You can reject invalid filenames by passing them to the OS.

On Linux:

Path('\0').is_file() --> ValueError

On Windows:

Path('n!ul:').is_file() --> OSError
Path('nul').is_file() --> OSError

(too lazy to remember the os.path equivalents but you get the idea).

Invalid inputs could either trigger an exception (I guess there's some value in erroring early... perhaps?) or be ignored (i.e. the default filename remains). Or we could just let the error happen later and ignore all this... which may be just fine. (I would suggest NOT trying to sanitize the filename, that's a can of worms I don't want to open... see e.g. https://bugs.python.org/issue27827)

Edit: Upon further thought, given that the only point of the default filename is to initialize a GUI dialog, we should just let the dialog do its own normalization of it, and not worry at all about invalid names.

In fact I would suggest even keeping the spaces in. It's really not that bad.

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I think we should err on the side of letting users shoot them selves in the foot here.

This will change the default filename from 'figure_1.png' to 'Figure_1.png'. Not sure how big of a deal this is.

Or 'Figure 1.png'?

@anntzer, the code is still replacing spaces with underscores, as it should. I would strongly oppose supplying a default filename with spaces in it. Regarding the switch to starting the default with upper case, I would say "no big deal", so I merged it.

I agree with @efiring.
I think it would be sensible to raise a warning if the filename is changed though.

If we warned an changing the file name we would warn by default as the default figure name in 'Figure N', I think silently mapping ' ' -> '_' is an opinionated, but reasonable, thing to do. We do not muck with the names that come in via fig.save_fig, this is only the default that pops up in the save UI.

Backported to v2.0.x as 2262d53.