title: change specific values in dataframe if one cell in a row is null
body: <p>I have the following dataframe in pandas:</p>
  <pre><code>&gt;&gt;&gt;name   food  beverage  age
  0  Ruth   Burger Cola      23
  1 Dina    Pasta  water     19
  2 Joel    Tuna   water     28
  3 Daniel  null   soda      30
  4 Tomas   null   cola     10
  </code></pre>
  <p>I want to put condistion that if value in food column is null, the age and beverage will change into ' ' (blank as well),</p>
  <p>I have wrote this code for that:</p>
  <pre><code>if df[(df['food'].isna())]:
    df['beverage']=' '
    df['age']=' '
  </code></pre>
  <p>but I keep getting error:</p>
  <blockquote>
  <p>ValueError: The truth value of a DataFrame is ambiguous. Use a.empty,
  a.bool(), a.item(), a.any() or a.all().</p>
  </blockquote>
  <p>I have playes with the location of the ([   but didn't help, what do I do wrong?</p>