feat: Add str.pad_{start,end}

[camriddell]

Description

Adds str.pad_start and str.pad_end to pad strings within a column.

import narwhals as nw
import pandas as pd
df_native = pd.DataFrame({"a": ["cow", "monkey", "hippopotamus", None]})
df = nw.from_native(df_native)
print(
    df.with_columns(
        pad_start=nw.col("a").str.pad_start(8, "*"),
        pad_end=nw.col("a").str.pad_end(8, "*"),
    )
)
# ┌───────────────────────────────────────────┐
# |            Narwhals DataFrame             |
# |-------------------------------------------|
# |              a     pad_start       pad_end|
# |0           cow      *****cow      cow*****|
# |1        monkey      **monkey      monkey**|
# |2  hippopotamus  hippopotamus  hippopotamus|
# |3          None          None          None|
# └───────────────────────────────────────────┘

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes [Enh]: Add str.pad_start, str.pad_end #3388

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

[FBruzzesi]

Thanks @camriddell , just a couple of minor details to take care of

[dangotbanned]

Looking good @camriddell!

Only suggestion from me is to try stealing some test cases from polars and see if everything is still happy

• py-polars/tests/unit/operations/namespaces/string/test_pad.py

The unicode tests look funky (pola-rs/polars#12008)

[camriddell]

Looking good @camriddell!

Only suggestion from me is to try stealing some test cases from polars and see if everything is still happy

• py-polars/tests/unit/operations/namespaces/string/test_pad.py

The unicode tests look funky (pola-rs/polars#12008)

Yeah, fingers crossed that these are handled somewhat consistently/supported across backends. I'll pull these tests over and see where we land!

[camriddell]

@dangotbanned those unicode tests fit into place without any poking from me :)

[dangotbanned]

@dangotbanned those unicode tests fit into place without any poking from me :)

Happy to hear it, thanks @camriddell!

I'm jealous of how small this PR is 😉