title: Adding file name in a column in pandas
body: <p>I have a single csv file chr1.step1.csv, say it has five columns. I can read this as follows:</p>
  <pre><code>df=pd.read_csv('chr1.step1.csv',sep='\t',header=None)
  df.head()
  0 1 2 3 4 5
  a1 b1 c1 d1 e1
  a2 b2 c2 d2 e2
  a3 b3 c3 d3 e3
  </code></pre>
  <p>How can I add 6th column as file name <code>chr1.step1.csv</code>, such as:</p>
  <pre><code>0 1 2 3 4 5 6
  a1 b1 c1 d1 e1 chr1.step1.csv
  a2 b2 c2 d2 e2 chr1.step1.csv
  a3 b3 c3 d3 e3 chr1.step1.csv
  </code></pre>
  <p>Its only one file, not multiple files.</p>