[Python] Groupby sum string

In pandas, I can do

In [33]: df = pd.DataFrame({'a': [1, 1, 2], 'b': ['foo', 'bar', 'foo']})

In [34]: df
Out[34]:
a b
0 1 foo
1 1 bar
2 2 foo

In [35]: df.groupby('a')['b'].sum()
Out[35]:
a
1 foobar
2 foo
Name: b, dtype: object


and have the strings concatenated when I do groupby.sum

In polars, however:

In [36]: df = pl.DataFrame({'a': [1, 1, 2], 'b': ['foo', 'bar', 'foo']})

In [37]: df
Out[37]:
shape: (3, 2)
┌─────┬─────┐
│ a ┆ b │
│ --- ┆ --- │
│ i64 ┆ str │
╞═════╪═════╡
│ 1 ┆ foo │
│ 1 ┆ bar │
│ 2 ┆ foo │
└─────┴─────┘

In [38]: df.group_by('a').agg(pl.col('b').sum())
Out[38]:
shape: (2, 2)
┌─────┬──────┐
│ a ┆ b │
│ --- ┆ --- │
│ i64 ┆ str │
╞═════╪══════╡
│ 2 ┆ null │
│ 1 ┆ null │
└─────┴──────┘


Is there a way to concatenate all the strings in each group in polars?

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