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Import a submodule in a Python module

I have already read Python: importing a sub‑package or sub‑module but this:
file content
============================================================================
main.py from mymodule.a import A; A()
mymodule/submodule/__init__.py (empty)
mymodule/submodule/b.py class B: pass
mymodule/__init__.py (empty)
mymodule/a.py (see below)
mymodule/a.py:
import submodule
class A:
def __init__(self):
self.b = submodule.B()
fails with:
File "a.py", line 1, in
import submodule
ModuleNotFoundError: No module named 'submodule'
when lauching the script main.py.
Question: how should I import submodule in mymodule/a.py?

TLDR:
mymodule/a.py:
from . import submodule
# or
import mymodule.submodule as submodule
Once you run your main.py, python adds to your sys.path the path to main.py folder. Python can now search subfolders to find modules trying to be imported. Once you import a.py, python DOES NOT add anything else to sys.path, therefore, to be able import subfolders, you need to do either relative importing (from . import submodule), which you can do because you aren't running a.py as your main file, OR do a full import, doing import mymodule.submodule, since python can search starting on main.py folder.

Using custom module import error on python3 version

I've created a custom module called config which have the following structure:
config/
__init__.py
config.py
util.py
Each file is defined like:
config.py
CONTROL_VAR = "A"
util.py
from config import CONTROL_VAR
if CONTROL_VAR == "A":
VAR_ONE = "A1"
else:
VAR_ONE = "B1"
I've setup my workspace like the following:
workspace_folder
config/
my_script.py
In order tu run my script I invoke it directly by running:
python my_script.py
my_script.py has the following code:
from config.config import CONTROL_VAR
from config.util import VAR_ONE
if __name__ == "__main__":
print (CONTROL_VAR)
print (VAR_ONE)
I am able to run the code with python 2.7 but not with 3.8, at first I thought i was having a circular dependency issue but now it seems like I have something wrong with my import syntax.
Can I make this work for both python2 and python3?

When you perform from config import CONTROL_VAR inside config/util.py that module (util.py) doesn't know that it's located inside the config package and hence the resolution will start from my_script.py's directory. That means even the import happens inside a module which is located inside config, it attempts to resolve that import from the perspective outside of config.
In order to change that behavior, you can prefix config with a . which tells Python that the config.py module is expected to be in the same directory as the util.py module:
from .config import CONTROL_VAR
The from .foo import bar and from ..foo import bar syntax instructs Python so resolve imports relative to the importing module. A plain from foo import bar on the other hand resolves foo as a "global" module (i.e. not relative to anything).

Using import statements in a submodule's init.py file in Python

I have the following folder structure:
- a/
- test.py
- b/
- __init__.py (empty)
- c/
__init.py (imports classes from x.py and z.py)
x.py
z.py
test.py content:
from b.c import SomeClient
I'm trying to import c in test.py, but receiving the following error:
ModuleNotFoundError: No module named 'c'
The contents of the __init__.py under the c/ folder looks like this:
from c.x import SomeClient
from c.z import AnotherClient
I tried removing the c. pointer from the import statements, but it starts throwing errors saying there is no module named x or z.

The file a/b/c/__init__.py imports files relative to c. The problem is that the usage of implicit relative imports (and also in this reference) has already been removed in Python3:
There is no longer any implicit import machinery
Either use absolute imports in your a/b/c/__init__.py
from b.c.x import SomeClient
from b.c.z import AnotherClient
Or explicit relative imports
from .x import SomeClient
from .z import AnotherClient
If that is not possible, just set PYTHONPATH to point to where the c directory can be found so that Python can see the modules in it when imported. If in Linux:
export PYTHONPATH=${PYTHONPATH}:/path/to/a/b

Python unittest failing to resolve import statements

I have a file structure that looks like the following
project
src
__init__.py
main.py
module.py
secondary.py
test
test_module.py
module.py
import secondary
x = False
secondary.py
pass
test_module.py
from unittest import TestCase
from src import module
class ModuleTest(TestCase):
def test_module(self):
self.assertTrue(module.x)
Invoking python3 -m unittest discover in /project/ gives an error:
File "/Users/Me/Code/project/test/test_module.py", line 6, in <module>
from src import module
File "/Users/Me/Code/project/src/module.py", line 1, in <module>
import secondary
ImportError: No module named 'secondary'
What can I do so that secondary.py is imported without error?

In Python 3 (and Python 2 with from __future__ import absolute_import), you must be explicit about what module you want when importing another module from the same package. The syntax you're using in module.py (import secondary) only works if secondary is a top-level module in a folder in the Python module search path.
To explicitly request a relative import from your own package, use from . import secondary instead. Or, make an absolute import, using the name of the package as well as the module (from src import secondary, or import src.secondary and use src.secondary elsewhere the module instead of just secondary).

