Nested for Loops

[sammay]

Hey everybody,
Hope everything goes well,
As I am a newcomer in Python, I have difficulty in understanding the way Python calculate nested loops.

Here is my exact question:
I'm trying to print triangle with stars,

line 1: for num in range (6):
line 2: stars = ' '
line 3: for x in range(num):
line 4: stars += '*'
line 5: print(stars)

Output:

line 1: null
line 2: *
line 3: **
line 4: ***
line 5: ****
line 6: *****
--------------------------------------------------------------------
What I've found is that when I write line 1, the Python considers 6 row, and for line 3, Python considers 5 columns. when num=0 , x=null, and for num=1 , x=0, and so on.
why for num=0 Python collect x=null?

Thank you so much in advance for your consideration

buran write Jan-09-2021, 05:53 PM:
Please, use proper tags when post code, traceback, output, etc.
See BBcode help for more info.

Also, don't include line numbers with your code