title: How to (re)name an empty column header in a pandas dataframe without exporting to csv
body: <p>I have a pandas dataframe <code>df1</code> with an index column and an unnamed series of values. I want to assign a name to the unnamed series. </p>
  
  <p>The only way to do this that I know so far is to export to <code>df1.csv</code> using: </p>
  
  <p><code>df1.to_csv("df1.csv", header = ["Signal"])</code></p>
  
  <p>and then re-import using: </p>
  
  <p><code>pd.read_csv("df1.csv", sep=",")</code></p>
  
  <p>However, this costs time and storage space. How to do this in-memory? </p>
  
  <p>When I do <code>df2 = df1.rename(columns = {"" : "Signal"}, inplace = True)</code></p>
  
  <p>I yield:</p>
  
  <p><code>AttributeError: "Series" object has no attribute "Signal"</code>.</p>