[Python-Dev] PEP 327: Decimal Data Type

[Batista, Facundo]

Michael Chermside wrote:

#- Imagine the following senario. I write some code which uses
#- Decimal, and which sets the context to get the desired rounding
#- behavior. Somewhere in my code I call a routine written by
#- somebody else... perhaps some library code... which also does
#- some calculations using Decimal.

This subject already has been discussed in c.l.p:


Emile van Sebille:
    
    ...from which I infer that Context exists in part to limit/round
    calculated results.  Even if it were possible for me, as a user of
    Decimal, to set Context appropriately to achieve these ends, what
    if I use a library that also changes Context?  The integrity of my
    calculations may be impacted.
    
Aahz:

    That's correct.  There needs to be a social convention that libraries
    intended for use by other people *CANNOT* muck with Context, and if they
    do so for their own internal calculations, they must save and restore
    Context.  You can probably find additional information about this in
    Cowlishaw.

Emile van Sebille:

    Enter the context stack.

Aahz:

    Well, sure.  And it won't be hard to add given that Decimal will already
    need to track thread-specific Context.  But modules changing Context
    will still need to explicitly push and pop Context because usually you
    will just want to replace the current Context.

Tim Peters:

    They have another choice, because Guido had a brilliant idea:  the
    arithmetic operations in Eric's implementation are methods *of* a
context
    object (because Guido suggested that).  So a maximally robust library
    doesn't *have* to change the thread context at all:  it can create
whatever
    private context object(s) it needs, and spell arithmetic as explicit
method
    calls on its private context object(s), so that the default thread
context
    object is neither consulted nor modified.  This is very robust, and in
small
    doses is quite bearable.


I don't have any problem to implement a context stack. But I think that it's
no clear to be useful, so I think it's better to go for the sure staff, and
when Decimal gets heavy use, if everybody agrees to add a context stack,
we'll go for it.

What do you think?

.	Facundo





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