for循环内外的变量

Question

The question itself lies inside the code (or questions themselves lie , since there are several question marks).

Code:

i = -3  # Where am I? Am I not the 'i' that should be inside the for loop?
print("before the cycle:", i)
for i in range(5):  # Why am I starting from 0, not from -3?
    print("cycle variable:", i)
    i += 1  # How come I change nothing in terms of amount of loop iterations?
    print("(?) variable:", i)  # And why do I start from the same 0?

Output:

before the cycle: -3
cycle variable: 0
(?) variable: 1
cycle variable: 1
(?) variable: 2
cycle variable: 2
(?) variable: 3
cycle variable: 3
(?) variable: 4
cycle variable: 4
(?) variable: 5

Where should I look up the information about such variable behavior?

My guess is that the question about loop start is somehow related to LEGB rule, and i is both declared and defined (?!) right after for keyword.

But again, I seek knowledge, not guesses.

Answer 1

You are mistaken for the functionality of range . It is a sequence generator which starts with 0 by default and hence it produces 0, 1, 2, 3, 4 for range(5) .

If you want to step from -3 in 5 steps you could do:

i = -3
range(i, i+5) 

Answer 2

Your questions boil down to the mechanics of generators :

for i in range(5):  # Why am I starting from 0, not from -3?

In this line, the range(5) is a generator that will produce values: 0, 1, 2, 3, 4, sequentially. Even if you changed the value of i before the for loop, or you will change the value of i inside the for loop, the for loop will take the next value from the generator and will execute until the generator raises StopIteration , which basically means that it ran out of values to generate.

Answer 3

I think you've understood more or less correctly. "for i" assigns the items yielded by the range generator to i. "range(5)" defaults to a range from 0 to 5. You could add a second parameter, for instance

range(-3, 5)

Or if you want to iterate from a number stored in an earlier variable, you could give it a different name, and pass it as the first argument:

x = 2

for i in range(x, 5):