?
Edit: clarifies how the program is run

Make sure that your parser is not shadowing a built-in or third-party package/module/library.
I am not 100% sure about the specifics of how this name conflict would be resolved, but it seems like you can potentially a). have your module overridden by the existing module (which seems like it might be happening in your Windows case), or b). override the existing module, which could cause bugs down the road. It seems like b is what commonly trips people up.
If you think this might be happening with one of your modules (which seems fairly likely with a name like parser), try renaming your module.
See this very nice article for more details and more common Python "import traps".

Put run.py outside root folder, so you'll have run.py next to root folder, then create __init__.py inside root folder, and change imports to:
import root.parser.parser as p
Or just rename your parser module.
Anyway you should be careful with naming, because you can simply mess your own stuff someday.

Importing Python class from sibling directory

My Directory structure in /VM/repo/project is:
__init__.py
scripts/
getSomething.py
__init__.py
classes/
project.py
db.py
__init__.py
getSomething.py
from ..classes import project
from ..classes import db
project.py
class PROJECT:
def __init__(self):
stuff
db.py
class DB:
def __init__(self):
stuff
When I try to run
python getSomething.py
I get the error
Traceback (most recent call last):
File "scripts/getSomething.py", line 4, in < module >
from ..classes import project
ValueError: Attempted relative import in non-package
What am I missing here?

As stated in the error, you're running getSomething as a main module. But you can't do package relative imports when you aren't in a package. The main module is never in a package. So, if you were to import getSomething as part of a package...:
# /VM/repo/main.py
from project.scripts import getSomething
Then you would not have the import errors.
Perhaps it is helpful to have a quick discussion on python modules and packages. In general, a file that contains python source code and has the .py extension is a module. Typically that module's name is the name of the file (sans extension), but if you run it directly, that module's name is '__main__'. So far, this is all well known and documented. To import a module you just do import module or import package.module and so on. This last import statement refers to something else (a "package") which we'll talk about now...
Packages are directories that you can import. Many directories can't be imported (e.g. maybe they have no python source files -- modules -- in them). So to resolve this ambiguity, there is also the requirement that the directory has an __init__.py file. When you import a directory on your filesystem, python actually imports the __init__.py module and creates the associated package from the things in __init__.py (if there are any).
Putting all of this together shows why executing a file inside a directory that has an __init__.py is not enough for python to consider the module to be part of a package. First, the name of the module is __main__, not package.filename_sans_extension. Second, the creation of a package depends not just on the filesystem structure, but on whether the directory (and therefore __init__.py was actually imported).
You might be asking yourself "Why did they design it this way?" Indeed, I've asked myself that same question on occasion. I think that the reason is because the language designers want certain guarantees to be in place for a package. A package should be a unit of things which are designed to work together for a specific purpose. It shouldn't be a silo of scripts that by virtue of a few haphazard __init__.py files gain the ability to walk around the filesystem to find the other modules that they need. If something is designed to be run as a main module, then it probably shouldn't be part of the package. Rather it should be a separate module that imports the package and relies on it.

