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bpo-32792: Preserve mapping order in ChainMap() #5586
I'm fine with merging this in time for 3.7.0b2. While changing the behavior to preserve mapping order would have been the right thing to do for 3.6.0, I question whether it is appropriate to make such a behavior change in a 3.6.x maintenance release. As best I read it, the 3.6 ChainMap docs make no commitment one-way or the other about iteration order. And in previous similar discussions about ordering guarantees, my take on Guido's intent is that, at this point, the status quo wins in 3.6.x (for example, https://mail.python.org/pipermail/python-dev/2017-December/151321.html or bpo-32690). So let's merge for master and 3.7. Thanks!