Make sure that setting `PY_PYTHON` to an invalid value is somehow discoverable about python-launcher

[brettcannon]

Here is the solution I'm planning on unless people object.

Without PYLAUNCH_DEBUG.

❯ PY_PYTHON3=3.8.10 cargo run -- -3 --version
   Compiling python-launcher v0.16.0 (/Users/brettcannon/Repositories/brettcannon/python-launcher)
    Finished dev [unoptimized + debuginfo] target(s) in 1.84s
     Running `target/debug/py -3 --version`
Error parsing '8.10' as an integer: invalid digit found in string

With PYLAUNCH_DEBUG

❯ PY_PYTHON3=3.8.10 PYLAUNCH_DEBUG=1 cargo run -- -3 --version
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/py -3 --version`
Checking the PY_PYTHON3 environment variable
PY_PYTHON3 = '3.8.10'
Error parsing '8.10' as an integer: invalid digit found in string

You can see the referenced PR for the planned documentation changes.

[brettcannon]

Should probably also make sure the help and man page are clear on the format.

[brettcannon]

No error in the normal case.

❯ PY_PYTHON=3.8.10 py --version
Python 3.9.5

Nothing obvious in the debug log case either:

❯ PYLAUNCH_DEBUG=1 PY_PYTHON=3.8.10 py --version
Checking for VIRTUAL_ENV environment variable
Searching for a venv in /private/tmp and parent directories
Checking /private/tmp/.venv/bin/python
Checking /private/.venv/bin/python
Checking /.venv/bin/python
Checking "--version" for a shebang
Checking for PY_PYTHON environment variable
PY_PYTHON set to 3.8.10
Checking PATH environment variable
PATH: ["/usr/local/sbin", "/Users/brettcannon/.local/bin", "/Users/brettcannon/.cargo/bin", "/usr/local/bin", "/usr/local/opt/[email protected]/bin", "/usr/local/opt/[email protected]/bin", "/usr/bin", "/bin", "/usr/sbin", "/sbin"]
Found executables: ["/usr/local/opt/[email protected]/bin/python3.8", "/usr/bin/python2.7", "/usr/local/opt/[email protected]/bin/python3.7", "/usr/local/bin/python3.9"]
Executing /usr/local/bin/python3.9 with ["--version"]

Neither --help or the man page explicitly state what the format should be.

[brettcannon]

OK, there are a couple of ways to play this. Let me know opinions.

Print a message with PYLAUNCH_DEBUG, but continue on

❯ PY_PYTHON3=3.8.10 PYLAUNCH_DEBUG=1 cargo run -- -3 --version
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/py -3 --version`
Checking for PY_PYTHON3 environment variable
PY_PYTHON3 = '3.8.10'
'3.8.10' cannot be parsed (must be formatted as 'X.Y')
Checking PATH environment variable
PATH: ["/Users/brettcannon/.cargo/bin", "/usr/local/sbin", "/Users/brettcannon/.local/bin", "/Users/brettcannon/.cargo/bin", "/usr/local/bin", "/usr/local/opt/[email protected]/bin", "/usr/local/opt/[email protected]/bin", "/usr/bin", "/bin", "/usr/sbin", "/sbin"]
Found executables: ["/usr/local/bin/python3.9", "/usr/local/opt/[email protected]/bin/python3.8", "/usr/local/opt/[email protected]/bin/python3.7", "/usr/bin/python2.7"]
Executing /usr/local/bin/python3.9 with ["--version"]
Python 3.9.5

Print a message about the error, but continue on

❯ PY_PYTHON3=3.8.10 cargo run -- -3 --version
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/py -3 --version`
PY_PYTHON3 cannot be parsed; must be formatted as 'X.Y', not '3.8.10'
Python 3.9.5

Error message shows up, more details with PYLAUNCH_DEBUG

I can enhance the initial error message to suggest running with PYLAUNCH_DEBUG for more details.

❯ PY_PYTHON3=3.8.10 cargo run -- -3 --version
   Compiling python-launcher v0.16.0 (/Users/brettcannon/Repositories/brettcannon/python-launcher)
    Finished dev [unoptimized + debuginfo] target(s) in 1.83s
     Running `target/debug/py -3 --version`
Error parsing a version component: invalid digit found in string

python-launcher on  main [!] is 📦 v0.16.0 via 🐍 v3.9.5 via 🦀 v1.52.1 
❯ PY_PYTHON3=3.8.10 PYLAUNCH_DEBUG=1 cargo run -- -3 --version
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/py -3 --version`
Checking the PY_PYTHON3 environment variable
Error parsing a version component: invalid digit found in string

Error message that is custom to this situation

Not shown because I have not coded it up. 😄

[treyhunner]

I'd vote for either the second or third one because it would have been easier for me to quickly guess that the format wasn't right. 👍

I assume having an invalid PY_PYTHON variable wouldn't have any use so I see no reason not to at least note the error before continuing on.