title: DataFrame of objects `astype(float)` behaviour different depending if lists or arrays
body: <p>I'll preface this with the statement that I wouldn't do this in the first place and that I ran across this helping a friend.</p>
  
  <p>Consider the data frame <code>df</code></p>
  
  <pre><code>df = pd.DataFrame(pd.Series([[1.2]]))
  
  df
  
         0
  0  [1.2]
  </code></pre>
  
  <p>This is a data frame of objects where the objects are lists.  In my friend's code, they had:</p>
  
  <pre><code>df.astype(float)
  </code></pre>
  
  <p>Which breaks as I had hoped</p>
  
  <blockquote>
  <pre><code>ValueError: setting an array element with a sequence.
  </code></pre>
  </blockquote>
  
  <p>However, if those values were numpy arrays instead:</p>
  
  <pre><code>df = pd.DataFrame(pd.Series([np.array([1.2])]))
  
  df
  
         0
  0  [1.2]
  </code></pre>
  
  <p>And I tried the same thing:</p>
  
  <pre><code>df.astype(float)
  
       0
  0  1.2
  </code></pre>
  
  <p>It's happy enough to do something and convert my 1-length arrays to scalars.  This feels very dirty!</p>
  
  <p>If instead they were not 1-length arrays</p>
  
  <pre><code>df = pd.DataFrame(pd.Series([np.array([1.2, 1.3])]))
  
  df
  
              0
  0  [1.2, 1.3]
  </code></pre>
  
  <p>Then it breaks</p>
  
  <blockquote>
  <pre><code>ValueError: setting an array element with a sequence.
  </code></pre>
  </blockquote>
  
  <hr>
  
  <p><strong>Question</strong><br>
  Please tell me this is a bug and we can fix it.  Or can someone explain why and in what world this makes sense?</p>
  
  <hr>
  
  <p><strong>Response to @root</strong><br>
  You are right.  Is this worth an issue?  Do you expect/want this?</p>
  
  <pre><code>a = np.empty((1,), object)
  a[0] = np.array([1.2])
  
  a.astype(float)
  
  array([ 1.2])
  </code></pre>
  
  <p>And</p>
  
  <pre><code>a = np.empty((1,), object)
  a[0] = np.array([1.2, 1.3])
  
  a.astype(float)
  </code></pre>
  
  <blockquote>
  <pre><code>ValueError: setting an array element with a sequence.
  </code></pre>
  </blockquote>