How does Python’s comma operator work during assignment?

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All the expressions to the right of the assignment operator are evaluated before any of the assignments are made.

From the Python tutorial: First steps towards programming:

The first line contains a multiple assignment: the variables a and b simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.

Emphasis mine.

Your code is functionally equivalent to the following:

a, b = 5 + 4, 5
print a, b

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