1 files changed, 164 insertions, 0 deletions
diff --git a/config.py b/config.py
new file mode 100644
index 0000000..bc0dcdf
--- /dev/null
+++ b/config.py
@@ -0,0 +1,164 @@
+#!/usr/bin/env python3
+
+"""Global configuration driven by commandline arguments (even across
+different modules).  Usage:
+
+    module.py:
+    ----------
+    import config
+
+    parser = config.add_commandline_args(
+        "Module",
+        "Args related to module doing the thing.",
+    )
+    parser.add_argument(
+        "--module_do_the_thing",
+        type=bool,
+        default=True,
+        help="Should the module do the thing?"
+    )
+
+    main.py:
+    --------
+    import config
+
+    def main() -> None:
+        parser = config.add_commandline_args(
+            "Main",
+            "A program that does the thing.",
+        )
+        parser.add_argument(
+            "--dry_run",
+            type=bool,
+            default=False,
+            help="Should we really do the thing?"
+        )
+        config.parse()   # Very important, this must be invoked!
+
+    If you set this up and remember to invoke config.parse(), all commandline
+    arguments will play nicely together:
+
+    % main.py -h
+    usage: main.py [-h]
+                   [--module_do_the_thing MODULE_DO_THE_THING]
+                   [--dry_run DRY_RUN]
+
+    Module:
+      Args related to module doing the thing.
+
+      --module_do_the_thing MODULE_DO_THE_THING
+                   Should the module do the thing?
+
+    Main:
+      A program that does the thing
+
+      --dry_run
+                   Should we really do the thing?
+
+    Arguments themselves should be accessed via config.config['arg_name'].  e.g.
+
+    if not config.config['dry_run']:
+        module.do_the_thing()
+"""
+
+import argparse
+import pprint
+import re
+import sys
+from typing import Dict, Any
+
+# Note: at this point in time, logging hasn't been configured and
+# anything we log will come out the root logger.
+
+
+class LoadFromFile(argparse.Action):
+    """Helper to load a config file into argparse."""
+    def __call__ (self, parser, namespace, values, option_string = None):
+        with values as f:
+            buf = f.read()
+            argv = []
+            for line in buf.split(','):
+                line = line.strip()
+                line = line.strip('{')
+                line = line.strip('}')
+                m = re.match(r"^'([a-zA-Z_\-]+)'\s*:\s*(.*)$", line)
+                if m:
+                    key = m.group(1)
+                    value = m.group(2)
+                    value = value.strip("'")
+                    if value not in ('None', 'True', 'False'):
+                        argv.append(f'--{key}')
+                        argv.append(value)
+            parser.parse_args(argv, namespace)
+
+
+# A global parser that we will collect arguments into.
+args = argparse.ArgumentParser(
+    description=f"This program uses config.py ({__file__}) for global, cross-module configuration.",
+    formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    fromfile_prefix_chars="@"
+)
+config_parse_called = False
+
+# A global configuration dictionary that will contain parsed arguments
+# It is also this variable that modules use to access parsed arguments
+config: Dict[str, Any] = {}
+
+
+def add_commandline_args(title: str, description: str = ""):
+    """Create a new context for arguments and return a handle."""
+    return args.add_argument_group(title, description)
+
+
+group = add_commandline_args(
+    f'Global Config ({__file__})',
+    'Args that control the global config itself; how meta!',
+)
+group.add_argument(
+    '--config_loadfile',
+    type=open,
+    action=LoadFromFile,
+    metavar='FILENAME',
+    default=None,
+    help='Config file from which to read args in lieu or in addition to commandline.',
+)
+group.add_argument(
+    '--config_dump',
+    default=False,
+    action='store_true',
+    help='Display the global configuration on STDERR at program startup.',
+)
+group.add_argument(
+    '--config_savefile',
+    type=str,
+    metavar='FILENAME',
+    default=None,
+    help='Populate config file compatible --config_loadfile to save config for later use.',
+)
+
+
+def parse() -> Dict[str, Any]:
+    """Main program should call this early in main()"""
+    global config_parse_called
+    config_parse_called = True
+    config.update(vars(args.parse_args()))
+
+    if config['config_savefile']:
+        with open(config['config_savefile'], 'w') as wf:
+            wf.write("\n".join(sys.argv[1:]))
+
+    if config['config_dump']:
+        dump_config()
+
+    return config
+
+
+def has_been_parsed() -> bool:
+    global config_parse_called
+    return config_parse_called
+
+
+def dump_config():
+    """Print the current config to stdout."""
+    print("Global Configuration:", file=sys.stderr)
+    pprint.pprint(config, stream=sys.stderr)