zope.i18n.translate overrides mapping if passed a message

I usually do:

X = _('some ${data}', mapping=dict(data=1))
zope.i18n.translate(X, context=self.request)

But indeed when you use X as a constant, you don't have necessary the mapping right away.
In this case, what I do is to create a new Message object:

from zope.i18nmessageid import Message
zope.i18n.translate(Message(X, mapping=dict(data=1)), context=self.request)

This is the preferred way to do it right now. You lose the domain and default message when you do unicode(X).

Indeed, it may be less verbose to write:
zope.i18n.translate(X, mapping=dict(data=1), context=self.request)

If somebody change the code of the translate function so you can override mapping like this, I think in this case you should be able to override domain and default as well.

Christian: Do we replace or update the mapping that's on the message object?

When you create a Message object from a Message object, it replaces the mapping completely, so I'd say we should do the same here, replace the mapping.

Currently it is only possible to override the existing mapping but it might be helpful (in some cases) to update it.
I see no need to remove keys from the mapping, so it would be always perfect to update the mapping.

This needs more discussion. My analysis and discussion is here:
https://mail.zope.org/pipermail/zope-dev/2010-October/041906.html

Now tracked on Github:

  https://github.com/zopefoundation/zope.i18n/issues/9