add a method to the config object to dynamically add a value to an (line-type) ini-value

_pytest/config.py

@@ -83,6 +83,7 @@ class Parser:
         self._inidict[name] = (help, type, default)
         self._ininames.append(name)
 
+
 class OptionGroup:
     def __init__(self, name, description="", parser=None):
         self.name = name
@@ -346,6 +347,14 @@ class Config(object):
             args.append(py.std.os.getcwd())
         self.args = args
 
+    def addinivalue_line(self, name, line):
+        """ add a line to an ini-file option. The option must have been
+        declared but might not yet be set in which case the line becomes the
+        the first line in its value. """
+        x = self.getini(name)
+        assert isinstance(x, list)
+        x.append(line) # modifies the cached list inline
+
     def getini(self, name):
         """ return configuration value from an ini file. If the
         specified name hasn't been registered through a prior ``parse.addini``

testing/test_config.py

@@ -208,6 +208,40 @@ class TestConfigAPI:
         l = config.getini("a2")
         assert l == []
 
+    def test_addinivalue_line_existing(self, testdir):
+        testdir.makeconftest("""
+            def pytest_addoption(parser):
+                parser.addini("xy", "", type="linelist")
+        """)
+        p = testdir.makeini("""
+            [pytest]
+            xy= 123
+        """)
+        config = testdir.parseconfig()
+        l = config.getini("xy")
+        assert len(l) == 1
+        assert l == ["123"]
+        config.addinivalue_line("xy", "456")
+        l = config.getini("xy")
+        assert len(l) == 2
+        assert l == ["123", "456"]
+
+    def test_addinivalue_line_new(self, testdir):
+        testdir.makeconftest("""
+            def pytest_addoption(parser):
+                parser.addini("xy", "", type="linelist")
+        """)
+        config = testdir.parseconfig()
+        assert not config.getini("xy")
+        config.addinivalue_line("xy", "456")
+        l = config.getini("xy")
+        assert len(l) == 1
+        assert l == ["456"]
+        config.addinivalue_line("xy", "123")
+        l = config.getini("xy")
+        assert len(l) == 2
+        assert l == ["456", "123"]
+
 def test_options_on_small_file_do_not_blow_up(testdir):
     def runfiletest(opts):
         reprec = testdir.inline_run(*opts)