Python3: How to increment a string value within a "for" loop - string

Question

I have a tabular.text file (Named "xfile"). An example of its contents is attached below.
Scaffold2_1 WP_017805071.1 26.71 161 97
Scaffold2_1 WP_006995572.1 26.36 129 83
Scaffold2_1 WP_005723576.1 26.92 130 81
Scaffold3_1 WP_009894856.1 25.77 245 43
Scaffold8_1 WP_017805071.1 38.31 248 145
Scaffold8_1 WP_006995572.1 38.55 249 140
Scaffold8_1 WP_005723576.1 34.88 258 139
Scaffold9_1 WP_005645255.1 42.54 446 144
Note how each line begins with Scaffold(y)_1, with y being a number. I have written the following code to print each line beginning with the following terms, Scaffold2 and Scaffold8.
with open("xfile", 'r') as data:
for line in data.readlines():
if "Scaffold2" in line:
a = line
print(a)
elif "Scaffold8" in line:
b = line
print(b)
I was wondering, is there a way you would recommend incrementing the (y) portion of Scaffold() in the if and elif statements?
The idea would be to allow the script to search for each line containing "Scaffold(y)" and storing each line with a specific number (y) in its own variable to be then printed. This would obviously be much faster than entering in each number manually.

Answer 1

You can try this, it is quite easier than using Regex. If this isn't what you expect, let me know, I'll change the code.
for line in data.readlines():
if line[0:8] == "Scaffold" and line[8].isdigit():
print(line)
I'm just checking the 9th Position in your line, i.e. (8th index). If it's a digit, I'm printing the line. Like you said, I'm printing if your "y" is a digit. I'm not incrementing it. The work of incrementation is already been done by your for loop.

Answer 2

Ok it seems like you want to get something in format like:
entries = {y1: ['Scaffold(y1)_...', 'Scaffold(y1)_...'], y2: ['Scaffold(y2)_...', 'Scaffold(y2)_...'], ...}
Then you can do something like that (I assume all of your lines start in the same manner as you have shown, so the y value is always the 8th position in the string):
entries = dict()
for line in data.readlines():
if not line[8] in entries.keys():
entries.update({line[8]: [line]})
else:
entries[line[8]].append(line)
print(entries)
This way you will have a dictionary in the format I have shown you above - output:
{'2': ['Scaffold2_1 WP_017805071.1 26.71 161 97', 'Scaffold2_1 WP_006995572.1 26.36 129 83', 'Scaffold2_1 WP_005723576.1 26.92 130 81'], '3': ['Scaffold3_1 WP_009894856.1 25.77 245 43'], '8': ['Scaffold8_1 WP_017805071.1 38.31 248 145', 'Scaffold8_1 WP_006995572.1 38.55 249 140', 'Scaffold8_1 WP_005723576.1 34.88 258 139'], '9': ['Scaffold9_1 WP_005645255.1 42.54 446 144']}
EDIT: tbh I still don't fully understand why would you need that tho.