[SOLVED] Python Polars: How to add columns in one lazyframe to another lazyframe?

Python Polars: How to add columns in one lazyframe to another lazyframe?

I have a Polars LazyFrame and would like to add to it columns from another LazyFrame. The two LazyFrames have the same number of rows and different columns.

I have tried the following, which doesn't work as with_columns expects an iterable.

def append_columns(df:pl.LazyFrame):
    df2 = pl.LazyFrame([1,2])
    return df.with_columns(df2)

Solution

For this, pl.concat setting how="horizontal" might be used.

import polars as pl

df = pl.LazyFrame({
    "a": [1, 2, 3],
    "b": [4, 5, 6],
})

other = pl.LazyFrame({
    "c": [9, 10, 11],
    "d": [12, 13, 14],
    "e": [15, 16, 17],
})

result = pl.concat((df, other.select("c", "d")), how="horizontal")

The resulting pl.LazyFrame then looks as follows.

shape: (3, 4)
┌─────┬─────┬─────┬─────┐
│ a   ┆ b   ┆ c   ┆ d   │
│ --- ┆ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╪═════╡
│ 1   ┆ 4   ┆ 9   ┆ 12  │
│ 2   ┆ 5   ┆ 10  ┆ 13  │
│ 3   ┆ 6   ┆ 11  ┆ 14  │
└─────┴─────┴─────┴─────┘