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Meir Gabay edited this page Nov 4, 2020 · 23 revisions

Requirements

  1. You are familiar with Packages and Modules
  2. It would be easier to understand the examples if you run the examples by yourself - use this project as a template

Getting Started

Executing modules from the project's root directory (top-level package)

  • main.py and src/appy/__main__.py

    (python-project) $ python main.py
My Path: python-project/main.py
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
Jaguars are the only big cats that don't roar.


(python-project) $ python -m src.appy
My Path: python-project/src/appy/__main__.py
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
A cat has more bones than a human being; humans have 206 and the cat has 230 bones.

  • src/appy/core/app.py

    # Contains relative imports - `..utils.message`
(python-project) $ python src/appy/core/app.py
Traceback (most recent call last):
  File "src/appy/core/app.py", line 1, in <module>
    from ..utils import message, img_ascii
ImportError: attempted relative import with no known parent package

(python-project) $ python -m src.appy.core.app
My Path: python-project/src/appy/core/app.py
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
Siamese kittens are born white because of the heat inside the mother's uterus before birth. This heat keeps the kittens' hair from 
darkening on the points.

  • src/appy/utils/message.py

    # Doesn't contain relative imports, so no exceptions were raised
(python-project) $ python src/appy/utils/message.py
My Path: python-project/src/appy/utils/message.py

(python-project) $ python -m src.appy.utils.message
python-project/src/appy/utils/message.py

Is it possible to invoke a function from the terminal?

  • Short answer - it depends

  • Trying to invoke a function from the terminal, such as appy.core.app.main(), will raise the ModuleNotFound exception. A package must be imported before invoking one of its functions.

    (python-project) $ python -m src.appy.core.app.main
/Users/meirgabay/.pyenv/versions/3.8.2/Python.framework/Versions/3.8/bin/python: Error while finding module specification for 'src.appy.core.app.main' (ModuleNotFoundError: __path__ attribute not found on 'src.appy.core.app' while trying to find 'src.appy.core.app.main')

  • Since you can't invoke main() directly from the terminal, calling it from the if __main__ block enables executing it from the terminal. It's possible to pass arguments, but it's a bit ugly, read the docs to learn how. The following example attempts to execute the module appy.core.app, which in turn call its if __main__ block

    (python-project) $ python -m src.appy.core.app
My Path: python-project/src/appy/core/app.py
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
Cats sleep 16 to 18 hours per day. When cats are asleep, they are still alert to incoming stimuli. If you poke the tail of a sleeping cat, it will respond accordingly.

  • If the PWD is a subdirectory of the project, such as python-project/appy, an attempt to execute a module which contains relative imports, will raise the exception below. Remember, your PWD should always be the project's root directory, in this case it's python-project.

    # PWD is `src/appy`
(python-project/src/appy) $ python -m core.app

Traceback (most recent call last):
  File "/Users/meirgabay/.pyenv/versions/3.8.2/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Users/meirgabay/.pyenv/versions/3.8.2/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/Users/meirgabay/python-project/src/appy/core/app.py", line 1, in <module>
    from ..utils import message, img_ascii
ValueError: attempted relative import beyond top-level package

  • It doesn't happen when invoking message, since message doesn't use relative imports

    (python-project/src/appy) $ python utils/message.py
My Path: python-project/src/appy/utils/message.py


(python-project/src/appy) $ python -m utils.message
My Path: python-project/src/appy/utils/message.py

  • Invoking a function from the terminal is also possible by using the -c flag. Surprise, it's possible to pass arguments in a more intuitive way, for example app.main(my_arg1, my_arg2)

    (python-project) $ python -c "import src.appy.core.app as app; app.main()"
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
Cats can be right-pawed or left-pawed.

What are the available command-line flags in Python?

  • In this tutorial, we used both -c and -m flags
  • Read the docs to learn about more flags - Using cmdline

Why is it possible to execute python -m appy?

The src/appy/__main__.py file acts like the if __main__ code snippet, but on packages. This enables the appy package to be executed with python -m or with runpy

(python-project) $ python -m src.appy
My Path: python-project/src/appy/__main__.py
Created the file: /Users/meirgabay/python-project/meirg-ascii.txt
Insert your name: willy wonka

Hello Willy Wonka, here's the cat fact of the day:
One reason that kittens sleep so much is because a growth hormone is released only during sleep.

What's runpy and why do you use it in main.py?

The runpy package provides the ability to run modules from a module (Python script).

main.py

import runpy


def main():
    appy_package = runpy.run_module(
        mod_name="src.appy", init_globals=globals())
    appy_package['message'].script_path(__file__)
    appy_package['main']()


if __name__ == "__main__":
    main()

What's globals()?

The official definition from the docs

Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).

(python-project) $ python
Python 3.8.2 (default, Jun 30 2020, 19:04:41)
[Clang 11.0.3 (clang-1103.0.32.59)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> globals()
{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>}
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