The following code explains the issue

>>> import numpy
>>> print numpy.__version__
1.10.1
>>> a = [["a", 1], [2, 3]]
>>> b = numpy.array(a)
>>> print repr(a)
[['a', 1], [2, 3]]
>>> print repr(b)
array([['a', '1'],
       ['2', '3']],
      dtype='|S1')

I would have expected that not specifying a dtype would have given the result as if I had specify a dtype(object) (to be able to hold both ints and strings)

>>> b = numpy.array(a, dtype=numpy.dtype(object))
>>> print repr(b)
array([['a', 1],
       [2, 3]], dtype=object)

as the doc http://docs.scipy.org/doc/numpy/reference/generated/numpy.array.html describes

dtype : data-type, optional
The desired data-type for the array. If not given, then the type will be determined as the minimum type required to hold the objects in the sequence. This argument can only be used to ‘upcast’ the array. For downcasting, use the .astype(t) method.

But I may have missed some point in the docs