polars full join default null value

Question

https://docs.pola.rs/api/python/stable/reference/dataframe/api/polars.DataFrame.join.html

Can I specify the default NULL value for full joins? Like 0?

Answer 1

The join method does not currently have an option for setting a default value for nulls. However, there is an easy way to accomplish this.

Let's say we have this data:

import polars as pl

df1 = pl.DataFrame({"key": ["a", "b", "d"], "var1": [1, 1, 1]})

df2 = pl.DataFrame({"key": ["a", "b", "c"], "var2": [2, 2, 2]})

df1.join(df2, on="key", how="full")

shape: (4, 4)
┌──────┬──────┬───────────┬──────┐
│ key  ┆ var1 ┆ key_right ┆ var2 │
│ ---  ┆ ---  ┆ ---       ┆ ---  │
│ str  ┆ i64  ┆ str       ┆ i64  │
╞══════╪══════╪═══════════╪══════╡
│ a    ┆ 1    ┆ a         ┆ 2    │
│ b    ┆ 1    ┆ b         ┆ 2    │
│ null ┆ null ┆ c         ┆ 2    │
│ d    ┆ 1    ┆ null      ┆ null │
└──────┴──────┴───────────┴──────┘

To create a different value for the null values, simply use this:

df1.join(df2, on="key", how="full").with_columns(pl.exclude("key", "key_right").fill_null(0))

shape: (4, 4)
┌──────┬──────┬───────────┬──────┐
│ key  ┆ var1 ┆ key_right ┆ var2 │
│ ---  ┆ ---  ┆ ---       ┆ ---  │
│ str  ┆ i64  ┆ str       ┆ i64  │
╞══════╪══════╪═══════════╪══════╡
│ a    ┆ 1    ┆ a         ┆ 2    │
│ b    ┆ 1    ┆ b         ┆ 2    │
│ null ┆ 0    ┆ c         ┆ 2    │
│ d    ┆ 1    ┆ null      ┆ 0    │
└──────┴──────┴───────────┴──────┘