introduce attrs as dependency

as a initial class to use it i picked FixtureFunctionMarker and made it immutable

_pytest/fixtures.py

@@ -7,6 +7,7 @@
 import warnings
 
 import inspect
+import attr
 import _pytest
 from _pytest._code.code import TerminalRepr
 from _pytest.compat import (
@@ -611,7 +612,6 @@ def scope2index(scope, descr, where=None):
                 scope)
         )
 
-
 class FixtureLookupError(LookupError):
     """ could not return a requested Fixture (missing or invalid). """
 
@@ -823,13 +823,21 @@ def pytest_fixture_setup(fixturedef, request):
     return result
 
 
-class FixtureFunctionMarker:
-    def __init__(self, scope, params, autouse=False, ids=None, name=None):
-        self.scope = scope
-        self.params = params
-        self.autouse = autouse
-        self.ids = ids
-        self.name = name
+def _ensure_immutable_ids(ids):
+    if ids is None:
+        return
+    if callable(ids):
+        return ids
+    return tuple(ids)
+
+
+@attr.s(frozen=True)
+class FixtureFunctionMarker(object):
+    scope = attr.ib()
+    params = attr.ib(convert=attr.converters.optional(tuple))
+    autouse = attr.ib(default=False)
+    ids = attr.ib(default=None, convert=_ensure_immutable_ids)
+    name = attr.ib(default=None)
 
     def __call__(self, function):
         if isclass(function):

setup.py

@@ -43,7 +43,11 @@ def has_environment_marker_support():
 
 
 def main():
-    install_requires = ['py>=1.4.33', 'setuptools']  # pluggy is vendored in _pytest.vendored_packages
+    install_requires = [
+        'py>=1.4.33',
+        'setuptools',
+        'attrs',
+    ]  # pluggy is vendored in _pytest.vendored_packages
     extras_require = {}
     if has_environment_marker_support():
         extras_require[':python_version=="2.6"'] = ['argparse', 'ordereddict']