Add example of how to combine configparser and argparse

Author: gjbex

Diff for: source-code/README.md

@@ -18,6 +18,7 @@ to create it.  There is some material not covered in the presentation as well.
 
 ## What is it?
 
+1. `application`: illustration of how to combine `configparser` and `argparse`.
 1. `cmd`: illustration of how to create a repl application.
 1. `code-evaluation`: illustrates how to evaluate a string containing
     Python code at runtime.

Diff for: source-code/application/README.md

@@ -0,0 +1,22 @@
+# Application
+
+Example of a very simple application that prints random integers between
+two given values.  It uses argparse and configparser to handle configuration
+files and command line arguments.
+
+The parameters are
+* `n`: the number of random values to generate,
+* `a`: the smallest value to generate, and
+* `b`: the largest value to generate.
+
+The point is to illustrate specifying the parameters via
+1. defaults in the application,  `n = 1`, `a = 0`, `b = 6`,
+1. a "system" configuration file, `etc/my_app.conf`, `n = 2`, `a = 1`, `b = 6`,
+1. optionally, a configurtion file specified as a command line option, i.e.,
+   `--conf my_app.conf`, `n = 3`, `a = 2`,
+1. command line options.
+
+## What is it?
+1. `my_apps.py`: Python script to print random integer values.
+1. `etc/my_app.conf`: "system" level configuration file.
+1. `my_app.conf`: optional "user" level configuration file.

Diff for: source-code/application/etc/my_app.conf

@@ -0,0 +1,4 @@
+[defaults]
+n = 2
+a = 1
+b = 6

Diff for: source-code/application/my_app.conf

@@ -0,0 +1,3 @@
+[defaults]
+n = 3
+a = 2

Diff for: source-code/application/my_app.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+
+from argparse import ArgumentParser
+from configparser import ConfigParser
+from pathlib import Path
+import random
+import sys
+
+
+def generate_data(n, a=0, b=1):
+    return [random.randint(a, b) for _ in range(n)]
+
+def main():
+    arg_parser = ArgumentParser(description='test application')
+    arg_parser.add_argument('--conf',
+                            help='configuration file to use')
+    arg_parser.add_argument('--verbose', action='store_true',
+                            help='verbose output')
+    options, remaining_options = arg_parser.parse_known_args()
+    system_conf = Path.cwd() / 'etc' / 'my_app.conf'
+    cfg = ConfigParser()
+    cfg['defaults'] = {'n': '1', 'a': '0', 'b': '6'}
+    if options.verbose:
+        print('application default values:', file=sys.stderr)
+        cfg.write(sys.stderr)
+    if system_conf.exists():
+        cfg.read('etc/my_app.conf')
+        if options.verbose:
+            print('system configuration file values:', file=sys.stderr)
+            cfg.write(sys.stderr)
+    else:
+        print(f'missing configuration file {system_conf}', file=sys.stderr)
+    if options.conf:
+        cfg.read(options.conf)
+        if options.verbose:
+            print('user configuration file values:', file=sys.stderr)
+            cfg.write(sys.stderr)
+    cfg_opts = dict(cfg['defaults'])
+    arg_parser.set_defaults(**cfg_opts)
+    arg_parser.add_argument('n', type=int, nargs='?',
+                            help='number of random values to generate')
+    arg_parser.add_argument('--a', type=int, help='smallest value')
+    arg_parser.add_argument('--b', type=int, help='largest value')
+    arg_parser.parse_args(remaining_options, options)
+    if options.verbose:
+        print('final options:', file=sys.stderr)
+        print(f'n = {options.n}\na = {options.a}\nb = {options.b}', end='\n\n',
+              file=sys.stderr)
+    values = generate_data(options.n, a=options.a, b=options.b)
+    print('\n'.join(str(value) for value in values))
+    return 0
+
+if __name__ == '__main__':
+    status = main()
+    sys.exit(status)