No as the only place the length of each string is known is by the string. So you have to find out from every string what its length is.

If you store the string in a numpy array of dtype object, then you can't get at the size of the objects (strings) without looping. However, if you let np.array decide the dtype, then you can find out the length of the longest string by peeking at the dtype:

In [64]: a = np.array(['hello','world','!','Oooh gaaah booo gaah?'])

In [65]: a.dtype
Out[65]: dtype('|S21')

In [72]: a.dtype.itemsize
Out[72]: 21

max(a, key=len) gives you the longest string (and len(max(a, key=len)) gives you its length) without requiring you to code an explicit loop, but of course max will do its own looping internally, as it couldn't possibly identify "the longest string" in any other way.