`merge_ordered()` doesn't validate `fill_method`

[wjandrea]

The docs say fill_method is the "Interpolation method for data", but I just tried pd.merge_ordered(..., fill_method='linear') and it didn't interpolate.

Looking closer at the docs, fill_method can only be either 'ffill' or None, but apparently that's not actually checked; for example, fill_method='carrot' also works.

fill_method : {'ffill', None}, default None
Interpolation method for data.

Looks like this code is responsible:

pandas/pandas/core/reshape/merge.py

Lines 1859 to 1868 in a167f13

	if self.fill_method == "ffill":
	if left_indexer is None:
	raise TypeError("left_indexer cannot be None")
	left_indexer = cast("npt.NDArray[np.intp]", left_indexer)
	right_indexer = cast("npt.NDArray[np.intp]", right_indexer)
	left_join_indexer = libjoin.ffill_indexer(left_indexer)
	right_join_indexer = libjoin.ffill_indexer(right_indexer)
	else:
	left_join_indexer = left_indexer
	right_join_indexer = right_indexer

It should probably be changed to something like:

if self.fill_method == "ffill": 
    if left_indexer is None:
    ...
elif self.fill_method is None: 
    left_join_indexer = left_indexer
    right_join_indexer = right_indexer
else:
    raise ValueError(f"fill_method must be one of ['ffill']. Got '{self.fill_method}' instead.")

(ValueError text based on equivalent df.interpolate error)

[tshehadeh9]

Hi, new contributor here -- can I work on this task?

[lithomas1]

PRs would be welcome for this.