argparsing: Parser: allow to forward prog to argparse

Ref: pytest-dev/pytest-xdist#388.

src/_pytest/config/argparsing.py

@@ -18,6 +18,8 @@ class Parser(object):
         there's an error processing the command line arguments.
     """
 
+    prog = None
+
     def __init__(self, usage=None, processopt=None):
         self._anonymous = OptionGroup("custom options", parser=self)
         self._groups = []
@@ -82,7 +84,7 @@ def parse(self, args, namespace=None):
     def _getparser(self):
         from _pytest._argcomplete import filescompleter
 
-        optparser = MyOptionParser(self, self.extra_info)
+        optparser = MyOptionParser(self, self.extra_info, prog=self.prog)
         groups = self._groups + [self._anonymous]
         for group in groups:
             if group.options:
@@ -319,12 +321,13 @@ def _addoption_instance(self, option, shortupper=False):
 
 
 class MyOptionParser(argparse.ArgumentParser):
-    def __init__(self, parser, extra_info=None):
+    def __init__(self, parser, extra_info=None, prog=None):
         if not extra_info:
             extra_info = {}
         self._parser = parser
         argparse.ArgumentParser.__init__(
             self,
+            prog=prog,
             usage=parser._usage,
             add_help=False,
             formatter_class=DropShorterLongHelpFormatter,

testing/test_parseopt.py

@@ -24,6 +24,12 @@ def test_no_help_by_default(self, capsys):
         out, err = capsys.readouterr()
         assert err.find("error: unrecognized arguments") != -1
 
+    def test_custom_prog(self, parser):
+        """Custom prog can be set for `argparse.ArgumentParser`."""
+        assert parser._getparser().prog == os.path.basename(sys.argv[0])
+        parser.prog = "custom-prog"
+        assert parser._getparser().prog == "custom-prog"
+
     def test_argument(self):
         with pytest.raises(parseopt.ArgumentError):
             # need a short or long option