Polars apply function to check if a row value is a substring of another string

Question

I'm trying to check if string_1 = "this example string" contains a column value as a substring.

For example the first value in Col B should be True since "example" is a substring of string_1

string_1 = "this example string"

df = pl.from_repr("""
┌────────┬─────────┬────────────┐
│ Col A  ┆ Col B   ┆ Col C      │
│ ---    ┆ ---     ┆ ---        │
│ str    ┆ str     ┆ str        │
╞════════╪═════════╪════════════╡
│ 448220 ┆ example ┆ 7101936801 │
│ 518398 ┆ 99999   ┆ 9999900091 │
│ 557232 ┆ 424570  ┆ 4245742060 │
└────────┴─────────┴────────────┘
""")

This is what I have tried so far, but it's returning the following error:

df=df.with_columns(pl.col("Col B").apply(lambda x: x in string_1).alias("new_col"))

AttributeError: 'Expr' object has no attribute 'apply'

Answer 1

It's always better to avoid using python functions and use native polars expressions.

• pl.lit() to create a dummy column from string_1
• str.contains() to check if string contains a column value.

(
    df
    .with_columns(pl.lit(string_1).str.contains(pl.col('Col B')).alias('new_col')
)

Or you can use name.keep() if you want to check all columns.

(
    df
   .with_columns(pl.lit(string_1).str.contains(pl.all()).name.keep())
)

┌───────┬───────┬───────┐
│ Col A ┆ Col B ┆ Col C │
│ ---   ┆ ---   ┆ ---   │
│ bool  ┆ bool  ┆ bool  │
╞═══════╪═══════╪═══════╡
│ false ┆ true  ┆ false │
│ false ┆ false ┆ false │
│ false ┆ false ┆ false │
└───────┴───────┴───────┘

or something like this if you need all new columns:

• name.suffix()

(
    df
   .with_columns(pl.lit(string_1).str.contains(pl.all()).name.suffix('_match'))
)

┌────────┬─────────┬────────────┬─────────────┬─────────────┬─────────────┐
│ Col A  ┆ Col B   ┆ Col C      ┆ Col A_match ┆ Col B_match ┆ Col C_match │
│ ---    ┆ ---     ┆ ---        ┆ ---         ┆ ---         ┆ ---         │
│ str    ┆ str     ┆ str        ┆ bool        ┆ bool        ┆ bool        │
╞════════╪═════════╪════════════╪═════════════╪═════════════╪═════════════╡
│ 448220 ┆ example ┆ 7101936801 ┆ false       ┆ true        ┆ false       │
│ 518398 ┆ 99999   ┆ 9999900091 ┆ false       ┆ false       ┆ false       │
│ 557232 ┆ 424570  ┆ 4245742060 ┆ false       ┆ false       ┆ false       │
└────────┴─────────┴────────────┴─────────────┴─────────────┴─────────────┘