Introduce support for user-provided extensions

This change introduces support for discovering and executing
user-provided extensions to the program. Extensions are useful for
allowing users to provide additional functionality on top of the
nitrocli proper. Implementation wise we stick to an approach similar to
git or cargo subcommands in nature: we find executables in the PATH
environment variable whose file name starts with "nitrocli-" and allow
for them to be invoked as a nitrocli command.

1 files changed, 52 insertions, 0 deletions

diff --git a/nitrocli/src/tests/extension_model_test.py b/nitrocli/src/tests/extension_model_test.py
new file mode 100644
index 0000000..651c8e7
--- /dev/null
+++ b/nitrocli/src/tests/extension_model_test.py
@@ -0,0 +1,52 @@
+#!/usr/bin/env python
+
+from argparse import (
+  ArgumentParser,
+)
+from enum import (
+  Enum,
+)
+from sys import (
+  argv,
+  exit,
+)
+
+
+class Action(Enum):
+  """An action to perform."""
+  NITROCLI = "nitrocli"
+  MODEL = "model"
+  VERBOSITY = "verbosity"
+
+  @classmethod
+  def all(cls):
+    """Return the list of all the enum members' values."""
+    return [x.value for x in cls.__members__.values()]
+
+
+def main(args):
+  """The extension's main function."""
+  parser = ArgumentParser()
+  parser.add_argument(choices=Action.all(), dest="what")
+  parser.add_argument("--nitrocli", action="store", default=None)
+  parser.add_argument("--model", action="store", default=None)
+  # We deliberately store the argument to this option as a string
+  # because we can differentiate between None and a valid value, in
+  # order to verify that it really is supplied.
+  parser.add_argument("--verbosity", action="store", default=None)
+
+  namespace = parser.parse_args(args[1:])
+  if namespace.what == Action.NITROCLI.value:
+    print(namespace.nitrocli)
+  elif namespace.what == Action.MODEL.value:
+    print(namespace.model)
+  elif namespace.what == Action.VERBOSITY.value:
+    print(namespace.verbosity)
+  else:
+    return 1
+
+  return 0
+
+
+if __name__ == "__main__":
+  exit(main(argv))