So, you have two lists:
>>> a = [1,2,3,4,5]
>>> b = [6,7,8,9,0]
... and you want to iterate over a and b
. So what is a and b
, exactly?
>>> a and b
[6, 7, 8, 9, 0]
That might look odd, but it's the result of two facts about Python:
Every object is either True
-ish or False
-ish. For example:
>>> bool(a)
True
>>> bool(b)
True
In fact, all lists except the empty list []
are True
-ish.
Python uses short-circuit evaluation, which means that, for a and b
, it:
Checks whether a
is True
-ish or False
-ish
If a
is False
-ish, evaluates to a
If a
is True
-ish, evaluates to b
Following those rules, you should be able to see why a and b
evaluates to [6, 7, 8, 9, 0]
in your case (and following the same rules for combinations of the actual values True
and False
will show you that short-circuit evaluation does make sense).
If what you want to actually do is iterate trough the items in a
and then those in b
, you can just use the +
operator to concatenate them:
>>> for item in a + b:
... print item,
...
1 2 3 4 5 6 7 8 9 0
As for your followup questions:
What might happen with other operators, such as or
and not
?
or
's rules for short-circuit evaluation are different (you can look them up for yourself or just follow the link above), and in your case a or b
evaluates to [1, 2, 3, 4, 5]
(in other words, a
).
not
always returns True
for a False
-ish value and False
for a True
-ish value, and since you can't iterate over True
or False
, you'll get a TypeError
.
Is this proper usage, even? If so, is it messy, unsafe, or frowned upon?
Well, there's nothing illegal about it, but as you can see, it doesn't do what you want. There are circumstances where (ab)using short-circuit evaluation to choose an iterable over which to iterate might be helpful, but this isn't one of them.