title: What is the Polars equivalent of Pandas `.isna()` method?
body: <p>I'm trying to replace Pandas with <a href="https://github.com/pola-rs/polars" rel="nofollow noreferrer">Polars</a> in production code, for better memory performance.</p>
  <p>What would be the Polars equivalent of Pandas <code>.isna()</code> method? I couldn't find any good equivalent in the doc.</p>