title: How to do column string concatenation including space separator in Pandas dataframe?
body: <p>I am a Pandas DataFrame as follows:</p>
  
  <pre><code>df = pd.DataFrame({
      'id': [1,2 ,3],
      'txt1': ['Hello there1', 'Hello there2', 'Hello there3'],
      'txt2': ['Hello there4', 'Hello there5', 'Hello there6'],
      'txt3': ['Hello there7', 'Hello there8', 'Hello there9']
  })
  df
  
  id  txt1            txt2            txt3
  1   Hello   there1  Hello there4    Hello there7
  2   Hello   there2  Hello there5    Hello there8
  3   Hello   there3  Hello there6    Hello there9
  </code></pre>
  
  <p>I want to concatenate column <code>txt1</code>, <code>txt2</code>, and <code>txt3</code>. So far I am able to achieve it as follows:</p>
  
  <pre><code>df['alltext'] = df['txt1']  + df['txt2'] + df['txt3']
  df
  
  id  txt1            txt2            txt3            alltext
  1   Hello there1    Hello there4    Hello there7    Hello there1Hello there4Hello there7
  2   Hello there2    Hello there5    Hello there8    Hello there2Hello there5Hello there8
  3   Hello there3    Hello there6    Hello there9    Hello there3Hello there6Hello there9
  </code></pre>
  
  <p>but how to introduce <strong>space</strong> character between the two column strings while concatenating in Pandas?</p>
  
  <p>I have just started learning Pandas.</p>