How do I convert a MultiIndex to type string

Question

consider the MultiIndex idx

idx = pd.MultiIndex.from_product([range(2013, 2016), range(1, 5)])

When I do

idx.to_series().str.join(' ')

I get

2013  1   NaN
      2   NaN
      3   NaN
      4   NaN
2014  1   NaN
      2   NaN
      3   NaN
      4   NaN
2015  1   NaN
      2   NaN
      3   NaN
      4   NaN
dtype: float64

This happens because the dtypes of the different levels are int and not str. join expects a str. How do I convert the whole idx to str?

I've done

join = lambda x, delim=' ': delim.join([str(y) for y in x])
idx.to_series().apply(join, delim=' ')

2013  1    2013 1
      2    2013 2
      3    2013 3
      4    2013 4
2014  1    2014 1
      2    2014 2
      3    2014 3
      4    2014 4
2015  1    2015 1
      2    2015 2
      3    2015 3
      4    2015 4
dtype: object

I expect there is a simpler way that I'm overlooking.

Answer 1

Something like this?

idx.to_series().apply(lambda x: '{0}-{1}'.format(*x))

Answer 2

I'm not sure it's the most elegant way, but it should work:

idx.get_level_values(0).astype(str).values + ' ' + idx.get_level_values(1).astype(str).values