Assume positional arguments are required

The 'positional' keyword specifically applies to oslo.config's argparse
support. Unlike oslo.config, argparse assumes that all positional
arguments are required by default, and you have to explicitly tell it
that a positional argument is optional if you'd like to opt into that
behavior.

This patch adopts that same behavior for oslo.config. When you define an
option to be non-positional (oslo.config's default, designed for config
files), then oslo.config makes that option optional:

However, when you define an option to be positional, oslo.config assumes
that the option is primarily going to be used on the CLI and thus sets
it as required, by default.

This change in behavior has the side effect of allowing argparse to
enforce required arguments on the CLI *while* parsing arguments, instead
of depending on oslo.config to detect the condition *after* argparse has
been allowed to parse "invalid" arguments. argparse correctly raises a
SystemExit in this case, and prints the actual command usage and a "hey,
you forgot this required argument", instead of allowing oslo.config to
dump a backtrace to the CLI with a context-less error message
("context-less" in that no useful CLI usage information is dumped along
with the crash to help you correct the condition).

Change-Id: Ifdc6918444fe72f7e1649483c237cce64b4c72d8
Partial-Bug: 1676989

1 files changed, 12 insertions, 0 deletions

diff --git a/releasenotes/notes/positional-arguments-are-required-22ddca72e6f523bf.yaml b/releasenotes/notes/positional-arguments-are-required-22ddca72e6f523bf.yaml
new file mode 100644
index 0000000..6e7914c
--- /dev/null
+++ b/releasenotes/notes/positional-arguments-are-required-22ddca72e6f523bf.yaml
@@ -0,0 +1,12 @@
+---
+upgrade:
+  - |
+    Positional options are now required by default, to match argparse's default
+    behavior. To revert this behavior (and maintain optional positional
+    arguments), you need to explicitly specify ``positional=True,
+    required=False`` as part of the options definition.
+fixes:
+  - |
+    On the command line, oslo.config now returns command usage information from
+    argparse (instead of dumping a backtrace) when required arguments are
+    missing.