Improve interoperability of TimeSpan dates.

Attributes 'start_date' and 'end_date' must return a normal Python
`datetime.date`:class: instance under normal operation.

Returning an Infinity-comparable type should be regarded as an internal
implementation hack to facilitate the implementation of `__and__` and
`__or__`.

This allows normal code to do math with `start_date` and `end_date`.  For
instance::

   >>> from datetime import date
   >>> date.today() - timespan.start_end

That could fail in Python 2.7 because 'date' would not interoperate with the
subtype returned by 'start_date'.

tests/test_datetime.py

@@ -187,3 +187,22 @@ def test_timespans_are_representable(value):
 @given(time_span('none'))
 def test_generate_valid_timespans(ts):
     assert ts.valid
+
+
+@given(time_span('none'))
+def test_ts_returns_dates_not_subtypes(ts):
+    from datetime import date
+    assert type(ts.start_date) is date
+    assert type(ts.end_date) is date
+
+    from xoutil.context import context
+    from xoutil.datetime import infinity_extended_date, NEEDS_FLEX_DATE
+    with context(NEEDS_FLEX_DATE):
+        assert type(ts.start_date) is infinity_extended_date
+        assert type(ts.end_date) is infinity_extended_date
+
+
+@given(time_span('none'), strategies.dates())
+def test_operate_with_timespans(ts, d):
+    assert ts.start_date - d is not None
+    assert d - ts.start_date is not None

xoutil/datetime.py

@@ -455,9 +455,10 @@ def __init__(self, name, nullable=False):
         self.nullable = nullable
 
     def __get__(self, instance, owner):
+        from xoutil.context import context
         if instance is not None:
             res = instance.__dict__[self.name]
-            if res:
+            if res and NEEDS_FLEX_DATE in context:
                 return infinity_extended_date(res.year, res.month, res.day)
             else:
                 return res
@@ -563,10 +564,12 @@ def valid(self):
         Unbound time spans are always valid.
 
         '''
-        if self.bound:
-            return self.start_date <= self.end_date
-        else:
-            return True
+        from xoutil.context import context
+        with context(NEEDS_FLEX_DATE):
+            if self.bound:
+                return self.start_date <= self.end_date
+            else:
+                return True
 
     def __contains__(self, other):
         '''Test if we completely cover `other` time span.
@@ -651,14 +654,22 @@ def __and__(self, other):
         '''
         import datetime
         from .infinity import Infinity
+        from xoutil.context import context
         if isinstance(other, _EmptyTimeSpan):
             return other
         elif isinstance(other, datetime.date):
             other = TimeSpan.from_date(other)
         elif not isinstance(other, TimeSpan):
             raise TypeError
-        start = max(self.start_date or -Infinity, other.start_date or -Infinity)
-        end = min(self.end_date or Infinity, other.end_date or Infinity)
+        with context(NEEDS_FLEX_DATE):
+            start = max(
+                self.start_date or -Infinity,
+                other.start_date or -Infinity
+            )
+            end = min(
+                self.end_date or Infinity,
+                other.end_date or Infinity
+            )
         if start <= end:
             if start is -Infinity:
                 start = None
@@ -679,14 +690,22 @@ def __or__(self, other):
         'Return the union of both time spans.'
         import datetime
         from .infinity import Infinity
+        from xoutil.context import context
         if isinstance(other, _EmptyTimeSpan):
             return self
         elif isinstance(other, datetime.date):
             other = TimeSpan.from_date(other)
         elif not isinstance(other, TimeSpan):
             raise TypeError
-        start = min(self.start_date or -Infinity, other.start_date or -Infinity)
-        end = max(self.end_date or Infinity, other.end_date or Infinity)
+        with context(NEEDS_FLEX_DATE):
+            start = min(
+                self.start_date or -Infinity,
+                other.start_date or -Infinity
+            )
+            end = max(
+                self.end_date or Infinity,
+                other.end_date or Infinity
+            )
         if start <= end:
             if start is -Infinity:
                 start = None
@@ -779,3 +798,8 @@ def __repr__(self):
 EmptyTimeSpan = _EmptyTimeSpan()
 
 _EmptyTimeSpan.__new__ = None  # Disallow creating more instances
+
+
+# A context to switch on/off returning a subtype of date from DateFields.
+# Used within TimeSpan to allow comparison with Infinity.
+NEEDS_FLEX_DATE = object()