BUG: interpolate with ffill/bfill method, axis=1, multi-block

[jbrockmendel]

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Reproducible Example

import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.randn(3, 3), columns=["A", "B", "C"])
df["D"] = np.nan
df["E"] = 1.0

>>> df.interpolate(method="ffill", axis=1)
<stdin>:1: FutureWarning: DataFrame.interpolate with method=ffill is deprecated and will raise in a future version. Use obj.ffill() or obj.bfill() instead.
          A         B         C   D    E
0 -0.656239  0.898067  0.842284 NaN  1.0
1 -0.914892 -0.018121 -0.382542 NaN  1.0
2  1.818251 -0.148962  0.550328 NaN  1.0

Issue Description

This operates block-by-block and fails to fill across blocks.

I'd expect this to behave like df.T.interpolate(method="ffill").T That is what we do in NDFrame._pad_or_backfill. Note that it doesn't play nicely with the "downcast" keyword.

This value for "method" is deprecated, but still might be worth doing something about in the interim.

Expected Behavior

NA

Installed Versions

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