Python relative import: ModuleNotFoundError: No module named 'sib1'

Question

I have a project hierarchy like below, when I run `python src/bot/main` I didn't get error. While if I run `python -m src.bot.main` I got an error. Why? This is my file hierarchy: ``` MyProject └── src ├── __init__.py ├── bot │ ├── __init__.py │ ├── main.py │ └── sib1.py └── mod ├── __init__.py └── module1.py ``` This is the content of `main.py`: ``` import sys if __name__ == "__main__": # sys.path will print the parent folder. print(sys.path, end="

") # my problem is on this line. import sib1 sib1.test() ``` The error: ``` Traceback (most recent call last): File "/usr/local/Caskroom/miniconda/base/envs/test/lib/python3.9/runpy.py", line 197, in _run_module_as_main return _run_code(code, main_globals, None, File "/usr/local/Caskroom/miniconda/base/envs/test/lib/python3.9/runpy.py", line 87, in _run_code exec(code, run_globals) File "/Users/me/Desktop/test_py/src/bot/main.py", line 16, in <module> import sib1 ModuleNotFoundError: No module named 'sib1' ``` Some conclusion I've made…

Accepted Answer

I will try my best to clarify each of my confusions in a Q&A form, and organize @Brain's comments along the way:

Q1: when I run `python src/bot/main` I got no error.

The `sys.path` will include the current directory containing the file `main.py`, i.e. the interpreter will see the file `MyProject/src/bot`:

```
import sib1
```

is logically equivalent to:

```
import "MyProject/src/bot" + "/sib1.py"
```

Hence, no error.

Q2: While if I run `python -m src.bot.main` I got error. Why?

Now it's time to quote @Brain's valuable (first) comment:

Using `python -m src.bot.main` tells Python that `src` is a top-level package. Everything below src in the directory structure will be considered submodules/subpackages of src. The proper name for `sib1` under that organization is `src.bot.sib1`. No top-level module named sib1 exists as far as Python is concerned.

(emphasis by me)

So:

- The `-m` option will define the scope (and thus the top-level package) for the file you're going to run.

- All packages not starting from `src.` will be regarded as built-in or third-party libraries installed in your virtual environment. Since I didn't install any package called `sib1`, you got the error. (the `sys.path` is intentionally ignored in this case, as we there is no implicitly relative import starting from Python 3.3)

- All your relative imports should not go outside of the top-level package `src` (by prepending too many `.`'s in the `import`).

For example, this will work:

```
from . import sib1
from ..mod import module1    # The `..` is equivalent to the MyProject/src.

module1.hello()
sib1.test()
```

Finally, don't test your package by inserting many `if __name__ == '__main__'`. Do so by professional tools:

If you only need to run submodules for testing purpose, you could consider using more robust testing tools like doctest (lightweight) or unittest (heavyweight).

This is a good read I had put 300 bounty on it years ago, you must find a time to read it.

Python relative import: ModuleNotFoundError: No module named 'sib1'

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