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 @@ class FixtureManager:
                     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_yield_tests_deprecation(testdir):
         '*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*',
+    ])