Re: [Qemu-devel] [PATCH] iotests: handle TypeError for Python3 in test 242

[Cleber Rosa <crosa@redhat.com>]

On 2/18/19 4:25 PM, Philippe Mathieu-Daudé wrote:
> On 2/18/19 9:05 PM, Eric Blake wrote:
>> [adding Eduardo for some python 2-vs-3 advice]
> 
> And Cleber.
> 
>>
>> On 2/18/19 1:59 PM, Andrey Shinkevich wrote:
>>> To write one byte to disk, Python2 may use 'chr' type.
>>> In Python3, conversion to 'byte' type is required.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>> ---
>>>  tests/qemu-iotests/242 | 9 +++++++--
>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tests/qemu-iotests/242 b/tests/qemu-iotests/242
>>> index 16c65ed..6b1f7b8 100755
>>> --- a/tests/qemu-iotests/242
>>> +++ b/tests/qemu-iotests/242
>>> @@ -65,9 +65,14 @@ def toggle_flag(offset):
>>>      with open(disk, "r+b") as f:
>>>          f.seek(offset, 0)
>>>          c = f.read(1)
>>> -        toggled = chr(ord(c) ^ bitmap_flag_unknown)
>>> +        toggled = ord(c) ^ bitmap_flag_unknown
>>>          f.seek(-1, 1)
>>> -        f.write(toggled)
>>> +        try:
>>> +            # python2
>>> +            f.write(chr(toggled))
>>> +        except TypeError:
>>> +            # python3
>>> +            f.write(bytes([toggled]))
>>
>> Looks like it works, but I'm not enough of a python expert to know if
>> there is a more Pythonic elegant approach.
>>

Well, there's no way around the fact that bytes in Python 3 are very
different from bytes in Python 2 (just another name for a string).

What I'd recommend here is to not base the type on the exception, but
choose it depending on the Python version.  Something like:

if sys.version_info.major == 2:
   f.write(chr(toggled))
else:
   f.write(bytes([toggled])]

This is cheaper than raising/catching exceptions, it's self documenting,
and follows the pattern on other tests.

Regards,
- Cleber.

>> If someone else picks it up before my next NBD pull request,
>> Acked-by: Eric Blake <eblake@redhat.com>
>>