add option to view failure stacktraces

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

docs/changelog/1572.feature.rst

@@ -0,0 +1,2 @@
+Add a CLI flag :option:`with-traceback` that allows displaying the stacktrace of the virtualenv when a failure occurs
+- by :user:`gaborbernat`.

src/virtualenv/__main__.py

@@ -1,19 +1,20 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
+import argparse
 import logging
 import sys
 from datetime import datetime
 
 
-def run(args=None):
+def run(args=None, options=None):
     start = datetime.now()
     from virtualenv.error import ProcessCallFailed
     from virtualenv.run import run_via_cli
 
     if args is None:
         args = sys.argv[1:]
     try:
-        run_via_cli(args)
+        run_via_cli(args, options)
     except ProcessCallFailed as exception:
         print("subprocess call failed for {}".format(exception.cmd))
         print(exception.out, file=sys.stdout, end="")
@@ -24,11 +25,16 @@ def run(args=None):
 
 
 def run_with_catch(args=None):
+    options = argparse.Namespace()
     try:
-        run(args)
+        run(args, options)
     except (KeyboardInterrupt, Exception) as exception:
-        logging.error("%s: %s", type(exception).__name__, exception)
-        sys.exit(1)
+        if options.with_traceback:
+            logging.shutdown()  # force flush of log messages before the trace is printed
+            raise
+        else:
+            logging.error("%s: %s", type(exception).__name__, exception)
+            sys.exit(1)
 
 
 if __name__ == "__main__":

src/virtualenv/config/cli/parser.py

@@ -11,7 +11,7 @@ class VirtualEnvConfigParser(ArgumentParser):
     Custom option parser which updates its defaults by checking the configuration files and environmental variables
     """
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, options=None, *args, **kwargs):
         self.file_config = IniConfig()
         self.epilog_list = []
         kwargs["epilog"] = self.file_config.epilog
@@ -22,7 +22,7 @@ def __init__(self, *args, **kwargs):
         self._fixed = set()
         self._elements = None
         self._verbosity = None
-        self._options = None
+        self._options = options
         self._interpreter = None
 
     def _fix_defaults(self):

src/virtualenv/run/__init__.py

@@ -12,30 +12,37 @@
 from .plugin.seeders import SeederSelector
 
 
-def run_via_cli(args):
+def run_via_cli(args, options=None):
     """Run the virtual environment creation via CLI arguments
 
     :param args: the command line arguments
     :return: the creator used
     """
-    session = session_via_cli(args)
+    session = session_via_cli(args, options)
     session.run()
     return session
 
 
 # noinspection PyProtectedMember
-def session_via_cli(args):
-    parser = build_parser(args)
+def session_via_cli(args, options=None):
+    parser = build_parser(args, options)
     parser.parse_args(args, namespace=parser._options)
     creator, seeder, activators = tuple(e.create(parser._options) for e in parser._elements)  # create types
     session = Session(parser._verbosity, parser._interpreter, creator, seeder, activators)
     return session
 
 
 # noinspection PyProtectedMember
-def build_parser(args=None):
-    parser = VirtualEnvConfigParser()
+def build_parser(args=None, options=None):
+    parser = VirtualEnvConfigParser(options)
     add_version_flag(parser)
+    parser.add_argument(
+        "--with-traceback",
+        dest="with_traceback",
+        action="store_true",
+        default=False,
+        help="on failure also display the stacktrace internals of virtualenv",
+    )
     parser._options, parser._verbosity = _do_report_setup(parser, args)
     discover = get_discover(parser, args, parser._options)
     parser._interpreter = interpreter = discover.interpreter
@@ -73,6 +80,6 @@ def _do_report_setup(parser, args):
     verbosity = verbosity_group.add_mutually_exclusive_group()
     verbosity.add_argument("-v", "--verbose", action="count", dest="verbose", help="increase verbosity", default=2)
     verbosity.add_argument("-q", "--quiet", action="count", dest="quiet", help="decrease verbosity", default=0)
-    options, _ = parser.parse_known_args(args)
+    options, _ = parser.parse_known_args(args, namespace=parser._options)
     verbosity_value = setup_report(options.verbose, options.quiet)
     return options, verbosity_value

tests/conftest.py

@@ -182,8 +182,8 @@ def coverage_env(monkeypatch, link):
         # we inject right after creation, we cannot collect coverage on site.py - used for helper scripts, such as debug
         from virtualenv import run
 
-        def via_cli(args):
-            session = prev_run(args)
+        def via_cli(args, options=None):
+            session = prev_run(args, options)
             old_run = session.creator.run
 
             def create_run():

tests/unit/create/test_creator.py

@@ -174,8 +174,8 @@ def list_to_str(iterable):
 
 @pytest.mark.skipif(not CURRENT.has_venv, reason="requires interpreter with venv")
 def test_venv_fails_not_inline(tmp_path, capsys, mocker):
-    def _session_via_cli(args):
-        session = session_via_cli(args)
+    def _session_via_cli(args, options=None):
+        session = session_via_cli(args, options)
         assert session.creator.can_be_inline is False
         return session