Enable custom error formatting

src/pytest_mypy.py

@@ -14,6 +14,14 @@
 nodeid_name = 'mypy'
 
 
+def default_file_error_formatter(item, results, errors):
+    """Create a string to be displayed when mypy finds errors in a file."""
+    return '\n'.join(errors)
+
+
+file_error_formatter = default_file_error_formatter
+
+
 def pytest_addoption(parser):
     """Add options for enabling and running mypy."""
     group = parser.getgroup('mypy')
@@ -158,7 +166,7 @@ def runtest(self):
         abspath = os.path.abspath(str(self.fspath))
         errors = results['abspath_errors'].get(abspath)
         if errors:
-            raise MypyError('\n'.join(errors))
+            raise MypyError(file_error_formatter(self, results, errors))
 
     def reportinfo(self):
         """Produce a heading for the test report."""

tests/test_pytest_mypy.py

@@ -286,3 +286,30 @@ def pytest_collection_modifyitems(session, config, items):
     testdir.mkdir(name)
     result = testdir.runpytest_subprocess('--mypy', *xdist_args, name)
     assert result.ret != 0
+
+
+def test_api_error_formatter(testdir, xdist_args):
+    """Ensure that the plugin can be configured in a conftest.py."""
+    testdir.makepyfile(bad='''
+        def pyfunc(x: int) -> str:
+            return x * 2
+    ''')
+    testdir.makepyfile(conftest='''
+        def custom_file_error_formatter(item, results, errors):
+            return '\\n'.join(
+                '{path}:{error}'.format(
+                    path=item.fspath,
+                    error=error,
+                )
+                for error in errors
+            )
+
+        def pytest_configure(config):
+            plugin = config.pluginmanager.getplugin('mypy')
+            plugin.file_error_formatter = custom_file_error_formatter
+    ''')
+    result = testdir.runpytest_subprocess('--mypy', *xdist_args)
+    result.stdout.fnmatch_lines([
+        '*/bad.py:2: error: Incompatible return value*',
+    ])
+    assert result.ret != 0