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up vote 2 down vote favorite

Scrolling through the python 2.7 docs I came across this snippet

def fib(n):    # write Fibonacci series up to n
    a, b = 0, 1
    while b < n:
        print a,
        a, b = b, a+b

But I don't understand the last line, and unsure of how I would google for it.

How should I read a, b = b, a+b, or, what does it mean ?

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2 Answers 2

up vote 5 down vote accepted

Python evaluates the right-hand side of assignments first. It evaluates 

b, a+b

from left to right. It then assigns the values to the variables a and b respectively.

So a, b = b, a+b is equivalent to 

c = b
d = a+b
a = c
b = d

except that it achieves the result without explicit temporary variables. See the docs on Python's evaluation order.

There is a subtle point here worth examining with an example. Suppose a = 1, b = 2.

a, b = b, a+b

is equivalent to 

a, b = 2, 1+2 
a, b = 2, 3

So a gets assign to 2, b is assigned to 3.

Notice that this is not equivalent to

a = b
b = a + b

Since the first line would assign

a = 2
b = 2 + 2 = 4

Notice that done this (wrong) way, b ends up equal to 4, not 3. That's why it is important to know that Python evaluates the right-hand side of assignments first (before any assignments are made).

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c is not useful. –  njzk2 Apr 1 at 19:34
    
@njzk2 Not in this case, but there may be cases where it would be, using this sort of assignment. –  StephenTG Apr 1 at 19:35
    
@njzk2: If you assign a = b first, then you've lost the original value of a. So a+b will be (in general) incorrect. Thus, you need a temporary variable c. I would agree that d is not strictly needed however. –  unutbu Apr 1 at 19:38
    
@both : that's right, in fact, c (and d) can be useful in the general case. –  njzk2 Apr 1 at 19:42
    
And I assume the order is important, so that a, b = b, a+b would be something like a = b b = a + b ? –  user1021726 Apr 1 at 19:44
up vote 4 down vote

It is setting a to b, and b to a + b, without needing an intermediate variable. It could also be accomplished with: 

temp = a
a = b
b = temp + b
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1  
Very good answer. The only reason you didn't get the voted answer is because I felt the comments and edit on the other answer provided a bit more information, but your answer definitely helped! :) –  user1021726 Apr 2 at 6:51

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