Does `pl.concat([lazyframe1, lazyframe2])` strictly preserve the order of the input dataframes?

Question

Suppose I create a polars Lazyframe from a list of csv files using pl.concat():

df = pl.concat([pl.scan_csv(file) for file in ['file1.csv', 'file2.csv']])

Is the data in the resulting dataframe guaranteed to have the exact order of the input files, or could there be a scenario where the query optimizer would mix things up?

ritchie46 · Answer

The order is maintained. The engine may execute them in a different order, but the final result will always have the same order as the lazy computations provided by the caller.

Does `pl.concat([lazyframe1, lazyframe2])` strictly preserve the order of the input dataframes?

1 Answers