Do not use setup.cfg file that does not have tox:tox namespace

docs/changelog/1045.bugfix.rst

@@ -0,0 +1 @@
+skip ``setup.cfg`` if it has no ``tox:tox`` namespace - by :user:`hroncok`

docs/config.rst

@@ -15,7 +15,7 @@ At the moment tox supports three configuration locations prioritized in the foll
 As far as the configuration format at the moment we only support standard ConfigParser_ "ini-style" format
 (there is a plan to add a pure TOML one soon).
 ``tox.ini`` and ``setup.cfg`` are such files. Note that ``setup.cfg`` requires the content to be under the
-``tox:tox`` section. ``pyproject.toml`` on the other hand is in TOML format. However, one can inline
+``tox:tox`` section and is otherwise ignored. ``pyproject.toml`` on the other hand is in TOML format. However, one can inline
 the *ini-style* format under the ``tool.tox.legacy_tox_ini`` key as a multi-line string.
 
 Below you find the specification for the *ini-style* format, but you might want to skim some

src/tox/config/__init__.py

@@ -271,7 +271,10 @@ def parseconfig(args, plugins=()):
                 content = toml_content["tool"]["tox"]["legacy_tox_ini"]
             except KeyError:
                 continue
-        ParseIni(config, config_file, content)
+        try:
+            ParseIni(config, config_file, content)
+        except SkipThisIni:
+            continue
         pm.hook.tox_configure(config=config)  # post process config object
         break
     else:
@@ -1045,13 +1048,22 @@ def make_hashseed():
     return str(random.randint(1, max_seed))
 
 
+class SkipThisIni(Exception):
+    """Internal exception to indicate the parsed ini file should be skipped"""
+
+
 class ParseIni(object):
     def __init__(self, config, ini_path, ini_data):  # noqa
         config.toxinipath = ini_path
         using("tox.ini: {} (pid {})".format(config.toxinipath, os.getpid()))
         config.toxinidir = config.toxinipath.dirpath()
 
         self._cfg = py.iniconfig.IniConfig(config.toxinipath, ini_data)
+
+        if ini_path.basename == "setup.cfg" and "tox:tox" not in self._cfg:
+            verbosity1("Found no [tox:tox] section in setup.cfg, skipping.")
+            raise SkipThisIni()
+
         previous_line_of = self._cfg.lineof
 
         self.expand_section_names(self._cfg)

tests/unit/config/test_config.py

@@ -3023,6 +3023,21 @@ def test_config_bad_pyproject_specified(initproj, capsys):
     assert "ERROR:" not in out
 
 
+def test_config_setup_cfg_no_tox_section(initproj, capsys):
+    setup_cfg = """
+        [nope:nope]
+        envlist = py37
+    """
+    initproj("setup_cfg_no_tox-0.1", filedefs={"setup.cfg": setup_cfg})
+    with pytest.raises(SystemExit):
+        parseconfig([])
+
+    out, err = capsys.readouterr()
+    msg = "ERROR: tox config file (either pyproject.toml, tox.ini, setup.cfg) not found\n"
+    assert err == msg
+    assert "ERROR:" not in out
+
+
 @pytest.mark.skipif(sys.platform == "win32", reason="no named pipes on Windows")
 def test_config_bad_config_type_specified(monkeypatch, tmpdir, capsys):
     monkeypatch.chdir(tmpdir)