Comment 36 for bug 525808

On 03/06/2012 12:12 PM, Numérigraphe wrote:
> Le 06/03/2012 10:26, Niels Huylebroeck a écrit :
>> This depends on whether or not the called function wants to alter the
>> behavior (...) of the calling function and thus seems optional and depends
>> on the case.

I tend to agree with Niels on this. Contrary to the mutable default parameter
which is an obscure Python caveat, developers cannot ignore when they are
mutating the request context.

> Usually in OpenERP the context is updated to adjust the behavior of
> subsequent function calls, not that of the caller. For example, there's no
> reason to mutate the context in an ORM method is there?

There are many subtle cases where changing the context is needed, sometimes for
callees only, but sometimes also for the entire request.
For example in the ORM the copy_data() method needs both:
- it needs to read a different translation of a column by setting
context['lang'] (for callees only)
- it needs to maintain something like a context['visited_records'] set, in
order to detect cycles in an object graph (for the entire request)

> Furthermore, there is no way a developer can know the impact of changing a
> key in the caller's context, so at first sight I'd consider it a mistake.

The impact is usually pretty obvious. If you're altering a known context value
such as 'lang' or 'tz' you obviously know what you're doing, and that it should
be done in a copy of the context in most cases (but not always).
If you're simply setting a custom context key that your method and its callees
agree upon, then you should properly namespace it in order to avoid name
conflicts with someone else's custom key. And it does not matter that much if
you do it in a copy of the context or not.

In most cases I think the meaning of altering the context is pretty clear and
developers are already taking care of copying it when needed.