Cast column of type list[] to str in polars

Question

Currently, using the polars' cast() method on columns of type list[] is not supported. It throws:

ComputeError: Cannot cast list type

Before I do as usual (use rows(), or convert to pandas, or work with apply()). Is there any trick or best practice to convert polars list[] to strings?

Here a quick snippet for you to replicate the error

df = pl.from_dict({'foo': [[1,2,3]], 'bar': 'Hello World'})
print(df)
'''
shape: (1, 2)
┌───────────┬─────────────┐
│ foo       ┆ bar         │
│ ---       ┆ ---         │
│ list[i64] ┆ str         │
╞═══════════╪═════════════╡
│ [1, 2, 3] ┆ Hello World │
└───────────┴─────────────┘
'''
df['foo'].cast(str)
# this other workaround wont work neither
df.select([pl.col('foo').str])

Here is what i expect to see:

'''
shape: (1, 2)
┌───────────┬─────────────┐
│ foo       ┆ bar         │
│ ---       ┆ ---         │
│ str       ┆ str         │
╞═══════════╪═════════════╡
│"[1, 2, 3]"┆ Hello World │
└───────────┴─────────────┘
'''

Answer 1

You can use the datatype pl.List(pl.String)

df.with_columns(pl.col("foo").cast(pl.List(pl.String)))

shape: (1, 2)
┌─────────────────┬─────────────┐
│ foo             | bar         │
│ ---             | ---         │
│ list[str]       | str         │
╞═════════════════╪═════════════╡
│ ["1", "2", "3"] | Hello World │
└─────────────────┴─────────────┘

To create an actual string - perhaps:

df.with_columns("[" + 
   pl.col("foo").cast(pl.List(pl.String)).list.join(", ") 
   + "]"
)

There's also pl.format()

df.with_columns(
   pl.format("[{}]",
      pl.col("foo").cast(pl.List(pl.String)).list.join(", ")))

shape: (1, 3)
┌───────────┬─────────────┬───────────┐
│ foo       | bar         | literal   │
│ ---       | ---         | ---       │
│ list[i64] | str         | str       │
╞═══════════╪═════════════╪═══════════╡
│ [1, 2, 3] | Hello World | [1, 2, 3] │
└───────────┴─────────────┴───────────┘