Preprocessing an OBJ file, what is the best way to?

[KeithSloan]

You could extract all the relevant lines with one pattern

import io
import re

file = io.StringIO('''\
v 6.806145 3.287188 34.507591
v 6.889372 3.314378 34.482964
...... repeat large number of times with different values
g groupName
usemtl groupMaterial
f 1 2 3
f 4 5 6
f 4 7 8
f 5 4 8
f 8 7 9
...... repeat a large number of times with different values
v -5.852165 2.924480 33.700558
v -5.861451 2.937897 33.763584
v -5.978782 2.973776 33.743969
v -5.635860 2.871064 33.685860
v -5.709496 2.899417 33.751705
v -5.442479 2.838525 33.676365
v -5.522288 2.871565 33.741238
v -5.261642 2.836869 33.678814
v -5.349662 2.853023 33.73287
.... repeat a large number with different values

g nextGroupName
usemtl nextGroupMaterial

... more v & f definitions''')

pattern = re.compile(r"^(?:[og]|usemtl)\s.*", re.MULTILINE)

print(pattern.findall(file.read()))

Output:
['g groupName', 'usemtl groupMaterial', 'g nextGroupName', 'usemtl nextGroupMaterial']

You could then split the result with more_itertools.bucket()

lines = pattern.findall(file.read())

from more_itertools import bucket
from operator import itemgetter

s = bucket(lines, itemgetter(0))
print(list(s['o']))
print(list(s['u']))
print(list(s['g']))

Output:
[]
['usemtl groupMaterial', 'usemtl nextGroupMaterial']
['g groupName', 'g nextGroupName']

Thanks that is VERY helpful.