Polars parse multiple datetime format [duplicate]

Question

I have string column in polars dataframe with multiple datetime formats and I am using following code to convert datatype of column from string into datetime.

import polars as pl

df = pl.from_dict({'a': ["2025-11-17 11:51:44 UTC", "17.11.2025 11:51:44 UTC"]})

expr = (
    pl.when(pl.col("a").str.starts_with("2025"))
    .then(pl.col("a").str.to_datetime("%F %T %Z"))
    .otherwise(pl.col("a").str.to_datetime("%d.%m.%Y %T %Z"))
    .alias("a")
)

df.with_columns(expr)

However I am getting following error.


InvalidOperationError: conversion from `str` to `datetime[μs]` failed in column 'a' for 1 out of 2 values: ["17.11.2025 11:51:44 UTC"]

You might want to try:
- setting `strict=False` to set values that cannot be converted to `null`
- using `str.strptime`, `str.to_date`, or `str.to_datetime` and providing a format string

Is there anything missing I am here? Is is okay to pass expression with then and otherwise?

when/then/otherwise executes all the expressions first, and the results are then masked. You could use non-strict parsing for each format with coalesce: stackoverflow.com/questions/76010212 — jqurious
– jqurious, Commented Nov 26 at 12:38

sanya arora · Accepted Answer · 2025-11-26 12:52:03Z

0

import polars as pl

df = pl.from_dict({
    "a": ["2025-11-17 11:51:44 UTC", "17.11.2025 11:51:44 UTC"]
})

result = df.with_columns([
    pl.when(pl.col("a").str.starts_with("2025"))
    .then(
        pl.col("a").str.strptime(pl.Datetime, "%Y-%m-%d %H:%M:%S %Z", strict=False)
    )
    .otherwise(
        pl.col("a").str.strptime(pl.Datetime, "%d.%m.%Y %H:%M:%S %Z", strict=False)
    )
    .alias("a")
])

print(result)

you can use this

Why this works

strict=False prevents errors and sets invalid conversions to null.
But because each branch selects correct rows, you will not get nulls in practice.
str.strptime is evaluated only on the matched rows → no cross-format parsing.

answered Nov 26 at 12:52

sanya arora

1

New contributor

Sign up to request clarification or add additional context in comments.

1 Comment

jqurious yesterday

The "Why this works" explanation is not true. strptime is evaluated on all rows in both cases.

Collectives™ on Stack Overflow

Polars parse multiple datetime format [duplicate]

1 Answer 1

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Linked

Related