beginner, idiomatic python

[bambam]

Ok, many environments are capable of cached evaluation
of functions without variable parameters so
    range(5)
is cached, but
    range(v) is re-evaluated every time. Is this defined
behaviour?

That is, is it defined what Python does for
    for i in f()
I'm sure it must be, but I haven't seen it yet. If I have
a user defined function returning a range, is it defined
that the range function is called on every loop? If I
have a function returning a range taking a parameter,
    for i in f(v)
is it defined that the variable is evaluated for every loop?

Steve.


"Paul McGuire" <ptmcg at austin.rr.com> wrote in message 
news:1188090322.663217.102750 at 22g2000hsm.googlegroups.com...
> On Aug 23, 11:50 pm, "bambam" <da... at asdf.asdf> wrote:
>> Thank you, so generallizing:
>>
>> (1) Python re-evaluates the loop range on every loop, and
>> (2) Python does short-circuit evaluation of conditions, in predictable
>> order.
>>
>> Sorry about the bad question.
>>
>
> A beginner would do well to work through the Python Tutorial (http://
> docs.python.org/tut/tut.html).  I think your first "insight" is
> actually incorrect, if I understand your wording.  If I wrote:
>
> for i in range(1000000000):
>    # do something with i
>
> I'm quite certain that range(1000000000) is not evaluated on every
> loop iteration.  In fact, the range call generates a list containing
> the values [0, 1, 2, ..., 999999999], and then for iterates over this
> list.  You can read this at 
> http://docs.python.org/tut/node6.html#SECTION006300000000000000000.
>
> On the other hand, if you are talking about a while loop, of course
> the condition is evaluated on every loop - otherwise such a loop, once
> entered, would never exit.
>
> Your second generalization is stated pretty plainly in
> http://docs.python.org/tut/node7.html#SECTION007700000000000000000.
> In fact, conditional short-circuiting is a consistent theme in most
> Python functions and structures.  The new any() and all() built-ins in
> Python 2.5 for example, evaluate a list of values for their boolean
> True/False-ness, any() returning True if any list entry is True,
> otherwise False; and all() returning True if all entries are True,
> otherwise False.  Both short-circuit their evaluation, so that if the
> first element of a billion element list gives a True value for any()
> (or a False value for all()), then the evaluation of the remaining
> billion-1 items is skipped.
>
> Best of luck in your new Python learning process,
> -- Paul
>