dtypes. Difference between S1 and S2 in Python

Question

I have two arrays of strings:

In [51]: r[\'Z\']
Out[51]: 
array([\'0\', \'0\', \'0\', ..., \'0\', \'0\', \'0\'], 
      dtype=\'|S1\')

In [52]: r[\'Y\']             


        

Answer 1

For storing strings of variable length in a numpy array you could store them as python objects. For example:

In [456]: x=np.array(('abagd','ds','asdfasdf'),dtype=np.object_)

In [457]: x[0]
Out[457]: 'abagd'

In [459]: map(len,x)
Out[459]: [5, 2, 8]

In [460]: x[1]=='ds'
Out[460]: True

In [461]: x
Out[461]: array([abagd, ds, asdfasdf], dtype=object)

In [462]: str(x)
Out[462]: '[abagd ds asdfasdf]'

In [463]: x.tolist()
Out[463]: ['abagd', 'ds', 'asdfasdf']

In [464]: map(type,x)
Out[464]: [str, str, str]

Answer 2

See the dtypes documentation.

The |S1 and |S2 strings are data type descriptors; the first means the array holds strings of length 1, the second of length 2. The | pipe symbol is the byteorder flag; in this case there is no byte order flag needed, so it's set to |, meaning not applicable.