Make _CountingAttr empty metadata unique

changelog.d/280.change.rst

@@ -0,0 +1,2 @@
+``attrib`` now defaults to a unique empty ``dict`` instance for the returned ``_CountingAttr`` instead of sharing a common empty ``dict`` for all.
+The singleton empty ``dict`` is still enforced by ``@attr.s``.

src/attr/_make.py

@@ -55,7 +55,7 @@ def __hash__(self):
 
 def attrib(default=NOTHING, validator=None,
            repr=True, cmp=True, hash=None, init=True,
-           convert=None, metadata={}, type=None):
+           convert=None, metadata=None, type=None):
     """
     Create a new attribute on a class.
 
@@ -134,6 +134,8 @@ def attrib(default=NOTHING, validator=None,
         raise TypeError(
             "Invalid value for hash.  Must be True, False, or None."
         )
+    if metadata is None:
+        metadata = {}
     return _CountingAttr(
         default=default,
         validator=validator,

tests/test_make.py

@@ -5,6 +5,7 @@
 from __future__ import absolute_import, division, print_function
 
 import inspect
+import itertools
 import sys
 
 from operator import attrgetter
@@ -26,7 +27,7 @@
 
 from .utils import (
     gen_attr_names, list_of_attrs, simple_attr, simple_attrs,
-    simple_attrs_without_metadata, simple_classes
+    simple_attrs_with_metadata, simple_attrs_without_metadata, simple_classes
 )
 
 
@@ -823,6 +824,34 @@ def test_empty_metadata_singleton(self, list_of_attrs):
         for a in fields(C)[1:]:
             assert a.metadata is fields(C)[0].metadata
 
+    @given(lists(simple_attrs_without_metadata, min_size=2, max_size=5))
+    def test_empty_countingattr_metadata_independent(self, list_of_attrs):
+        """
+        All empty metadata attributes are independent before ``@attr.s``.
+        """
+        for x, y in itertools.combinations(list_of_attrs, 2):
+            assert x.metadata is not y.metadata
+
+    @given(lists(simple_attrs_with_metadata(), min_size=2, max_size=5))
+    def test_not_none_metadata(self, list_of_attrs):
+        """
+        Non-empty metadata attributes exist as fields after ``@attr.s``.
+        """
+        C = make_class("C", dict(zip(gen_attr_names(), list_of_attrs)))
+
+        assert len(fields(C)) > 0
+
+        for cls_a, raw_a in zip(fields(C), list_of_attrs):
+            assert cls_a.metadata != {}
+            assert cls_a.metadata == raw_a.metadata
+
+    def test_not_none_metadata_force_coverage(self):
+        """
+        Force coverage of metadata is not None case even though other tests
+        should do so anyways.
+        """
+        attr.ib(metadata={})
+
 
 class TestClassBuilder(object):
     """

tests/utils.py

@@ -145,7 +145,7 @@ def ordereddict_of_class(tup):
                      attrs_and_classes.map(ordereddict_of_class))
 
 
-bare_attrs = st.just(attr.ib(default=None))
+bare_attrs = st.builds(attr.ib, default=st.none())
 int_attrs = st.integers().map(lambda i: attr.ib(default=i))
 str_attrs = st.text().map(lambda s: attr.ib(default=s))
 float_attrs = st.floats().map(lambda f: attr.ib(default=f))
@@ -164,7 +164,9 @@ def simple_attrs_with_metadata(draw):
     c_attr = draw(simple_attrs)
     keys = st.booleans() | st.binary() | st.integers() | st.text()
     vals = st.booleans() | st.binary() | st.integers() | st.text()
-    metadata = draw(st.dictionaries(keys=keys, values=vals))
+    metadata = draw(st.dictionaries(
+        keys=keys, values=vals, min_size=1, max_size=5))
+    print('metadata', metadata)
 
     return attr.ib(c_attr._default, c_attr._validator, c_attr.repr,
                    c_attr.cmp, c_attr.hash, c_attr.init, c_attr.convert,