Is the right-hand side of an assignment always evaluated before the assignment?

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As I mentioned in a comment, this test case can be reduced to:

x = {}
x[1] = len(x)

The question then becomes, is x[1] == 0, or is x[1] == 1?

Let’s look at the relevant 2.x documentation and 3.x documentation:

Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.

In the following lines, expressions will be evaluated in the arithmetic order of their suffixes:

expr3, expr4 = expr1, expr2

Therefore…

len(x) will be fully computed before we do x[1], so x[1] == 0 and this is well defined.

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