pythonpython-polars

Python-Polars. Method.find() returns an incorrect value for strings when using utf-8 characters

The behavior of the str.find() method in differs from str.find() in in Python. Is there a parameter for processing utf-8 characters? Or is it a bug?

Example code in python:

import polars as pl

# Define a custom function that wraps the str.find() method
def find_substring(s, substring):
    return int(s.find(substring))

# Test df
df = pl.DataFrame({
    "text": ["testтестword",None,'']
})

# Apply the custom function to the "text" column using map_elements()
substr = 'word'
df = df.with_columns(
    pl.col('text').str.find(substr,literal=True,strict=True).alias('in_polars'),   
    pl.col("text").map_elements(lambda s: find_substring(s, substr), return_dtype=pl.Int64).alias('find_check')
)

print(df)

Results:

img

There is no parameter in the documentation for setting the character encoding.
Using my function is a solution, but it's very slow.

Can you suggest something faster and without map_elements? Thanks.

pl.col("text").map_elements(lambda s: find_substring(s, substr), return_dtype=pl.Int64).alias('find_check')
Solution

  • There is an open issue on the Github tracker.

    I think we should update the docs to make clear that we return byte offsets.

    As for the actual goal - it seems you want split a string into 2 parts and take the right hand side.

    You could use regex e.g. with .str.extract()

    df.with_columns(after = pl.col("url").str.extract(r"\?(.*)"))

    shape: (1, 2)
┌────────────────────────────────────┬──────────────┐
│ url                                ┆ after        │
│ ---                                ┆ ---          │
│ str                                ┆ str          │
╞════════════════════════════════════╪══════════════╡
│ https://тестword.com/?foo=bar&id=1 ┆ foo=bar&id=1 │
└────────────────────────────────────┴──────────────┘

    .str.splitn() could be another option.

    df = pl.DataFrame({
    "url": ["https://тестword.com/?foo=bar&id=1"]
})

df.with_columns(
    pl.col("url").str.splitn("?", 2)
      .struct.rename_fields(["before", "after"])
      .struct.unnest()
)

    shape: (1, 3)
┌────────────────────────────────────┬───────────────────────┬──────────────┐
│ url                                ┆ before                ┆ after        │
│ ---                                ┆ ---                   ┆ ---          │
│ str                                ┆ str                   ┆ str          │
╞════════════════════════════════════╪═══════════════════════╪══════════════╡
│ https://тестword.com/?foo=bar&id=1 ┆ https://тестword.com/ ┆ foo=bar&id=1 │
└────────────────────────────────────┴───────────────────────┴──────────────┘

    It returns a Struct which we rename/unnest into columns.