Expression contexts

You cannot use an expression anywhere. An expression needs a context, the available contexts are:

• selection: df.select([..])
• groupy aggregation: df.groupby(..).agg([..])
• hstack/ add columns: df.with_columns([..])

Syntactic sugar

The reason for such a context, is that you actually are using the Polars lazy API, even if you use it in eager. For instance this snippet:

df.groupby("foo").agg([pl.col("bar").sum()])

actually desugars to:

(df.lazy().groupby("foo").agg([pl.col("bar").sum()])).collect()

This allows Polars to push the expression into the query engine, do optimizations, and cache intermediate results.

Select context

In the select context the selection applies expressions over columns. The expressions in this context must produce Series that are all the same length or have a length of 1.

A Series of a length of 1 will be broadcasted to match the height of the DataFrame. Note that a select may produce new columns that are aggregations, combinations of expressions, or literals.

Selection context

out = df.select(
    [
        pl.sum("nrs"),
        pl.col("names").sort(),
        pl.col("names").first().alias("first name"),
        (pl.mean("nrs") * 10).alias("10xnrs"),
    ]
)
print(out)

shape: (5, 4)
┌─────┬───────┬────────────┬────────┐
│ nrs ┆ names ┆ first name ┆ 10xnrs │
│ --- ┆ ---   ┆ ---        ┆ ---    │
│ i64 ┆ str   ┆ str        ┆ f64    │
╞═════╪═══════╪════════════╪════════╡
│ 11  ┆ null  ┆ foo        ┆ 27.5   │
├╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
│ 11  ┆ egg   ┆ foo        ┆ 27.5   │
├╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
│ 11  ┆ foo   ┆ foo        ┆ 27.5   │
├╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
│ 11  ┆ ham   ┆ foo        ┆ 27.5   │
├╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
│ 11  ┆ spam  ┆ foo        ┆ 27.5   │
└─────┴───────┴────────────┴────────┘

Add columns

Adding columns to a DataFrame using with_columns is also the selection context.

df = df.with_columns(
    [
        pl.sum("nrs").alias("nrs_sum"),
        pl.col("random").count().alias("count"),
    ]
)
print(out)

shape: (5, 6)
┌──────┬───────┬──────────┬────────┬─────────┬───────┐
│ nrs  ┆ names ┆ random   ┆ groups ┆ nrs_sum ┆ count │
│ ---  ┆ ---   ┆ ---      ┆ ---    ┆ ---     ┆ ---   │
│ i64  ┆ str   ┆ f64      ┆ str    ┆ i64     ┆ u32   │
╞══════╪═══════╪══════════╪════════╪═════════╪═══════╡
│ 1    ┆ foo   ┆ 0.154163 ┆ A      ┆ 11      ┆ 5     │
├╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ 2    ┆ ham   ┆ 0.74     ┆ A      ┆ 11      ┆ 5     │
├╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ 3    ┆ spam  ┆ 0.263315 ┆ B      ┆ 11      ┆ 5     │
├╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null ┆ egg   ┆ 0.533739 ┆ C      ┆ 11      ┆ 5     │
├╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ 5    ┆ null  ┆ 0.014575 ┆ B      ┆ 11      ┆ 5     │
└──────┴───────┴──────────┴────────┴─────────┴───────┘

Groupby context

In the groupby context expressions work on groups and thus may yield results of any length (a group may have many members).

out = df.groupby("groups").agg(
    [
        pl.sum("nrs"),  # sum nrs by groups
        pl.col("random").count().alias("count"),  # count group members
        # sum random where name != null
        pl.col("random").filter(pl.col("names").is_not_null()).sum().suffix("_sum"),
        pl.col("names").reverse().alias(("reversed names")),
    ]
)
print(out)

shape: (3, 5)
┌────────┬──────┬───────┬────────────┬────────────────┐
│ groups ┆ nrs  ┆ count ┆ random_sum ┆ reversed names │
│ ---    ┆ ---  ┆ ---   ┆ ---        ┆ ---            │
│ str    ┆ i64  ┆ u32   ┆ f64        ┆ list [str]     │
╞════════╪══════╪═══════╪════════════╪════════════════╡
│ B      ┆ 8    ┆ 2     ┆ 0.263315   ┆ [null, "spam"] │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ C      ┆ null ┆ 1     ┆ 0.533739   ┆ ["egg"]        │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ A      ┆ 3    ┆ 2     ┆ 0.894213   ┆ ["ham", "foo"] │
└────────┴──────┴───────┴────────────┴────────────────┘

Besides the standard groupby, groupby_dynamic, and groupby_rolling are also entrances to the groupby context.