Edoardo Abati

[Enh]: consider switching to `src` layout

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### We would like to learn about your use case. For example, if this feature is needed to adopt Narwhals in an open source project, could you please enter the...

fix: unify `ColumnNotFound` for `duckdb` and `pyspark`

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## What type of PR is this? (check all applicable) - [ ] 💾 Refactor - [ ] ✨ Feature - [x] 🐛 Bug Fix - [ ] 🔧 Optimization...

MNT Narwhals in `utils._input_validations`

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## Description Towards #1522 (thanks @StefanieSenger for your refactoring PR, made this work way easier :) ) This PR makes the `utils _input_validations.py` narwhals compatible. Since all the rest doesn't...