On 11/05/13 07:19, Jim Mooney wrote:
If I'm using a variable-dependent range in a for loop, is Python smart
enough to figure the variable once so I don't have to hoist it up?
That is for c in range(0,x+2), is x+2 figured once or every time
through the loop? I'm assuming it's once but I like to verify.
Yes, it is calculated once.
Remember that range() is not a syntax feature, it is a function that returns an
object. Since it is a function, its argument must be figured *before* the
function is called, and once the range function has returned, it is done.
So you can write something like this in Python 3:
py> x = 3
py> values = range(0, x+2)
py> print(values)
range(0, 5)
py> for c in values:
... print(c)
...
0
1
2
3
4
Python 2 is the same, except that instead of getting that mysterious "range(0,
5)" line printed, you get an actual list of values [0, 1, 2, 3, 4].
By the way, you don't need to include the starting value of 0 for range.
range(0, x+2) is exactly the same as range(x+2).
Here are a couple of "monkey-patching" tricks for testing things like this.
Let's patch the range function to display when it is called, and make a chatty little
object that prints when it is added to.
=== cut ===
saved_range = range
def range(*args):
print("calling range function")
return saved_range(*args)
class Chatty(int):
def __add__(self, other):
print("adding %s and %s" % (self, other))
return int.__add__(self, other)
x = Chatty(3)
for c in range(x+2):
print(c)
=== cut ===
If you copy the code between the "cut" lines, and paste into an interactive
interpreter, you will see what Python does when executing this sort of for-loop.
--
Steven
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