Made _guess avoid a couple cases in which Tables are instantiated and then ignored - #2892

Haven't had a close look at this yet, but would it supersede #2880?

Oh yeah, I guess it would. I suppose either approach works, although it's probably optimal to just avoid instantiation altogether. That's a separate issue from the _apply_include_exclude_names fix anyway, so if we go with #2880 instead I could just reduce this PR to only that change.

This approach looks good to me. I'll look at the code more closely later, but on first inspection this approach is even better than the advertised "not instantiating a table". In fact it often avoids even reading the data for the table if the header doesn't parse into acceptable columns. Previously if you had a long CSV file then the basic reader would read the whole thing as a single column table with column name "a,b,c" and string data values like "1,2,3". From that perspective this is short-circuiting in a very good way (assuming my initial understanding of the code changes is correct).

True, and that's definitely an advantage of this approach. Of course this still doesn't get around the fact that the data still has to be read and split into lines (although not split into fields) for each reader...now that I think about it, this can be easily avoided by reading the (potential) file in _guess before going through each reader. I think I'll wait until #2716 is merged before opening a PR for that, just to avoid rebasing.

@AmrAS1 @taldcroft - what is the status of this PR?

I don't have any more changes in mind, but maybe @taldcroft is interested in further review?

@AmrAS1 - ok - btw, it looks like this needs rebasing

I'm OK with this, but it looks to be failing a couple of io.ascii tests including test_guess_with_format_arg.

Oops, I made a mistake while rebasing. This should be fixed now--waiting on Travis.

Ah, I see...there was an issue with the fast reader because FastBasic wasn't doing any strict_names/guessing checks. This should be fixed now--waiting on Travis again.