title: Why does using "==" return a Series instead of bool in pandas?
body: <p>I just can't figure out what "==" means at the second line:<br>
  - It is not a test, there is no if statement...<br>
  - It is not a variable declaration...</p>
  
  <p>I've never seen this before, the thing is <code>data.ctage==cat</code> is a pandas Series and not a test...</p>
  
  <pre><code>for cat in data["categ"].unique():
      subset = data[data.categ == cat] # Création du sous-échantillon
      print("-"*20)
      print('Catégorie : ' + cat)
      print("moyenne:\n",subset['montant'].mean())
      print("mediane:\n",subset['montant'].median())
      print("mode:\n",subset['montant'].mode())
      print("VAR:\n",subset['montant'].var())
      print("EC:\n",subset['montant'].std())
      plt.figure(figsize=(5,5))
      subset["montant"].hist(bins=30) # Crée l'histogramme
      plt.show() # Affiche l'histogramme
  </code></pre>