Implement the "-c" command line switch that allows to explicitly specifiy the config file to load.

This feature was requested in issue #174.

--HG--
branch : explicit-ini-filename

_pytest/config.py

@@ -833,6 +833,14 @@ def exists(path, ignore=EnvironmentError):
         return False
 
 def getcfg(args, inibasenames):
+    if "-c" in args:
+        n = len(args)
+        for i in range(1, n):
+            if args[i - 1] == "-c" and not str(args[i]).startswith("-"):
+                iniconfig = py.iniconfig.IniConfig(args[i])
+                if 'pytest' in iniconfig.sections:
+                    return iniconfig['pytest']
+        return {}
     args = [x for x in args if not str(x).startswith("-")]
     if not args:
         args = [py.path.local()]

_pytest/main.py

@@ -38,6 +38,9 @@ def pytest_addoption(parser):
                help="exit after first num failures or errors.")
     group._addoption('--strict', action="store_true",
                help="run pytest in strict mode, warnings become errors.")
+    # This option is never used as such, see config.getcfg().
+    group._addoption("-c", metavar="file", type=str, dest="_inifilename",
+               help="load configuration from `file` instead of trying to locate one of the implicit configuration files.")
 
     group = parser.getgroup("collect", "collection")
     group.addoption('--collectonly', '--collect-only', action="store_true",

testing/test_config.py

@@ -79,6 +79,21 @@ class TestConfigCmdlineParsing:
         config = testdir.parseconfig()
         pytest.raises(AssertionError, lambda: config.parse([]))
 
+    def test_explicitly_specified_config_file_is_loaded(self, testdir):
+        testdir.makeconftest("""
+            def pytest_addoption(parser):
+                parser.addini("custom", "")
+        """)
+        testdir.makeini("""
+            [pytest]
+            custom = 0
+        """)
+        testdir.makefile(".cfg", custom = """
+            [pytest]
+            custom = 1
+        """)
+        config = testdir.parseconfig("-c", "custom.cfg")
+        assert config.getini("custom") == "1"
 
 class TestConfigAPI:
     def test_config_trace(self, testdir):