Try adding this to the top of the test file
sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
Note: Depending on the project structure we have to specify the root folder

Python imports with __init__.py

No matter how I structure the imports in the code files and in the __init__.py files, I can't seem to get it right for executing the program and running the tests using pytest. How do I need to write the imports when my project structure looks like this:
src/
__init__.py
VocableFileWriter.py
WordListHelper.py
WordListReader.py
XLDAttributeValueAdder.py
exceptions/
__init__.py
XMLInvalidException.py
XMLParseException.py
gui/
__init__.py
GTKSignal.py
XLDAttributeValueAdderWindow.py
test/
__init__.py
test_XLDAttributeValueAdder.py
xmlparser/
__init__.py
XMLParser.py
Currently I have them like this:
In the __init__.py files I have the imports like this (src/__init__.py):
from src import *
from src.exceptions import *
from src.xmlparser import *
and in a subpackage (src/xmlparser/__init__.py):
from src.xmlparser import *
So I guess those are project-absolute paths to the modules.
In the code files themselves I import like this:
import os
import sys
from VocableFileWriter import VocableFileWriter
from XLDAttributeValueAdder import XLDAttributeValueAdder
However, when I execute the code from the directory above src using:
./src/main.py
It tells me that:
Traceback (most recent call last):
File "./src/main.py", line 6, in <module>
from VocableFileWriter import VocableFileWriter
File "/home/xiaolong/Development/PycharmProjects/xld-attribute-value-adder/src/VocableFileWriter.py", line 2, in <module>
from XMLInvalidException import XMLInvalidException
ImportError: No module named 'XMLInvalidException'
It used to be a PyCharm project, but I couldn't get it to run with the imports structure PyCharm used when not using PyCharm but running it from the terminal, so I decided I wanted to take the whole import stuff into my own hands. So don't get confused by it being in a PyCharm directory.
I also want to be able to execute the tests using for example:
py.test src/test/test_XLDAttributeValueAdder.py
How do I solve this mess?
Edit#1:
I had the program running once, but then the test complained about missing modules, probably because it's in another subdirectory and I tried so many configurations, that I don't know how I had the program running anymore. If possible please add some explanation why a certain structure is correct and works for both, tests and the program itself.
EDIT#2:
I've managed to get the tests running now, but now the program doesn't run anymore.
I emptied all the __init__.py files and used only project-absolute paths in the code files like this (src/test/test_XLDAttributeValueAdder.py):
from src.VocableFileWriter import VocableFileWriter
from src.WordListHelper import WordListHelper
from src.WordListReader import WordListReader
from src.XLDAttributeValueAdder import XLDAttributeValueAdder
from src.exceptions.XMLInvalidException import XMLInvalidException
from src.exceptions.XMLParseException import XMLParserException
from src.xmlparser.XMLParser import XMLParser
from src.test.path_helper import go_up
from src.test.path_helper import go_in
and in the main.py:
from src.VocableFileWriter import VocableFileWriter
from src.XLDAttributeValueAdder import XLDAttributeValueAdder
the output of ./src/main.py:
./src/main.py
Traceback (most recent call last):
File "./src/main.py", line 6, in <module>
from src.VocableFileWriter import VocableFileWriter
ImportError: No module named 'src'
EDIT#3:
I've tried the relative import way like this:
main.py moved one directory up
The structure now looks like this:
main.py
src/
__init__.py
VocableFileWriter.py
WordListHelper.py
WordListReader.py
XLDAttributeValueAdder.py
exceptions/
__init__.py
XMLInvalidException.py
XMLParseException.py
gui/
__init__.py
GTKSignal.py
XLDAttributeValueAdderWindow.py
test/
__init__.py
test_XLDAttributeValueAdder.py
xmlparser/
__init__.py
XMLParser.py
__init__.py files empty
main.py file gets relative
Imports look like this:
import os
import sys
from .src.VocableFileWriter import VocableFileWriter
from .src.XLDAttributeValueAdder import XLDAttributeValueAdder
When I try to run the main.py file:
Traceback (most recent call last):
File "main.py", line 6, in <module>
from .src.VocableFileWriter import VocableFileWriter
SystemError: Parent module '' not loaded, cannot perform relative import
However, there is a __init__.py file in the same directory as the main.py file, also empty.

You'd better use relative imports.
When doing from src import * in src/__init__.py you'll only import anything you've defined before calling the import. Most likely that's just nothing. If you've defined the __all__ variable, you'll get the submodule from it, but maybe also an AttributeError if a variable hasn't been defined yet.
So instead import the modules you need explicitly where you need them like
from .VocableFileWriter import VocableFileWriter
from .exceptions.XMLInvalidException import XMLInvalidException
or lets say within src/gui/GTKSignal.py
from ..exceptions.XMLInvalidException import XMLInvalidException
Also you could use project-absolute paths like mentioned in your Edit#2.
Furthermore you've a problem with your path when calling ./src/main.py. In this case the directory containing src is not in the path. Instead ./src/ is in the path. Therefore python doesn't consider it a package anymore and since there's no other package with that name available, the import fails.
Instead put your main.py module in the same directory like the src package or call it as a module with python -m src.main. Doing so you could also rename it to src/main.py and call the package instad with python -m src.
When using the first approach, main should use an absolute import structure. Just think of it as a module you put anywhere on your computer, while your src package is somewhere else, where it can be found by Python, like any other module or package installed by the system. It's not different, when it's located besides the package itself, since the current directory is in sys.path.
main.py
import os
import sys
from src.VocableFileWriter import VocableFileWriter
from src.XLDAttributeValueAdder import XLDAttributeValueAdder