how does multiple assignment work internally

Learning the pythonic way to code makes your code elegant and easy to understand. Since it’s a different coding style among other languages, coder like me should understand completely about the underlying mechanism. In this post, we will talk about multiple assignment, a.k.a. tuple assignment.

1. Multiple Assignment

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a, b = b, a

It’s definitely naturally and elegant. According to the doc:

1. a, b = b, a demonstrates that the expressions on the right-hand side are all evaluated first before any of the assignments take place.
2. The right-hand side expressions are evaluated from the left to the right.

From above we can learn the basic principles of multiple assignment. Thus,

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a, b = 1, 3
a, b = a+b, a
a, b = 1+3, 1

2. Tuple Abbreviation

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a, b, c, d = (i*i for i in range(0,7) if i % 2 == 0)
# returns a=0, b=4, c=16, d=36

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import dis
def (a, b):
...     a, b = a+b, a
... 
dis.dis(foo)

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>>> dis.dis(swap)
  2           0 LOAD_FAST                1 (a)
              3 LOAD_FAST                0 (b)
              6 BINARY_ADD
              7 LOAD_FAST           	 0 (a)
             10 ROT_TWO					 
             11 STORE_FAST               0 (a)
             14 STORE_FAST               1 (b)
             17 RETURN_VALUE

I think it explains everything itself. And it’s consist with the basic principles mentioned above.If you want to know meanings of these term, check this link.

3. Acknowledgment

1. An Informal Introduction to Python
2. http://stackoverflow.com/questions/11502268/how-does-pythons-comma-operator-works-during-assignment
3. http://stackoverflow.com/questions/21047524/how-does-swapping-of-members-in-the-python-tuples-a-b-b-a-work-internally
4. https://www.quora.com/How-does-Pythons-comma-operator-work-in-assignments-with-expressions