Python: Why does this code execute?

Question

I'm a beginner in python and using v2.7.2 here's what i tried to execute in the command prompt

 p = 2
 while(p>0):
     for i in range(10):
          print i+1 , p
          p-=1

The expected output was

1 2
2 1

However the actual output is

1 2
2 1
3 0
4 -1
5 -2
6 -3
7 -4
8 -5
9 -6
10 -7

Why does this happen? and How do i achieve the expected behavior?

Answer 1

The while condition is only tested again after the for loop finishes. You could do this instead

p = 2
for i in range(10):
    if p <= 0:
        break
    print i+1 , p
    p-=1

Answer 2

This is the output I get:

1 2
2 1
3 0
4 -1
5 -2
6 -3
7 -4
8 -5
9 -6
10 -7

Your question as to why it runs. Your outer most conditional is a while loop, which is true upon the first execution, however it runs right into a nested for loop. When this happens the while, will not be checked until the for loop finishes its first execution (which is why p = -7).

What you want is this:

p = 2
for i in range(10):
    if p <= 0:
        break
    print i+1 , p
    p-=1

which gives output:

1 2
2 1