On Sat, Nov 23, 2013 at 08:57:54PM +0100, Rafael Knuth wrote:
> I have only one question left.
> Here's my original program again:
>
> for x in range(2, 10):
> for y in range(2, x):
> if x % y == 0:
> print(x, "equals", y, "*", x//y)
> break
> else:
> print(x, "is a prime number")
>
> So the first output of the outer loop is: 2.
> It's then passed to the inner loop:
>
> for y in range(2,x):
> if x % y == 0:
> ...
>
> And I was wondering what is happening inside that loop.
Absolutely nothing! The inner loop doesn't get executed. Python first
generates the range object range(2, 2) which is empty (it starts at 2
and stops at 2). Since y iterates over an empty list, the loop
immediately exits and the body gets executed zero times.
> The output of
>
> for y in range (2,2):
>
> should be ... none - correct?
Not quite "none", more like *nothing*. There's no output at all, because
the body isn't executed even once.
[...]
> Just want to understand how Python deals with "no values" within a program.
It doesn't. Things always have a value, if they are executed at all. But
if they don't get executed, then they don't exist at all.
if False:
# This code never runs!
print("This will never be printed")
x = 23
print(x)
At this point (unless x happened to have been defined even earlier in
the code), trying to print x will cause a NameError -- the name 'x'
doesn't exist.
--
Steven
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