Check for config existence

[LuminousPath]

Added lines at beginning of run_async to assign variables to cmd and shell_cmd if they exist in the config. In response to #248

[randy3k]

It will disallow overriding the variables via the function arguments. We will need to put the code around here https://github.com/randy3k/Terminus/blob/master/terminus/core.py#L148

[LuminousPath]

you can't check for whether cmd and shell_cmd exist before they're potentially assigned values, as it stands now, there are potential occasions when both cmd and shell_cmd are both null and this function doesn't work as intended.

I can add some gating if not cmd and if not shell_cmd lines to allow for overridding, but you have to put the assignment earlier in the function to be able to use shell_cmd the way that its written out here.

I'll make my proposed changes to the PR and you can take a look :)

[randy3k]

Unfortunately, it won't work either. The not cmd in https://github.com/LuminousPath/Terminus/blob/patch-1/terminus/core.py#L141 will never be triggered because cmd has a fallback value.

[randy3k]

We need to refactor the logic a little bit. Maybe also to rename some variables to avoid confusion.

Let a_xxx be the values from the arguments and c_xxx be the values from the config.

There are four variables a_cmd, a_shell_cmd, c_cmd, c_shell_cmd. Note that the default config and most other configs have non empty values of c_cmd.

The rules are:

1. if a_cmd or a_shell_cmd is non empty, ignore both c_cmd and c_shell_cmd.
2. raise error if both a_cmd and a_shell_cmd are specified
3. raise error if both c_cmd and c_shell_cmd are specified
4. a_cmd dominates c_cmd and a_shell_cmd dominates c_shell_cmd.

The following table lists the expected results.

a_cmd	a_shell_cmd	c_cmd	c_shell_cmd	result
0	0	0	0	error: need cmd or shell_cmd but not both
0	0	0	1	c_shell_cmd
0	0	1	0	c_cmd
0	0	1	1	error: need cmd or shell_cmd but not both
0	1	0	0	error: need cmd or shell_cmd but not both
0	1	0	1	a_shell_cmd
0	1	1	0	a_shell_cmd
0	1	1	1	error: need cmd or shell_cmd but not both
1	0	0	0	error: need cmd or shell_cmd but not both
1	0	0	1	a_cmd
1	0	1	0	a_cmd
1	0	1	1	error: need cmd or shell_cmd but not both
1	1	0	0	error: need cmd or shell_cmd but not both
1	1	0	1	error: need cmd or shell_cmd but not both
1	1	1	0	error: need cmd or shell_cmd but not both
1	1	1	1	error: need cmd or shell_cmd but not both

[LuminousPath]

I see what you're getting at, how does the following construct sound:

cmd = cmd or config["cmd"] if "cmd" in config else None
shell_cmd = shell_cmd or config["shell_cmd"] if "shell_cmd" in config else None

if not (bool(cmd) ^ bool(shell_cmd)):
  raise ValueError("need cmd or shell_cmd but not both")