title: Unexpectedly assign in pandas by df.loc and single-element list
body: <p>When I assign new value to DataFrame by df.loc method and a list containing single element, pandas behaves not the way I expected.</p>
  
  <p>For example (in python 3.6 and pandas 0.22.0):</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame([[1,2,3]],columns=["A", "B", "C"])
  
  # a list has two or more elements
  df.loc[0, ["A", "B"]] = ["X", "Y"]
  df
     A  B  C
  0  X  Y  3
  
  # a list has single element
  df.loc[0, ["C"]] = ["Z"]
  </code></pre>
  
  <p>as I expected</p>
  
  <pre><code>df
     A  B  C
  0  X  Y  Z
  </code></pre>
  
  <p>but the result is</p>
  
  <pre><code>df
     A  B    C
  0  X  Y  [Z]
  </code></pre>
  
  <p>Is this behavior according to pandas' implementation?</p>
  
  <p>I want to assign values the same way independently of how much elements a list has.</p>