import numpy as np
x = np.array(zip((1, 2), (3., 4.)), dtype=[('i', 'i4'), ('f', 'f4')])
y = np.array(zip(('a', 'b')), dtype=[('s', 'a10')])

z = np.hstack((x, y))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-def477e6c8bf> in <module>()
----> 1 z = np.hstack((x, y))
TypeError: invalid type promotion

>>> import numpy.lib.recfunctions as nlr
>>> x = np.array(zip((1, 2), (3., 4.)), dtype=[('i', 'i4'), ('f', 'f4')])
>>> y = np.array(zip(('a', 'b')), dtype=[('s', 'a10')])
>>> x
array([(1, 3.0), (2, 4.0)],
      dtype=[('i', '<i4'), ('f', '<f4')])
>>> y
array([('a',), ('b',)],
      dtype=[('s', '|S10')])
>>> z = nlr.merge_arrays([x, y], flatten=True)
>>> z
array([(1, 3.0, 'a'), (2, 4.0, 'b')],
      dtype=[('i', '<i4'), ('f', '<f4'), ('s', '|S10')])

import numpy
from numpy.lib.recfunctions import merge_arrays
from itertools import chain

a = numpy.empty(3, [("col1", int), ("col2", float)])
b = numpy.empty(3, [("col3", int), ("col4", "U1")])

%timeit [i for i in (row for row in merge_arrays([a,b], flatten=True))]
52.9 µs ± 2 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

%timeit [i for i in (row for row in (chain(i,k) for i,k in zip(a,b)))]
3.47 µs ± 52 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)