gh-116159: argparse: performance improvement parsing large number of options (#116162)

When parsing positional vs optional arguments, the use of min with a
list comprehension inside of a loop results in quadratic time based
on the number of optional arguments given. When combined with use of
prefix based argument files and a large number of optional flags, this
can result in extremely slow parsing behavior.

This replaces the min call with a simple loop with a short circuit to
break at the next optional argument.

Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 4200dd5..0dbdd67 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2153,10 +2153,11 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         while start_index <= max_option_string_index:
 
             # consume any Positionals preceding the next option
-            next_option_string_index = min([
-                index
-                for index in option_string_indices
-                if index >= start_index])
+            next_option_string_index = start_index
+            while next_option_string_index <= max_option_string_index:
+                if next_option_string_index in option_string_indices:
+                    break
+                next_option_string_index += 1
             if start_index != next_option_string_index:
                 positionals_end_index = consume_positionals(start_index)