How to convert numpy object array into str/unicode array?

Update: In lastest version of numpy (e.g., v1.8.1), this is no longer a issue. All the methods mentioned here now work as excepted.

Original question: Using object dtype to store string array is convenient sometimes, especially when one needs to modify the content of a large array without prior knowledge about the maximum length of the strings, e.g.,

>>> import numpy as np
>>> a = np.array([u'abc', u'12345'], dtype=object)

At some point, one might want to convert the dtype back to unicode or str. However, simple conversion will truncate the string at length 4 or 1 (why?), e.g.,

>>> b = np.array(a, dtype=unicode)
>>> b
array([u'abc', u'1234'], dtype='<U4')
>>> c = a.astype(unicode)
>>> c
array([u'a', u'1'], dtype='<U1')

Of course, one can always iterate over the entire array explicitly to determine the max length,

>>> d = np.array(a, dtype='<U{0}'.format(np.max([len(x) for x in a])))
array([u'abc', u'12345'], dtype='<U5')

Yet, this is a little bit awkward in my opinion. Is there a better way to do this?

Edit to add: According to this closely related question,

>>> len(max(a, key=len))

is another way to find out the longest string length, and this step seems to be unavoidable...