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The Ref Man doesn't explicitly say anything about 
Python's left-to-right evaluation order. Strict left-
to-right was Guido's intent, though, and it comes up a 
few times per year on c.l.py (indeed, I was just 
replying to a msg asking about it).
The docs should get fleshed out.
There's also a bug:
>>> counter = 0
>>> def i():
... global counter
... counter += 1
... return counter
...
>>> {i(): i()}
{2: 1}
>>> {i(): i(), i(): i()}
{4: 3, 6: 5}
>>>
That is, key:value *pairs* are evaluated left to 
right, but within a single key:value pair, the value 
gets evaluated first. When I asked Guido about that 
some years ago, he agreed it was a bug.