Please avoid calling asarray. This adds unnecessary overhead and does not guarantee that we get an array back if None is passed into it.

You can use assert if it is guaranteed, that left_indexer is not None here. Adjusting the return type of _get_join_info would be even better

@phofl Hi Patrick. I see what you mean about the overhead of np.asarray(). None seems to come from L957 and L962 in _get_join_info which comes from return value of _left_join_on_index L2071. Not sure what a good alternative np.ndarray alternative to None would be?

can we reason about the code s.t. we know we can never get here with left_indexer being None? if so, then can do a left_indexer = cast(np.ndarray, left_indexer). otherwise

if left_indexer is None:
     raise AppropriateException("Useful message")

@jbrockmendel thank you for the suggestion. I was unsure if it was okay to use typing.cast() Do these changes look okay?

is this ever reached? is it because the user passed something invalid?

@jbrockmendel I was wondering the same thing so I could write a better error message. Tbh I am not sure. I need to spend some more time to see how we get to this point

OK. this probably shouldn't have been merged until this was sorted out. NBD bc it isn't actively harmful here, but id appreciate it if you'd follow up on this

Will do

@jbrockmendel I am having a hard time finding how to get the exception to be thrown. I am working on it, but it seems it will take me a little time.

it seems that this exception situation only happens when the fill_method=='ffill'.
Is the fill_method argument only in pandas.merge_ordered()?

Is the fill_method argument only in pandas.merge_ordered()?

looks like it, yah. i dont know this part of the code particularly well so im just checking docstrings

@jbrockmendel i do not think we can ever get to the TyperError thrown on L1651 because the left_indexer is only None when L956 has self.right_index as True. But, pandas.merge_ordered() does not have a right_index argument. I might be misunderstanding what is happening but I do not understand how you can set the fill_method='ffill' that i think is only an argument for merge_ordered but it has no right_index argument?

I think you're right. In this case, the check should go right after the _get_join_info call since i dont think it depends on anything after it (double-check me on this pls)

If im looking at this correctly, i think we can also rule out the cases where either left_join_indexer or right_join_indexer is None?

I think it is possible for left_indexer and right_indexer to be None, but not when fill_method == "ffill".