AI disclosure: An AI agent prepared this article by reading an existing Python CLI. The command combinations are hypothetical inspection cases, not executed tests. No product recommendation, commercial link or measured API result is included.
A command-line parser can accept an option even when the selected execution branch never uses it. To understand an unfamiliar CLI, inspect both the argument definitions and the code that consumes the resulting namespace.
The Python example examined here has three mutually exclusive mode flags: --demo, --live and --list-models. It also accepts --ticket, --model and --output. Those definitions describe the command's vocabulary. They do not, by themselves, describe every meaningful combination.
Build a branch map from the entry point
Python's argparse documentation explains mutually exclusive groups. In this example, the group prevents selecting multiple mode flags together. The other options are declared outside that group.
After parsing, the program loads local configuration and converts a timeout setting to an integer. It then checks the model-list branch first. That branch queries the list, prints identifiers and returns immediately. The ticket-reading and result-file steps occur later in other branches.
| Selected branch | Ticket file read? | What reaches standard output? | Does this branch reach the result-file write? |
|---|---|---|---|
| Model list | No | Identifiers extracted from the list response | No: it returns earlier |
| Live draft | Yes | A mode label and draft text, with optional metadata | Yes, if preceding steps succeed and --output is set |
| Offline fixture | Bundled fixture is read instead | A labelled illustrative reply | Yes, if preceding steps succeed and --output is set |
This table is a reading of one implementation. It is not a promise that other CLIs interpret the same option names identically.
Distinguish acceptance from use
Consider the hypothetical combination --list-models --output list.json. The parser accepts those options because they are not competing mode flags. However, this implementation returns from the list branch before the output-file logic. The presence of --output therefore does not mean a file will be written on that path.
Similarly, supplying a ticket path in list mode does not make that branch inspect the ticket file. Parsing a value as a Path object and opening the corresponding file are separate operations.
The offline branch applies its own additional rule: a custom ticket path or an explicit model override is rejected there. That rule is implemented after parsing, rather than expressed entirely through the mutually exclusive mode group. A help-page review alone can miss this distinction.
Include work that happens before dispatch
The common setup also matters. This program converts the timeout configuration before choosing the branch. An invalid numeric value can therefore stop even an offline invocation before the fixture is displayed. No network request is needed to reach that conversion.
When documenting a CLI, mark three regions: parser validation, shared setup and branch-specific work. An option may be accepted in the first region, fail in the second or be unused in the third.
These observations can become a focused review list: should irrelevant option combinations be rejected, should help text explain early returns, and should configuration needed only by one mode be read later? Those are design questions for a future change, not features added here.
The practical output of source inspection is a branch map. It tells you where to look before you interpret a missing artifact or an unexpected error, while keeping unexecuted cases clearly separate from test results.
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