Warn if MPLBACKEND is invalid.

[anntzer]

I could be convinced to make this an error actually.

[NelleV]

Hi Anthony,
Do you mind having the warning list the possible values? Something like:

("%s is not a valid backend. Valid backends are %s." % (s, the list of backends)

Else it looks good. I'd also be fine raising an error.

[anntzer]

Actually the error message already contains the list of valid backends (you can see it by calling explicitly matplotlib.use("foo")); I'm just turning it into a warning.
Let's have a straw poll as to whether this should be an error or a warning?

[NelleV]

I finally understand what is going on.

I think a warning is fine for now: that's the current behaviour when the matplotlibrc ba kend is set to an unknown backend so it is at least consistent with the behaviour when using matplotlibrc.

In general, I think it might be worth be more strict and throw errors when options are set to wrong values, but this behaviour needs to be consistent.

[tacaswell]

I lean toward error as well.

[dopplershift]

"In the face of ambiguity, resist the temptation to guess." I'd say the zen argues for an error here.

[tacaswell]

Consensus seems to be to raise if invalid.

[anntzer]

That makes the patch even more trivial.

[NelleV]

Do you mind changing l. 1455 and l. 1105 (that will require more work) as well to have the error raised consistently.

[anntzer]

Changing l.1455 is probably a no-go (because other applications may be using matplotlib and using -dfoo themselves -- that's the whole reason -d$backend is deprecated I think).

Changing l.1105 comes down to switching the default value of fail_on_error to True, right?

[tacaswell]

I agree on 1455 having to stay as it is for being able to pass things through.

I ma not sure about L1105, that makes all rcparsing raise. That should probably get wider discussion.