Loosen restrictions on shebang.

[lorencarvalho]

This commit loosens shiv's requirements when constructing a shebang
line: rather than enforcing that the target interpreter must exist, we
simply accept what the user provides.

We no longer append -sE to the shebang as these arguments can be
provided to the value given to the --python argument.

Shiv will now exit(1) if the resulting shebang would exceed the BIN_PRM
restrictions present on most unix-y OSes.

Fixes #17

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After merging I'll cut a new release (0.0.26)

[warsaw]

What do you think about keeping -sE if sys.executable (the default) is used?

[lorencarvalho]

@warsaw I thought about that too, but ultimately felt like it would be confusing... say someone didn't want -sE, they'd have to explicitly set --python to be (effectively) the same as sys.executable. I feel like that would be weird 😕

[warsaw]

True, but OTOH if they just want the defaults (which should be the most common case, right?) they'll have to use --python to add the isolation.

I guess it's a 6 vs 1/2 dozen thing, so as long as the change is well communicated and the documentation is up to date, I don't have a strong opinion either way.

[warsaw]

Is this still true then?

[lorencarvalho]

Notice that I added "internally we..." 😉

[lorencarvalho]

I guess it's a 6 vs 1/2 dozen thing, so as long as the change is well communicated and the documentation is up to date, I don't have a strong opinion either way.

Yah, I actually feel like the behavior is more obvious now, since it's non-typical for folks to be using those flags.

[warsaw]

Re: Internally - will an outside consumer know what that means? Maybe just suggest that for better isolation from the system's and user's Python environment, these flags can be used?

Re: default use of the flags - I'm not sure whether it's non-typical because folks don't think about it, or whether they really do no want isolation in general.

None of this is a blocker for me though. Just thoughts about usability.

[lorencarvalho]

@warsaw I updated the wording in the history doc to be more clear, but rather than try and tack on -sE I think it will be better to leave that up to individual users. That way using --python vs not supplying it will be similar enough to not be confusing. (and imagine if someone supplied -sE without knowing that we add it by default! confusing!).

[dan-blanchard]

I did not actually know about -sE until looking at shiv, and thought it was a cool thing that it had. It might be nice to have an --isolated flag that appended that to the shebang.