Polars to_date conversion from str to date fails with proper format

by

Summary

The issue at hand is the failure to convert a string column in a Polars dataframe to a date format using the to_date method. The column values are in the format %YW%W, which represents the year and week number. Despite using the correct format string, the conversion fails, resulting in an InvalidOperationError.

Root Cause

The root cause of this issue is the incorrect usage of the format string in the to_date method. The format string "%Y%W" is not sufficient to parse the date string correctly. The main causes are:

  • Incorrect format string
  • Lack of consideration for the week number in the format string
  • Insufficient error handling

Why This Happens in Real Systems

This issue occurs in real systems due to:

  • Inadequate input validation: Failing to validate the input data before attempting to convert it to a date format.
  • Incorrect format string usage: Using an incorrect or incomplete format string, leading to parsing errors.
  • Lack of error handling: Not implementing proper error handling mechanisms to catch and handle conversion errors.

Real-World Impact

The real-world impact of this issue includes:

  • Data corruption: Failed conversions can result in corrupted or incorrect data, leading to downstream errors and inaccuracies.
  • System crashes: Unhandled errors can cause system crashes or failures, resulting in downtime and lost productivity.
  • Inaccurate analysis: Incorrectly parsed dates can lead to inaccurate analysis and decision-making.

Example or Code

import polars as pl

# Create a sample dataframe
df = pl.DataFrame({
    "year_week": ["2026W03", "2026W04", "2026W05"]
})

# Correctly parse the date string using the strptime method
df = df.with_column(pl.col("year_week").str.strptime(pl.Datetime, fmt="%GW%V"))

# Print the resulting dataframe
print(df)

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Using the correct format string: Utilizing the correct format string, such as "%GW%V", to parse the date string.
  • Implementing error handling: Catching and handling conversion errors using try-except blocks or error handling mechanisms.
  • Validating input data: Validating the input data before attempting to convert it to a date format.

Why Juniors Miss It

Juniors may miss this issue due to:

  • Lack of experience: Inadequate experience with date parsing and formatting.
  • Insufficient knowledge: Limited knowledge of format strings and their usage.
  • Inadequate testing: Failing to thoroughly test the code and handle potential errors.