allow_exact_matches parameter in polars.DataFrame.join_asof

[wangkev]

Problem description

pandas.merge_asof has an allow_exact_matches parameter. Can we add a similar parameter to polars.DataFrame.join_asof?

[cmdlineluser]

This did come up on stackoverflow a while ago.

https://stackoverflow.com/questions/75068390/is-it-possible-to-disallow-exact-matches-on-join-asof

The suggestion was an anti-join beforehand to get:

shape: (3, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 2   ┆ 1   │
│ 4   ┆ 3   │
│ 6   ┆ 5   │
└─────┴─────┘

allow_exact_matches=False is slightly different though, it doesn't discard the exact match, it just considers it a non-match and continues searching.

pl.from_pandas(
   pd.merge_asof(
      df1.to_pandas(), 
      df2.to_pandas(), 
      direction="backward",
      allow_exact_matches=False
   )
)

shape: (7, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ i64 ┆ f64  │
╞═════╪══════╡
│ 1   ┆ null │
│ 2   ┆ 1.0  │
│ 3   ┆ 1.0  │
│ 4   ┆ 3.0  │
│ 5   ┆ 3.0  │
│ 6   ┆ 5.0  │
│ 7   ┆ 5.0  │
└─────┴──────┘