Add pl.lit(list)

[mcrumiller]

Problem description

Stems from #6596. Literal lists would be very useful for series with dtype pl.List. Example:

import polars as pl

lit = pl.lit([1, 1])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\430015439\AppData\Local\Programs\Python\Python39\lib\site-packages\polars\internals\lazy_functions.py", line 1176, in lit
    return pli.wrap_expr(pylit(item, allow_object))
ValueError: could not convert value '[1, 1]' as a Literal

In a more useful context, it would be inferred, e.g:

df = pl.DataFrame({
    'a': [
        [1, 1],
        [2, 2]
    ]
})
df.filter(pl.col('a') == [1, 1])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\430015439\AppData\Local\Programs\Python\Python39\lib\site-packages\polars\internals\expr\expr.py", line 251, in __eq__
    return wrap_expr(self._pyexpr.eq(self._to_expr(other)._pyexpr))
  File "C:\Users\430015439\AppData\Local\Programs\Python\Python39\lib\site-packages\polars\internals\expr\expr.py", line 163, in _to_expr
    return pli.lit(other)
  File "C:\Users\430015439\AppData\Local\Programs\Python\Python39\lib\site-packages\polars\internals\lazy_functions.py", line 1176, in lit
    return pli.wrap_expr(pylit(item, allow_object))

[deanm0000]

The problem is that there aren't literal lists in dataframes

For instance, taking your example type(df[0,0]) is a polars.internals.series.series.Series not a list.

[mcrumiller]

@deanm0000 that makes sense, but since a pl.lit(pl.Series) is possible, surely this could be hidden from the user and work as intended?

[deanm0000]

touche.

So it seems what you're after is being able to do

df.with_columns(pl.Series([1,1]).alias('c')).filter(pl.col('a')==pl.col('c')) 

but without the, seemingly, superfluous with_columns
makes sense.

[MarcoGorelli]

I'd have thought concat_list would work here, but it looks like it returns both rows 🤔

In [1]: df = pl.DataFrame({
   ...:     'a': [
   ...:         [1, 1],
   ...:         [2, 2]
   ...:     ]
   ...: })
   ...: df.filter(pl.col('a') == pl.concat_list([1, 1]))
Out[1]:
shape: (2, 1)
┌───────────┐
│ a         │
│ ---       │
│ list[i64] │
╞═══════════╡
│ [1, 1]    │
│ [2, 2]    │
└───────────┘

In [2]: df = pl.DataFrame({
   ...:     'a': [
   ...:         [1, 1],
   ...:         [2, 2]
   ...:     ]
   ...: })
   ...: df.filter(pl.col('a') == pl.concat_list([1, 9]))
Out[2]:
shape: (0, 1)
┌───────────┐
│ a         │
│ ---       │
│ list[i64] │
╞═══════════╡
└───────────┘