Implemented the dynamic scope feature.

changelog/1682.deprecation.rst

@@ -0,0 +1,2 @@
+Passing arguments to pytest.fixture() as positional arguments is deprecated - pass them
+as a keyword argument instead.

changelog/1682.feature.rst

@@ -0,0 +1,3 @@
+The ``scope`` parameter of ``@pytest.fixture`` can now be a callable that receives
+the fixture name and the ``config`` object as keyword-only parameters.
+See `the docs <https://docs.pytest.org/en/fixture.html#dynamic-scope>`__ for more information.

doc/en/example/costlysetup/conftest.py

@@ -1,7 +1,7 @@
 import pytest
 
 
-@pytest.fixture("session")
+@pytest.fixture(scope="session")
 def setup(request):
     setup = CostlySetup()
     yield setup

doc/en/fixture.rst

@@ -301,6 +301,32 @@ are finalized when the last test of a *package* finishes.
     Use this new feature sparingly and please make sure to report any issues you find.
 
 
+Dynamic scope
+^^^^^^^^^^^^^
+
+In some cases, you might want to change the scope of the fixture without changing the code.
+To do that, pass a callable to ``scope``. The callable must return a string with a valid scope
+and will be executed only once - during the fixture definition. It will be called with two
+keyword arguments - ``fixture_name`` as a string and ``config`` with a configuration object.
+
+This can be especially useful when dealing with fixtures that need time for setup, like spawning
+a docker container. You can use the command-line argument to control the scope of the spawned
+containers for different environments. See the example below.
+
+.. code-block:: python
+
+    def determine_scope(fixture_name, config):
+        if config.getoption("--keep-containers"):
+            return "session"
+        return "function"
+
+
+    @pytest.fixture(scope=determine_scope)
+    def docker_container():
+        yield spawn_container()
+
+
+
 Order: Higher-scoped fixtures are instantiated first
 ----------------------------------------------------
 

src/_pytest/deprecated.py

@@ -29,3 +29,8 @@
     "--result-log is deprecated and scheduled for removal in pytest 6.0.\n"
     "See https://docs.pytest.org/en/latest/deprecations.html#result-log-result-log for more information."
 )
+
+FIXTURE_POSITIONAL_ARGUMENTS = PytestDeprecationWarning(
+    "Passing arguments to pytest.fixture() as positional arguments is deprecated - pass them "
+    "as a keyword argument instead."
+)

src/_pytest/fixtures.py

@@ -2,6 +2,7 @@
 import inspect
 import itertools
 import sys
+import warnings
 from collections import defaultdict
 from collections import deque
 from collections import OrderedDict
@@ -27,6 +28,7 @@
 from _pytest.compat import is_generator
 from _pytest.compat import NOTSET
 from _pytest.compat import safe_getattr
+from _pytest.deprecated import FIXTURE_POSITIONAL_ARGUMENTS
 from _pytest.outcomes import fail
 from _pytest.outcomes import TEST_OUTCOME
 
@@ -58,7 +60,6 @@ def pytest_sessionstart(session):
 
 scopename2class = {}  # type: Dict[str, Type[nodes.Node]]
 
-
 scope2props = dict(session=())  # type: Dict[str, Tuple[str, ...]]
 scope2props["package"] = ("fspath",)
 scope2props["module"] = ("fspath", "module")
@@ -792,6 +793,25 @@ def _teardown_yield_fixture(fixturefunc, it):
         )
 
 
+def _eval_scope_callable(scope_callable, fixture_name, config):
+    try:
+        result = scope_callable(fixture_name=fixture_name, config=config)
+    except Exception:
+        raise TypeError(
+            "Error evaluating {} while defining fixture '{}'.\n"
+            "Expected a function with the signature (*, fixture_name, config)".format(
+                scope_callable, fixture_name
+            )
+        )
+    if not isinstance(result, str):
+        fail(
+            "Expected {} to return a 'str' while defining fixture '{}', but it returned:\n"
+            "{!r}".format(scope_callable, fixture_name, result),
+            pytrace=False,
+        )
+    return result
+
+
 class FixtureDef:
     """ A container for a factory definition. """
 
@@ -811,6 +831,8 @@ def __init__(
         self.has_location = baseid is not None
         self.func = func
         self.argname = argname
+        if callable(scope):
+            scope = _eval_scope_callable(scope, argname, fixturemanager.config)
         self.scope = scope
         self.scopenum = scope2index(
             scope or "function",
@@ -995,7 +1017,57 @@ def __call__(self, function):
         return function
 
 
-def fixture(scope="function", params=None, autouse=False, ids=None, name=None):
+FIXTURE_ARGS_ORDER = ("scope", "params", "autouse", "ids", "name")
+
+
+def _parse_fixture_args(callable_or_scope, *args, **kwargs):
+    arguments = {
+        "scope": "function",
+        "params": None,
+        "autouse": False,
+        "ids": None,
+        "name": None,
+    }
+    kwargs = {
+        key: value for key, value in kwargs.items() if arguments.get(key) != value
+    }
+
+    fixture_function = None
+    if isinstance(callable_or_scope, str):
+        args = list(args)
+        args.insert(0, callable_or_scope)
+    else:
+        fixture_function = callable_or_scope
+
+    positionals = set()
+    for positional, argument_name in zip(args, FIXTURE_ARGS_ORDER):
+        arguments[argument_name] = positional
+        positionals.add(argument_name)
+
+    duplicated_kwargs = {kwarg for kwarg in kwargs.keys() if kwarg in positionals}
+    if duplicated_kwargs:
+        raise TypeError(
+            "The fixture arguments are defined as positional and keyword: {}. "
+            "Use only keyword arguments.".format(", ".join(duplicated_kwargs))
+        )
+
+    if positionals:
+        warnings.warn(FIXTURE_POSITIONAL_ARGUMENTS, stacklevel=2)
+
+    arguments.update(kwargs)
+
+    return fixture_function, arguments
+
+
+def fixture(
+    callable_or_scope=None,
+    *args,
+    scope="function",
+    params=None,
+    autouse=False,
+    ids=None,
+    name=None
+):
     """Decorator to mark a fixture factory function.
 
