no side effects

[Bengt Richter]

On 8 Jan 2003 05:10:04 -0800, mis6 at pitt.edu (Michele Simionato) wrote:

>I was surprised by the following code:
>
>>>> for i in [1,2,3]:
>...     print i,
>...     i=3
>
>I would have expected only 1 to be printed, but instead Python
>continues the loop without noticing that the value of i has
>changed. IOW, no side effect.

I'm not sure what you mean by "side effect" here, but UIAM "i"
is effectively being bound/assigned-to as if something like this
was happening: (I changed your 3 to 4 to show final value effect)

 >>> it = iter([1,2,3])
 >>> try:
 ...     while 1:
 ...         i = it.next()
 ...         print i,
 ...         i=4
 ... except StopIteration:
 ...     pass
 ...
 1 2 3
 >>> i
 4

 >>> for i in [1,2,3]:
 ...     print i,
 ...     i=4
 ...
 1 2 3
 >>> i
 4

In other words, "i" is not part of the loop control conditions, but
it does get assigned to. The i=4 value doesn't get used at all,
as it's just overwritten by the next value from the iterator --
except when the StopIteration exception happens, which doesn't
assign anything to i, so leaves the last value assigned elsewhere alone.

Regards,
Bengt Richter