problem with read() write()

Gertjan Klein gklein at
Sun Nov 1 12:07:03 CET 2009

Alf P. Steinbach wrote:

>* Gertjan Klein:
>> I reproduced (with Python 2.5.2 on WinXP) the code the OP wrote after
>> creating an empty (0-byte) test file; after the write() the read()
>> returns random garbage. I can't imagine why anyone would want that
>> behaviour. The file grew to be 4099 bytes after f.close(). I wrote
>> 'hello' to it, so the length of garbage added was 4094 bytes, which I
>> find a strange number also.
>Could you post (copy and paste) the code, and description of results?

The code is exactly the OP's code, with an f.close() after The
results are described above.

>"Shall not" means UB. This applies to C "FILE*" handling.

Yes. But Python is not C. I would have expected that Python shields me
from such bizarre results. The fact that, apparently, the Python file
object is a thin wrapper over a C library function is an explanation of
the behaviour, but not a justification.

>And for a language used by so many newbies (this is positive!) I agree that it 
>should ideally get rid of that UB (assuming that's what the problem is), or, if 
>it doesn't already, mention that in the Python documentation.

I'm not sure I can defend this statement, but I think Python should not
have undefined behaviour at all.


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