[Python-Dev] Why does _pyio.*.readinto have to work with 'b' arrays?

[Nikolaus Rath]

Hello,

The _pyio.BufferedIOBase class contains the following hack to make sure
that you can read-into array objects with format 'b':

        try:
            b[:n] = data
        except TypeError as err:
            import array
            if not isinstance(b, array.array):
                raise err
            b[:n] = array.array('b', data)

I am now wondering if I should implement the same hack in BufferedReader
(cf. issue 20578). Is there anything special about 'b' arrays that
justifies to treat them this way?

Note that readinto is supposed to work with any object implementing the
buffer protocol, but the Python implementation only works with
bytearrays and (with the above hack) 'b' arrays. Even using a 'B' array
fails:

>>> import _pyio
>>> from array import array
>>> buf = array('b', b'x' * 10)
>>> _pyio.open('/dev/zero', 'rb').readinto(buf) 
10
>>> buf = array('B', b'x' * 10)
>>> _pyio.open('/dev/zero', 'rb').readinto(buf)
Traceback (most recent call last):
  File "/home/nikratio/clones/cpython/Lib/_pyio.py", line 662, in readinto
    b[:n] = data
TypeError: can only assign array (not "bytes") to array slice

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/nikratio/clones/cpython/Lib/_pyio.py", line 667, in readinto
    b[:n] = array.array('b', data)
TypeError: bad argument type for built-in operation


It seems to me that a much cleaner solution would be to simply declare
_pyio's readinto to only work with bytearrays, and to explicitly raise a
(more helpful) TypeError if anything else is passed in.


Best,
-Nikolaus

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