     This decorator can be used, with or without parameters, to define a
@@ -1041,21 +1113,55 @@ def fixture(scope="function", params=None, autouse=False, ids=None, name=None):
                 ``fixture_<fixturename>`` and then use
                 ``@pytest.fixture(name='<fixturename>')``.
     """
-    if callable(scope) and params is None and autouse is False:
+    fixture_function, arguments = _parse_fixture_args(
+        callable_or_scope,
+        *args,
+        scope=scope,
+        params=params,
+        autouse=autouse,
+        ids=ids,
+        name=name
+    )
+    scope = arguments.get("scope")
+    params = arguments.get("params")
+    autouse = arguments.get("autouse")
+    ids = arguments.get("ids")
+    name = arguments.get("name")
+
+    if fixture_function and params is None and autouse is False:
         # direct decoration
-        return FixtureFunctionMarker("function", params, autouse, name=name)(scope)
+        return FixtureFunctionMarker(scope, params, autouse, name=name)(
+            fixture_function
+        )
+
     if params is not None and not isinstance(params, (list, tuple)):
         params = list(params)
     return FixtureFunctionMarker(scope, params, autouse, ids=ids, name=name)
 
 
-def yield_fixture(scope="function", params=None, autouse=False, ids=None, name=None):
+def yield_fixture(
+    callable_or_scope=None,
+    *args,
+    scope="function",
+    params=None,
+    autouse=False,
+    ids=None,
+    name=None
+):
     """ (return a) decorator to mark a yield-fixture factory function.
 
     .. deprecated:: 3.0
         Use :py:func:`pytest.fixture` directly instead.
     """
-    return fixture(scope=scope, params=params, autouse=autouse, ids=ids, name=name)
+    return fixture(
+        callable_or_scope,
+        *args,
+        scope=scope,
+        params=params,
+        autouse=autouse,
+        ids=ids,
+        name=name
+    )
 
 
 defaultfuncargprefixmarker = fixture()

testing/python/fixtures.py

@@ -2217,6 +2217,68 @@ def test_1(arg):
             ["*ScopeMismatch*You tried*function*session*request*"]
         )
 
+    def test_dynamic_scope(self, testdir):
+        testdir.makeconftest(
+            """
+            import pytest
+
+
+            def pytest_addoption(parser):
+                parser.addoption("--extend-scope", action="store_true", default=False)
+
+
+            def dynamic_scope(fixture_name, config):
+                if config.getoption("--extend-scope"):
+                    return "session"
+                return "function"
+
+
+            @pytest.fixture(scope=dynamic_scope)
+            def dynamic_fixture(calls=[]):
+                calls.append("call")
+                return len(calls)
+
+        """
+        )
+
+        testdir.makepyfile(
+            """
+            def test_first(dynamic_fixture):
+                assert dynamic_fixture == 1
+
+
+            def test_second(dynamic_fixture):
+                assert dynamic_fixture == 2
+
+        """
+        )
+
+        reprec = testdir.inline_run()
+        reprec.assertoutcome(passed=2)
+
+        reprec = testdir.inline_run("--extend-scope")
+        reprec.assertoutcome(passed=1, failed=1)
+
+    def test_dynamic_scope_bad_return(self, testdir):
+        testdir.makepyfile(
+            """
+            import pytest
+
+            def dynamic_scope(**_):
+                return "wrong-scope"
+
+            @pytest.fixture(scope=dynamic_scope)
+            def fixture():
+                pass
+
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(
+            "Fixture 'fixture' from test_dynamic_scope_bad_return.py "
+            "got an unexpected scope value 'wrong-scope'"
+        )
+
     def test_register_only_with_mark(self, testdir):
         testdir.makeconftest(
             """
@@ -4044,12 +4106,43 @@ def test_fixture_named_request(testdir):
     )
 
 
+def test_fixture_duplicated_arguments(testdir):
+    """Raise error if there are positional and keyword arguments for the same parameter (#1682)."""
+    with pytest.raises(TypeError) as excinfo:
+
+        @pytest.fixture("session", scope="session")
+        def arg(arg):
+            pass
+
+    assert (
+        str(excinfo.value)
+        == "The fixture arguments are defined as positional and keyword: scope. "
+        "Use only keyword arguments."
+    )
+
+
+def test_fixture_with_positionals(testdir):
+    """Raise warning, but the positionals should still works (#1682)."""
+    from _pytest.deprecated import FIXTURE_POSITIONAL_ARGUMENTS
+
+    with pytest.warns(pytest.PytestDeprecationWarning) as warnings:
+
+        @pytest.fixture("function", [0], True)
+        def fixture_with_positionals():
+            pass
+
+    assert str(warnings[0].message) == str(FIXTURE_POSITIONAL_ARGUMENTS)
+
+    assert fixture_with_positionals._pytestfixturefunction.scope == "function"
+    assert fixture_with_positionals._pytestfixturefunction.params == (0,)
+    assert fixture_with_positionals._pytestfixturefunction.autouse
+
+
 def test_indirect_fixture_does_not_break_scope(testdir):
     """Ensure that fixture scope is respected when using indirect fixtures (#570)"""
     testdir.makepyfile(
         """
         import pytest
-
         instantiated  = []
 
         @pytest.fixture(scope="session")