python import path: packages with the same name in different folders

I am developing several Python projects for several customers at the same time. A simplified version of my project folder structure looks something like this:
/path/
to/
projects/
cust1/
proj1/
pack1/
__init__.py
mod1.py
proj2/
pack2/
__init__.py
mod2.py
cust2/
proj3/
pack3/
__init__.py
mod3.py
When I for example want to use functionality from proj1, I extend sys.path by /path/to/projects/cust1/proj1 (e.g. by setting PYTHONPATH or adding a .pth file to the site_packages folder or even modifying sys.path directly) and then import the module like this:
>>> from pack1.mod1 import something
As I work on more projects, it happens that different projects have identical package names:
/path/
to/
projects/
cust3/
proj4/
pack1/ <-- same package name as in cust1/proj1 above
__init__.py
mod4.py
If I now simply extend sys.path by /path/to/projects/cust3/proj4, I still can import from proj1, but not from proj4:
>>> from pack1.mod1 import something
>>> from pack1.mod4 import something_else
ImportError: No module named mod4
I think the reason why the second import fails is that Python only searches the first folder in sys.path where it finds a pack1 package and gives up if it does not find the mod4 module in there. I've asked about this in an earlier question, see import python modules with the same name, but the internal details are still unclear to me.
Anyway, the obvious solution is to add another layer of namespace qualification by turning project directories into super packages: Add __init__.py files to each proj* folder and remove these folders from the lines by which sys.path is extended, e.g.
$ export PYTHONPATH=/path/to/projects/cust1:/path/to/projects/cust3
$ touch /path/to/projects/cust1/proj1/__init__.py
$ touch /path/to/projects/cust3/proj4/__init__.py
$ python
>>> from proj1.pack1.mod1 import something
>>> from proj4.pack1.mod4 import something_else
Now I am running into a situation where different projects for different customers have the same name, e.g.
/path/
to/
projects/
cust3/
proj1/ <-- same project name as for cust1 above
__init__.py
pack4/
__init__.py
mod4.py
Trying to import from mod4 does not work anymore for the same reason as before:
>>> from proj1.pack4.mod4 import yet_something_else
ImportError: No module named pack4.mod4
Following the same approach that solved this problem before, I would add yet another package / namespace layer and turn customer folders into super super packages.
However, this clashes with other requirements I have to my project folder structure, e.g.
Development / Release structure to maintain several code lines
other kinds of source code like e.g. JavaScript, SQL, etc.
other files than source files like e.g. documents or data.
A less simplified, more real-world depiction of some project folders looks like this:
/path/
to/
projects/
cust1/
proj1/
Development/
code/
javascript/
...
python/
pack1/
__init__.py
mod1.py
doc/
...
Release/
...
proj2/
Development/
code/
python/
pack2/
__init__.py
mod2.py
I don't see how I can satisfy the requirements the python interpreter has to a folder structure and the ones that I have at the same time. Maybe I could create an extra folder structure with some symbolic links and use that in sys.path, but looking at the effort I'm already making, I have a feeling that there is something fundamentally wrong with my entire approach. On a sidenote, I also have a hard time believing that python really restricts me in my choice of source code folder names as it seems to do in the case depicted.
How can I set up my project folders and sys.path so I can import from all projects in a consistent manner if there are project and packages with identical names ?

This is the solution to my problem, albeit it might not be obvious at first.
In my projects, I have now introduced a convention of one namespace per customer. In every customer folder (cust1, cust2, etc.), there is an __init__.py file with this code:
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)
All the other __init__.py files in my packages are empty (mostly because I haven't had the time yet to find out what else to do with them).
As explained here, extend_path makes sure Python is aware there is more than one sub-package within a package, physically located elsewhere and - from what I understand - the interpreter then does not stop searching after it fails to find a module under the first package path it encounters in sys.path, but searches all paths in __path__.
I can now access all code in a consistent manner criss-cross between all projects, e.g.
from cust1.proj1.pack1.mod1 import something
from cust3.proj4.pack1.mod4 import something_else
from cust3.proj1.pack4.mod4 import yet_something_else
On a downside, I had to create an even deeper project folder structure:
/path/
to/
projects/
cust1/
proj1/
Development/
code/
python/
cust1/
__init__.py <--- contains code as described above
proj1/
__init__.py <--- empty
pack1/
__init__.py <--- empty
mod1.py
but that seems very acceptable to me, especially considering how little effort I need to make to maintain this convention. sys.path is extended by /path/to/projects/cust1/proj1/Development/code/python for this project.
On a sidenote, I noticed that of all the __init__.py files for the same customer, the one in the path that appears first in sys.path is executed, no matter from which project I import something.

You should be using the excellent virtualenv and virtualenvwrapper tools.

What happens if you accidentally import code from one customer/project in another and don't notice? When you deliver it will almost certainly fail. I would adopt a convention of having PYTHONPATH set up for one project at a time, and not try to have everything you've ever written be importable at once.
You can use a wrapper script per-project to set PYTHONPATH and start python, or use scripts to switch environments when you switch projects.
Of course some projects well have dependencies on other projects (those libraries you mentioned), but if you intend for the customer to be able to import several projects at once then you have to arrange for the names to not clash. You can only have this problem when you have multiple projects on the PYTHONPATH that aren't supposed to be used together.