So far, we've been writing our code all in Jupyter. But when it comes time to write code that we want to reuse, we want to put it into a standalone *.py file.

Then we can load it on in python (or Jupyter) and use the capabilities it provides or make it a standalone program that can be run from the command line.

Jupyter is great for interactive explorations and sharing your workflow with others
in a self-contained way.  But if there is an operation that you do over and over,
you should put it into a separate module that you import.  That way you only need to
maintain and debug a single instance of the function, and all your workflows can reuse it.

There are a number of popular editors for writing python source. Some popular ones include:

• spyder: https://www.spyder-ide.org/

• VS Code: https://code.visualstudio.com/

• emacs / vi

Here's a very simply module (lets call it hello.py):

def hello():
    print("hello")

if __name__ == "__main__":
    hello()

There are two ways we can use this.

• Inside of python (or IPython), we can do:

  import hello
  hello.hello()

• From the command line, we can do:

  python hello.py

Additionally, on a Unix system, we can add:

#!/usr/bin/env python3

to the top and then mark the file as executable, via:

chmod a+x hello.py

allowing us to execute it simply as:

./hello.py

Here we see how the `__name__` variable is treated by python:

* If we import our module into python, then `__name__` is set to the module name

* If we run the module from the command line, then `__name__` is set to `__main__`

If we make changes to our module file, then we need to re-import it. This can be done as:

import importlib
example = importlib.reload(example)