Add example of a directory entry point

Author: gjbex

source-code/README.md

@@ -28,6 +28,8 @@ to create it.  There is some material not covered in the presentation as well.
     configuration files.
 1. `data-formats`: illustrates how to deal with data formats such as CSV
     files, binary data and XML.
+1. `directory-entrypoint`: illustrates how to define an application entry point in
+   a directory.
 1. `enviroment-variables`: illustrates how to use environment variables to set paths
    in a script.
 1. `hydra`: Facebook Hydra application framework illustration.

source-code/directory-entrypoint/README.md

@@ -0,0 +1,11 @@
+# Directory entry point
+
+Using `__main__.py` as an entry point, the Python interpreter can run a
+directory.
+
+## What is it?
+
+1. `messenger`: directory containing the application.
+1. `messenger/__main__.py`: Python application entry point.
+1. `messenger/messenger.py`: Python module defining some functions for the
+   application.

source-code/directory-entrypoint/messenger/__main__.py

@@ -0,0 +1,18 @@
+import argparse
+import messenger
+import sys
+
+
+arg_parser = argparse.ArgumentParser(description='say something to the user')
+arg_parser.add_argument('--hello', action='store_true', help='say hello')
+arg_parser.add_argument('--bye', action='store_true', help='say bye')
+arg_parser.add_argument('name', help='name to talk to')
+options = arg_parser.parse_args()
+
+if options.hello:
+    messenger.say_hello(options.name)
+if options.bye:
+    messenger.say_bye(options.name)
+if not (options.hello or options.bye):
+    print(f"don't know what  to say to {options.name}", file=sys.stderr)
+    sys.exit(1)

source-code/directory-entrypoint/messenger/messenger.py

@@ -0,0 +1,5 @@
+def say_hello(name):
+    print(f'hello {name}')
+
+def say_bye(name):
+    print(f'bye {name}')