title: Python [[0]] meaning
body: <p>I am running a Python script (Kaggle script).  It works in a 3.4.5 virtualenv, but not in 3.5.2</p>
  
  <p>I am not sure why and I am not familiar with the [[0]] syntax.  Below is the snippet.</p>
  
  <pre><code>import pandas as pd
  data = pd.read_csv(r'path\train.csv')
  labels_flat = data[[0]].values.ravel()
  </code></pre>
  
  <p>It should produce a list of values from the csv's first column.</p>
  
  <p>In 3.5.2 I get this error:</p>
  
  <pre><code>KeyError: '[0] not in index'
  </code></pre>
  
  <p>I tried to replicate the value with</p>
  
  <pre><code>labels_flat = []
  lf = data.values.tolist()
  for row in lf:
      labels_flat.append(row[0])
  </code></pre>
  
  <p>But I don't think it is the same thing.</p>