Deprecate support for passing command-line as string to pytest.main()

Fixes pytest-dev#1723

CHANGELOG.rst

@@ -203,6 +203,9 @@ time or change existing behaviors in order to make them less surprising/more use
   removed in pytest-4.0 (`#1684`_).
   Thanks `@nicoddemus`_ for the PR.
 
+* Passing a command-line string to ``pytest.main()`` is considered deprecated and scheduled
+  for removal in pytest-4.0. It is recommended to pass a list of arguments instead (`#1723`_).
+
 * Rename ``getfuncargvalue`` to ``getfixturevalue``. ``getfuncargvalue`` is
   still present but is now considered deprecated. Thanks to `@RedBeardCode`_ and `@tomviner`_
   for the PR (`#1626`_).
@@ -282,6 +285,7 @@ time or change existing behaviors in order to make them less surprising/more use
 .. _#1633: https://github.com/pytest-dev/pytest/pull/1633
 .. _#1664: https://github.com/pytest-dev/pytest/pull/1664
 .. _#1684: https://github.com/pytest-dev/pytest/pull/1684
+.. _#1723: https://github.com/pytest-dev/pytest/pull/1723
 
 .. _@DRMacIver: https://github.com/DRMacIver
 .. _@RedBeardCode: https://github.com/RedBeardCode

_pytest/config.py

@@ -98,6 +98,7 @@ def get_plugin_manager():
     return get_config().pluginmanager
 
 def _prepareconfig(args=None, plugins=None):
+    warning = None
     if args is None:
         args = sys.argv[1:]
     elif isinstance(args, py.path.local):
@@ -106,6 +107,10 @@ def _prepareconfig(args=None, plugins=None):
         if not isinstance(args, str):
             raise ValueError("not a string or argument list: %r" % (args,))
         args = shlex.split(args, posix=sys.platform != "win32")
+        # we want to remove this way of passing arguments to pytest.main()
+        # in pytest-4.0
+        warning = ('passing a string to pytest.main() is deprecated, '
+                   'pass a list of arguments instead.')
     config = get_config()
     pluginmanager = config.pluginmanager
     try:
@@ -115,6 +120,8 @@ def _prepareconfig(args=None, plugins=None):
                     pluginmanager.consider_pluginarg(plugin)
                 else:
                     pluginmanager.register(plugin)
+        if warning:
+            config.warn('C1', warning)
         return pluginmanager.hook.pytest_cmdline_parse(
                 pluginmanager=pluginmanager, args=args)
     except BaseException:

testing/acceptance_test.py

@@ -795,3 +795,17 @@ def test_funcarg_prefix(value):
          'Please remove the prefix and use the @pytest.fixture decorator instead.'),
         '*1 passed*',
     ])
+
+
+def test_str_args_deprecated(tmpdir, testdir):
+    """Deprecate passing strings to pytest.main(). Scheduled for removal in pytest-4.0."""
+    warnings = []
+
+    class Collect:
+        def pytest_logwarning(self, message):
+            warnings.append(message)
+
+    ret = pytest.main("%s -x" % tmpdir, plugins=[Collect()])
+    testdir.delete_loaded_modules()
+    assert warnings == ['passing a string to pytest.main() is deprecated, pass a list of arguments instead.']
+    assert ret == EXIT_NOTESTSCOLLECTED