Do not allow abbreviated arguments

changelog/1149.feature.rst

@@ -0,0 +1,4 @@
+Pytest no longer accepts prefixes of command-line arguments.
+This was previously allowed where the ``ArgumentParser`` thought it was unambigious,
+because this could be incorrect due to delayed parsing of options for plugins.
+See for example issues #1149, #3413, and #4009.

extra/get_issues.py

@@ -74,7 +74,7 @@ def report(issues):
 if __name__ == "__main__":
     import argparse
 
-    parser = argparse.ArgumentParser("process bitbucket issues")
+    parser = argparse.ArgumentParser("process bitbucket issues", allow_abbrev=False)
     parser.add_argument(
         "--refresh", action="store_true", help="invalidate cache, refresh issues"
     )

scripts/release.py

@@ -105,7 +105,7 @@ def changelog(version, write_out=False):
 
 def main():
     init(autoreset=True)
-    parser = argparse.ArgumentParser()
+    parser = argparse.ArgumentParser(allow_abbrev=False)
     parser.add_argument("version", help="Release version")
     options = parser.parse_args()
     pre_release(options.version)

src/_pytest/config/argparsing.py

@@ -1,5 +1,6 @@
 import argparse
 import warnings
+from gettext import gettext
 
 import py
 
@@ -328,6 +329,7 @@ def __init__(self, parser, extra_info=None, prog=None):
             usage=parser._usage,
             add_help=False,
             formatter_class=DropShorterLongHelpFormatter,
+            allow_abbrev=False,
         )
         # extra_info is a dict of (param -> value) to display if there's
         # an usage error to provide more contextual information to the user
@@ -355,6 +357,43 @@ def parse_args(self, args=None, namespace=None):
             getattr(args, FILE_OR_DIR).extend(argv)
         return args
 
+    def _parse_optional(self, arg_string):
+        # This method is entirely copied from `argparse.ArgumentParser`, with a
+        # one-line bugfix for https://bugs.python.org/issue26967 so we can disable
+        # argument abbreviation without breaking flag combination (e.g. -vv).
+        if not arg_string:
+            return None
+        if not arg_string[0] in self.prefix_chars:
+            return None
+        if arg_string in self._option_string_actions:
+            action = self._option_string_actions[arg_string]
+            return action, arg_string, None
+        if len(arg_string) == 1:
+            return None
+        if "=" in arg_string:
+            option_string, explicit_arg = arg_string.split("=", 1)
+            if option_string in self._option_string_actions:
+                action = self._option_string_actions[option_string]
+                return action, option_string, explicit_arg
+        # The "or startswith" part is all we needed to patch!
+        if self.allow_abbrev or not arg_string.startswith("--"):
+            option_tuples = self._get_option_tuples(arg_string)
+            if len(option_tuples) > 1:
+                # (OK, plus tidying this up a bit and the explicit `gettext`)
+                options = ", ".join(option for _, option, _ in option_tuples)
+                args = {"option": arg_string, "matches": options}
+                msg = gettext("ambiguous option: %(option)s could match %(matches)s")
+                self.error(msg % args)
+            elif len(option_tuples) == 1:
+                option_tuple, = option_tuples
+                return option_tuple
+        if self._negative_number_matcher.match(arg_string):
+            if not self._has_negative_number_optionals:
+                return None
+        if " " in arg_string:
+            return None
+        return None, arg_string, None
+
 
 class DropShorterLongHelpFormatter(argparse.HelpFormatter):
     """shorten help for long options that differ only in extra hyphens

testing/example_scripts/perf_examples/collect_stats/generate_folders.py

@@ -4,7 +4,7 @@
 HERE = pathlib.Path(__file__).parent
 TEST_CONTENT = (HERE / "template_test.py").read_bytes()
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(allow_abbrev=False)
 parser.add_argument("numbers", nargs="*", type=int)
 
 

testing/test_parseopt.py

@@ -200,7 +200,7 @@ def defaultget(option):
 
     def test_drop_short_helper(self):
         parser = argparse.ArgumentParser(
-            formatter_class=parseopt.DropShorterLongHelpFormatter
+            formatter_class=parseopt.DropShorterLongHelpFormatter, allow_abbrev=False
         )
         parser.add_argument(
             "-t", "--twoword", "--duo", "--two-word", "--two", help="foo"