Row descriptions
Create readable labels by combining several columns in a projection.
Polars · Function
pl.format
Creates a Polars string expression by filling a template with literal values or expression results. It supports sequential, indexed, and column-name placeholders.
How it works
pl.format creates a string expression from a template and replacement expressions. Placeholders can consume arguments by position or refer directly to column names, and the resulting expression can be used in contexts such as select or with_columns.
Syntax
polars.format(*f_string: str, **args: IntoExpr) → ExprParameters
| Name | Type | Description |
|---|---|---|
f_string | str | Template string containing {}, indexed, or named placeholders. |
args | IntoExpr | Expressions that supply values for the template placeholders. |
Returns
Expr
Key points
- The function builds a string-valued Polars expression from a template.
- Use
{}placeholders to consume supplied arguments from left to right. - Indexed placeholders let the template select a specific argument by position.
- ASCII identifier placeholders can refer directly to columns by name.
- Replacement values are expressions, allowing formatting to be evaluated per row.
Common mistakes
- Literal braces must be doubled in the format string, such as
{{and}}. - A null in any input expression produces a null formatted value for that row.
Example: Format row values
import polars as pl
df = pl.DataFrame({"name": ["Ada", "Linus"], "age": [36, 55]})
result = df.select(
pl.format("{} is {} years old", pl.col("name"), pl.col("age")).alias("description")
)
print(result)Builds one formatted string per row from two column expressions.
When to use it
Composite keys
Construct standardized identifiers from columns and fixed text.
Display strings
Embed column values in report-friendly messages.
Interview questions
QWhat does pl.format return, and what are its main inputs?
pl.format returns a Polars Expr that formats expression values as strings. Its first argument is the template, and subsequent keyword or positional arguments supply placeholder values.
Follow-up: How can a template address arguments without relying on their order? Use indexed placeholders such as {0}, or named placeholders that refer directly to column names.
QWhat placeholder forms does pl.format support?
An empty {} consumes the next argument, while {index} selects the argument at that position. A placeholder containing an ASCII identifier refers directly to a column name.
Follow-up: What happens when a referenced input is null? The result is null for that row, rather than a partially formatted string.
QHow are literal braces represented in a pl.format template?
The template must escape literal braces by doubling them. For example, a JSON-like literal can use {{"json": 42}} in the format string.
Follow-up: How would you preserve row-wise behavior? Keep the inputs as Polars expressions so each row is evaluated independently.
Related
polars.Exprpolars.colpolars.DataFrame.selectpolars.DataFrame.with_columns