numpy frombuffer - AttributeError: 'str' object has no attribute 'buffer'

Question

Python Version: 3.5.2 Numpy Version : 1.12.1

Error:

import numpy as np
s = 'Hello World'
np.frombuffer(s, dtype='S1')
AttributeError: 'str' object has no attribute '__buffer__'

Things Tried:

Tried Online Ideone compiler, got same error in Python3.xx.

Referred scipy faqs for numpy and python compatible version, which states "NumPy support the Python 2.x series, (versions 2.6 and 2.7), as well as Python 3.2 and newer. The first release of NumPy to support Python 3 was NumPy 1.5.0."

Can't figure out the issue, tried stackoverflow for same issue but nothing found, may be i had missed it. Any suggestions or leads on why the error and how to resolve it in python3.xx.

So at first i was like oh.. it's a simple Dtype error. then i tried it, then i tried harder. Now i have anxiety. — axwr
– axwr, Commented Apr 12, 2017 at 7:54
A string isn't a buffer, especially not in py3, where strings are unicode. What array do you want? Why are you using frombuffer? That's not a beginners tool. — hpaulj
– hpaulj, Commented Apr 12, 2017 at 8:24
frombuffer docs has an example like this, but it needs to be refined for py3 use. — hpaulj
– hpaulj, Commented Apr 12, 2017 at 8:33
stackoverflow.com/q/22236749 is the only related SO question, indicating how rare and specialized frombuffer is. Creating s as a bytestring might work. I can't test from here. — hpaulj
– hpaulj, Commented Apr 12, 2017 at 8:38
@hpaulj thanks for your precious comments and link. Beginners usually starts with documentation which is the way i went in order to understand the frombuffer and become more knowledgeable on the subject. — JkShaw
– JkShaw, Commented Apr 12, 2017 at 8:50

hpaulj · Accepted Answer · 2017-04-12 16:05:03Z

In a PY3 session:

In [62]: np.frombuffer('hello world')
...
AttributeError: 'str' object has no attribute '__buffer__'
In [63]: np.frombuffer(b'hello world')
...
ValueError: buffer size must be a multiple of element size
In [64]: np.frombuffer(b'hello world',dtype='S1')
Out[64]: 
array([b'h', b'e', b'l', b'l', b'o', b' ', b'w', b'o', b'r', b'l', b'd'],  dtype='|S1')

In PY3, the default string type is unicode. The b is used to create and display bytestrings.

The np.frombuffer docs should be updated to reflect the difference. The 'hello world' example only works with PY2 or with PY3 bytestrings.

As I noted in the comments, there are few SO questions regarding frombuffer, indicating that it is rarely used. np.array is by far the most common way of making an array, even from strings:

In [80]: np.array('hello')
Out[80]: 
array('hello', 
      dtype='<U5')

or use list to split the string into characters:

In [81]: np.array(list('hello'))
Out[81]: 
array(['h', 'e', 'l', 'l', 'o'], 
      dtype='<U1')

In [82]: np.array(b'hello')
Out[82]: 
array(b'hello', 
      dtype='|S5')
In [83]: np.array(list(b'hello'))
Out[83]: array([104, 101, 108, 108, 111])

In [85]: np.fromiter('hello','S1')
Out[85]: 
array([b'h', b'e', b'l', b'l', b'o'], 
      dtype='|S1')
In [86]: np.fromiter('hello','U1')
Out[86]: 
array(['h', 'e', 'l', 'l', 'o'], 
      dtype='<U1')*

I created a bug issue: https://github.com/numpy/numpy/issues/8933

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