Beginner's assignment question

[Lorenzo Gatti]

On Mar 1, 3:39 pm, Schizoid Man <sc... at lon.don> wrote:
> As in variable assignment, not homework assignment! :)
>
> I understand the first line but not the second of the following code:
>
> a, b = 0, 1
> a, b = b, a + b
>
> In the first line a is assigned 0 and b is assigned 1 simultaneously.
>
> However what is the sequence of operation in the second statement? I;m
> confused due to the inter-dependence of the variables.

The expressions of the right of the assignment operator are evaluated
before assigning any new values, to the destinations on the left side
of the assignment operator.
So substitutig the old values of a and b the second assignment means

a, b = 0, 0 + 1

Simplifying the Python Reference Manual ("6.3 Assignment Statements")
a little :

assignment_stmt ::= target_list "="+ expression_list

An assignment statement evaluates the expression list (remember that
this can be a single expression or a comma-separated list, the latter
yielding a tuple) and assigns the single resulting object to each of
the target lists, from left to right.

[...]

WARNING: Although the definition of assignment implies that overlaps
between the left-hand side and the right-hand side are `safe' (for
example "a, b = b, a" swaps two variables), overlaps within the
collection of assigned-to variables are not safe! For instance, the
following program prints "[0, 2]":

x = [0, 1]
i = 0
i, x[i] = 1, 2
print x

Lorenzo Gatti