Why does pl.col('B').top_k_by('x', 1).over('A') cause a window size error in Polars?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I have the following DataFrame:

PYTHON
pl.DataFrame({
    'A': ['a0', 'a0', 'a1', 'a1'],
    'B': ['b1', 'b2', 'b1', 'b2'],
    'x': [0, 10, 5, 1]
})

I want to retrieve, for each group defined by column A, the value from column B that corresponds to the maximum value in column x (similar to the solution in this question).

Although I know that using pl.Expr.get() and pl.Expr.arg_max() works, I prefer to use pl.Expr.top_k_by(). However, when I set k to 1, I get the following error:

PYTHON
df.with_columns(
    pl.col.B.top_k_by("x", 1).over("A").alias("y")
)

PYTHON
ComputeError: the length of the window expression did not match that of the group

Error originated in expression: 'col("B").top_k_by([dyn int: 1, col("x")]).over([col("A")])'

Interestingly, it works fine for k = 2. Is this behavior a bug?

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The behavior you're encountering isn’t a bug—it’s due to the fact that top_k_by with k = 1 returns one value per group as a list, but when using an over‑window, Polars expects a value for each row in the group. In your case with k = 2, the output list length (2 per group) happens to match the group size (2 rows), so no error is raised.

To get the scalar value (the top value) per group, you can extract it from the list using an additional expression (for example, using arr.first()). For instance:

PYTHON
import polars as pl

df = pl.DataFrame({
    'A': ['a0', 'a0', 'a1', 'a1'],
    'B': ['b1', 'b2', 'b1', 'b2'],
    'x': [0, 10, 5, 1]
})

result = df.with_columns(
    pl.col("B")
      .top_k_by("x", 1)
      .over("A")
      .arr.first()  # Extract the scalar from the list
      .alias("y")
)

print(result)

This approach extracts the top value from the list so that each row gets the correct scalar value.

In summary, it’s not a bug but an expected consequence of how the expression returns lists in an over‑window operation.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by AstroAdventurer119 • 1 month ago

The error message produced when running your code without the window function gives is a bit more explicit and hints at a solution.

PYTHON
 df.with_columns(
     pl.col("B").top_k_by("x", 1)
)

PYTHON
InvalidOperationError: Series B, length 1 doesn't match the DataFrame height of 4

If you want expression: col("B").top_k_by([dyn int: 1, col("x")]) to be broadcasted, ensure it is a scalar (for instance by adding '.first()').

Especially, pl.Expr.first can be used to allow for proper broadcasting here.

PYTHON
df.with_columns(
     pl.col("B").top_k_by("x", 1).first().over("A").alias("y")
)

PLAINTEXT
shape: (4, 4)
┌─────┬─────┬─────┬─────┐
│ A   ┆ B   ┆ x   ┆ y   │
│ --- ┆ --- ┆ --- ┆ --- │
│ str ┆ str ┆ i64 ┆ str │
╞═════╪═════╪═════╪═════╡
│ a0  ┆ b1  ┆ 0   ┆ b2  │
│ a0  ┆ b2  ┆ 10  ┆ b2  │
│ a1  ┆ b1  ┆ 5   ┆ b1  │
│ a1  ┆ b2  ┆ 1   ┆ b1  │
└─────┴─────┴─────┴─────┘

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Why does pl.col('B').top_k_by('x', 1).over('A') cause a window size error in Polars?

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