seems like there's an extra space after {0}

I find it a bit odd that you specify that "features[i] must be in a range", but then show the user the value of just "i" in the error message.

I can't show features[i] in this case, since i is out of range of the array. However, i is the value from features[i] that caused the error because of the looping above.

It was a bit of a tricky issue here; perhaps the error message needs further revision. Just to make sure I understand correctly, features is an array of indices for the features whose names correspond to the elements of feature_names at features[i]. My goal here is to work out a way to show a message such that a user who hasn't fully read the documentation would understand it. Along those lines, showing the value for len(feature_names) is also a bit confusing, since both seem like they should contain at most the n_features elements (with the further restriction that features[i] is also bound to [0, n_features)). If a user didn't specify a name for every feature, then it seems this value would be wrong.

ah I see. perhaps @tetraptych would have an opinion about this, since he had to go through the process of debugging it?

There is a bit of history here that's missing, because I picked this up from an abandoned PR. The error message that I have here is, more or less, the same as that proposed in the previous PR, #7609 .

I think the original error message is (partially) correct, we just need to add some more description to it about what's going on here. It seems to me that there are 2 conditions that features[i] has to satisfy:

1. 0 <= features[i] < n_features because you can't enumerate more features than exist
2. 'features[i]' < len(feature_names) because features[i] contains indices to feature_names

thanks for linking that to me. yeah, I agree that the error message is mostly there. It just seemed extremely ambiguous to me when I first read it (which made me question its value to a user), but it's obvious in hindsight after I've seen the code in question.

It's slightly confusing that the i in features[i] is not i as far as I can tell.

Why not features contains {} which is too large for feature_names? or something like that?
Maybe "All entries of features must be <len(feature_names)={}, got {}".format(len(feature_names), features[i])"

Unless I'm missing something, after the features is converted by the prior code, the looping below causes i to be a value from the original features array that was passed in (if features containing integer values).

for fxs in features:
            l = []
            # explicit loop so "i" is bound for exception below
            for i in fxs:
                l.append(feature_names[i])
            names.append(l)
    except IndexError:
    ...

From a clarity standpoint, part of the issue may come from reassigning the features parameter that was passed in to tmp_features. After that, features[i] in the code takes a different meaning than what a user thinks of when reading features[i] in an error message.

That's why my error message doesn't contain features[i] ;)

Ahh sorry about that... I had a forum update issue and didn't see your last message : D. I'll push out another update later today.