fix(python): Issue correct PolarsInefficientMapWarning for lshift/rshift operations

[MarcoGorelli]

closes #12384

[cmdlineluser]

Perhaps #12090 (comment) could be suggested as an alternative.

As a workaround for now you can replace a << b with a * 2**b and a >> b with a.floordiv(2**b).

Consider alternative proposed by cmdlineuser

[MarcoGorelli]

nice! I'll give this a go

[MarcoGorelli]

That seems to work wonderfully, thanks!

Demo with the originally reported issue:

In [3]: 
   ...: df = pl.DataFrame({'ipvers': [4, 4], 'prefix': ['172.16.2.0/24', '172.16.2.101/32'], 'prefixlen': [24, 32]})
   ...: (
   ...:     df
   ...:     .filter(pl.col('ipvers') == 4)
   ...:     .with_columns(
   ...:         netmask = pl.col('prefixlen')
   ...:         .map_elements(lambda x: (0xffffffff >> (32 - x)) &
   ...:                       0xffffffff)
   ...:     )
   ...: )
<ipython-input-3-cf7f706f16ef>:7: PolarsInefficientMapWarning: 
Expr.map_elements is significantly slower than the native expressions API.
Only use if you absolutely CANNOT implement your logic otherwise.
In this case, you can replace your `map_elements` with the following:
  - pl.col("prefixlen").map_elements(lambda x: ...)
  + (4294967295 / (2**(32 - pl.col("prefixlen")))).cast(pl.Int64) & 4294967295

  .map_elements(lambda x: (0xffffffff >> (32 - x)) &
Out[3]: 
shape: (2, 4)
┌────────┬─────────────────┬───────────┬────────────┐
│ ipvers ┆ prefix          ┆ prefixlen ┆ netmask    │
│ ---    ┆ ---             ┆ ---       ┆ ---        │
│ i64    ┆ str             ┆ i64       ┆ i64        │
╞════════╪═════════════════╪═══════════╪════════════╡
│ 4      ┆ 172.16.2.0/24   ┆ 24        ┆ 16777215   │
│ 4      ┆ 172.16.2.101/32 ┆ 32        ┆ 4294967295 │
└────────┴─────────────────┴───────────┴────────────┘

In [4]: 
   ...: df = pl.DataFrame({'ipvers': [4, 4], 'prefix': ['172.16.2.0/24', '172.16.2.101/32'], 'prefixlen': [24, 32]})
   ...: (
   ...:     df
   ...:     .filter(pl.col('ipvers') == 4)
   ...:     .with_columns(
   ...:         netmask = (4294967295 / (2**(32 - pl.col("prefixlen")))).cast(pl.Int64) & 4294967295
   ...:     )
   ...: )
Out[4]: 
shape: (2, 4)
┌────────┬─────────────────┬───────────┬────────────┐
│ ipvers ┆ prefix          ┆ prefixlen ┆ netmask    │
│ ---    ┆ ---             ┆ ---       ┆ ---        │
│ i64    ┆ str             ┆ i64       ┆ i64        │
╞════════╪═════════════════╪═══════════╪════════════╡
│ 4      ┆ 172.16.2.0/24   ┆ 24        ┆ 16777215   │
│ 4      ┆ 172.16.2.101/32 ┆ 32        ┆ 4294967295 │
└────────┴─────────────────┴───────────┴────────────┘

[stinodego]

Great, thanks Marco!