title: Python dask program failing to produce output even though it seems to compute
body: <p>I am confused as to why my dask program is not producing any output, it simply hangs after submitting. I have specified to use processes instead of threads and can see all of the cores fire up upon submitting (as suggested here:<a href="https://stackoverflow.com/questions/35516305/dask-computation-not-executing-in-parallel">dask computation not executing in parallel</a>) so it seems to compute but never finishes.  I am just trying to run a simple regex over a list of long text files.  Am I missing something obvious?</p>
  
  <pre><code>import re
  from os import listdir
  
  import dask.bag as db
  import dask.multiprocessing
  dask.set_options(get=dask.multiprocessing.get)
  
  
  loc = 'D:\\...\\text_files\\'
  txts = [loc + i for i in listdir(loc)[:10]]
  
  #  Load data in parallel
  f = db.from_filenames(txts)
  f = f.repartition(3)
  
  # Define the regex
  regex = re.compile(r'\b[A-Z][a-z]+\b')
  
  # create function to parallelize
  def reg(text):
      return regex.findall(text)
  
  # distribute the function over cores
  output = f.map(reg).compute().concat()
  </code></pre>