Beginner's assignment question

[Schizoid Man]

Gabriel Genellina wrote:
> En Sun, 02 Mar 2008 08:25:49 -0200, Schizoid Man <schiz at lon.don> escribi�:
> 
>> Lorenzo Gatti wrote:
>>> 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
>>
>> Thank you for the explanation. I guess my question can be simplified as:
>>
>> First step: a, b = 0, 1
>> No problem here as a and b are assigned values.
>>
>> Second step: a, b = b, a + b
>>
>> Now my question is does b become a + b after a becomes 1 or while a
>> stays at 0?
>>
>> As the assignment occurs simultaneously I suppose the answer is while a
>> stays at 0.
> 
> Read the previous response carefully and you'll answer your question. 
> The right hand side is EVALUATED in full before values are assignated to 
> the left hand side. Evaluating b, a+b results in 1, 1. The, those values 
> are assigned to a, b.

Thank you very much. It's clear now.