[Python-Dev] Py2.5 issue: decimal context manager misimplemented, misdesigned, and misdocumented

[Phillip J. Eby]

At 05:57 PM 8/29/2006 -0700, Raymond Hettinger wrote:
>Phillip J. Eby wrote:
>
>>At 05:20 PM 8/29/2006 -0700, Raymond Hettinger wrote:
>>
>>>* The implementation's doc string examples were not tested and don't
>>>work (this is a deep error).  One reads:
>>>
>>>     with decimal.getcontext() as ctx:
>>>         ctx.prec += 2
>>>         s = ...
>>>     return +s
>>>
>>>
>>>To get this to work with the current implementation, it should read
>>>
>>>     with decimal.getcontext().copy().get_manager() as ctx:
>>>         ctx.prec += 2
>>>         s = ...
>>>     return +s
>>>
>>>This is horrid.  Please either revert the patch or fix it to match PEP-343.
>>
>>
>>Actually, as I read the code, that would be:
>>
>>     with decimal.getcontext().get_manager() as ctx:
>
>Given the current mis-implementation, the copy() step is absolutely 
>necessary.  Since context objects are mutable, the current context would 
>never get it precision and flags restored.
>
>Try running the example and printing out the current context precision 
>before and after the with-suite.  You'll see that the context has changed 
>(which defeats the whole purpose).

No need; now that you've explained the problem I see why the code is 
wrong.  This is definitely a bug in the decimal module.  It looks like the 
code is correct at first glance, but the .copy() definitely needs to be in 
the ContextManager class, not the get_manager() method.  Yuck.