Question

I’d like to know what is getting assigned to what in line 8.

# Iterators

class Fibs:
    def __init__(self):
        self.a = 0
        self.b = 1
    def next(self):
        self.a, self.b = self.b, self.a+self.b # <--- here
        return self.a
    def __iter__(self):
        return self

fibs = Fibs()

for f in fibs:
    if f > 1000:
        print f
        break

The rest of the program I really don’t need much explanation. I’m not sure what’s getting assigned to what.

Answer 1

It’s a pair assignment, a shorthand of

t = self.a
self.a = self.b
self.b = t+self.b

just to use an one-liner instead that two assignments.. to be precise i think that the left operand of the assignment is considered a tuple of two elements, so you are like assigning to tuple (self.a, self,b) the value (self.b, self.a+self.b) which does the same thing as the three separate assignments written before without the need of a temporary variable. This because, while without using tuple the assignments are executed sequentially, in your example they are resolved at the same time, preserving the value of self.a in second assignment.

As stated in documentation:

Assignment of an object to a target list is recursively defined as follows.

• If the target list is a single target: The object is assigned to that target.
• If the target list is a comma-separated list of targets (your case): The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets. (This rule is relaxed as of Python 1.5; in earlier versions, the object had to be a tuple. Since strings are sequences, an assignment like a, b = “xy” is now legal as long as the string has the right length.)