title: Merge branch coverage files python
body: <p>I have a multiprocessing application in python. And I am trying to get the coverage report after running my tests. I am trying to merge coverage reports but I am not able to do in a single shot.</p>
  
  <p>Following is the issue I am facing.
  My two tests have generated 4 coverage files. And when I run the command <strong>"coverage combine"</strong> I get the following error:</p>
  
  <pre><code>Can't combine line data with arc data
  </code></pre>
  
  <p>To merge the coverage files I need to run <strong>"coverage combine"</strong> command 4 times. But I am planning to add more tests and that will make combining the reports even difficult.</p>
  
  <p>So how can I combine all the coverage reports in a single go?</p>
  
  <p>PS:
  I have set the configuration file as follows:</p>
  
  <pre><code>[run]
  branch = True
  parallel = True
  concurrency = multiprocessing
  
  [report]
  # Regexes for lines to exclude from consideration
  exclude_lines =
      # Have to re-enable the standard pragma
      pragma: no cover
  </code></pre>
  
  <p>And I am able to get the combined report correctly for line coverage.</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>This is how I run my application to get the coverage</p>
  
  <pre><code>coverage --rcfile=coverage_rc_file tester_script.py test1 test2
  </code></pre>
  
  <p>The above command runs my app twice and generates 4 coverage files.</p>
  
  <p>Then I run the following command to combine the results:</p>
  
  <pre><code>coverage combine
  </code></pre>