Deprecate "pytest_funcarg__" prefix to declare fixtures

Fixes #1684

CHANGELOG.rst

@@ -188,7 +188,9 @@
 * ``yield``-based tests are considered deprecated and will be removed in pytest-4.0.
  Thanks `@nicoddemus`_ for the PR.
 
-*
+* Using ``pytest_funcarg__`` prefix to declare fixtures is considered deprecated and will be
+ removed in pytest-4.0 (`#1684`_).
+ Thanks `@nicoddemus`_ for the PR.
 
 *
 
@@ -262,6 +264,7 @@
 .. _#1632: https://github.com/pytest-dev/pytest/issues/1632
 .. _#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
 
 .. _@DRMacIver: https://github.com/DRMacIver
 .. _@RedBeardCode: https://github.com/RedBeardCode

_pytest/fixtures.py

@@ -865,6 +865,10 @@ def yield_fixture(scope="function", params=None, autouse=False, ids=None, name=N
  return FixtureFunctionMarker(scope, params, autouse, ids=ids, name=name)
 
 defaultfuncargprefixmarker = fixture()
+funcarg_prefix_warning = 'declaring fixtures using "pytest_funcarg__" prefix is deprecated ' \
+ 'and scheduled to be removed in pytest 4.0.\n' \
+ 'remove the prefix and use the @pytest.fixture decorator instead'
+
 
 
 @fixture(scope="session")
@@ -1043,6 +1047,7 @@ def parsefactories(self, node_or_obj, nodeid=NOTSET, unittest=False):
  continue
  marker = defaultfuncargprefixmarker
  name = name[len(self._argprefix):]
+ self.config.warn('C1', funcarg_prefix_warning)
  elif not isinstance(marker, FixtureFunctionMarker):
  # magic globals with __getattr__ might have got us a wrong
  # fixture attribute

testing/acceptance_test.py

@@ -777,3 +777,19 @@ def test_gen():
  '*yield tests are deprecated, and scheduled to be removed in pytest 4.0*',
  '*2 passed*',
  ])
+
+
+def test_funcarg_prefix_deprecation(testdir):
+ testdir.makepyfile("""
+ def pytest_funcarg__value():
+ return 10
+
+ def test_funcarg_prefix(value):
+ assert value == 10
+ """)
+ result = testdir.runpytest('-ra')
+ result.stdout.fnmatch_lines([
+ '*declaring fixtures using "pytest_funcarg__" prefix is deprecated and scheduled to be removed in pytest 4.0*',
+ '*remove the prefix and use the @pytest.fixture decorator instead*',
+ '*1 passed*',
+ ])