Extract string before underscore in python string

Regular expressions can remove a suffix including the underscore from a string. Using regular expressions you can match on exactly what you want to operate on.

import re
for s in ["AAA_X", "BBB_X", "CCC_X"]:
  print(s, ">>", re.sub(r"_.*", "", s))

Output:

AAA_X >> AAA
BBB_X >> BBB
CCC_X >> CCC

If want to output the prefix only when an underscore character is present in a string then can use the re.match() function. If no match such as input string "DDD" then the target string does not have a prefix followed by an underscore and will not be printed.

import re
for s in ["AAA_X", "BBB_X", "CCC_X", "DDD"]:
  if m := re.match(r"(w+)_", s):
     print(s, ">>", m.group(1))

The above code has identical output as the first code example.