Accept None for Date[time]Picker widgets

While hard to reproduce, there has been some reports about this being an issue, so we make an explicit check to cover this case.

Author: vidartf

python/ipywidgets/ipywidgets/widgets/widget_date.py

@@ -54,6 +54,8 @@ class DatePicker(DescriptionWidget, ValueWidget, CoreWidget):
     def _validate_value(self, proposal):
         """Cap and floor value"""
         value = proposal["value"]
+        if value is None:
+            return value
         if self.min and self.min > value:
             value = max(value, self.min)
         if self.max and self.max < value:
@@ -64,6 +66,8 @@ def _validate_value(self, proposal):
     def _validate_min(self, proposal):
         """Enforce min <= value <= max"""
         min = proposal["value"]
+        if min is None:
+            return min
         if self.max and min > self.max:
             raise TraitError("Setting min > max")
         if self.value and min > self.value:
@@ -74,6 +78,8 @@ def _validate_min(self, proposal):
     def _validate_max(self, proposal):
         """Enforce min <= value <= max"""
         max = proposal["value"]
+        if max is None:
+            return max
         if self.min and max < self.min:
             raise TraitError("setting max < min")
         if self.value and max < self.value:

python/ipywidgets/ipywidgets/widgets/widget_datetime.py

@@ -61,6 +61,8 @@ def _validate_tz(self, value):
     def _validate_value(self, proposal):
         """Cap and floor value"""
         value = proposal["value"]
+        if value is None:
+            return value
         value = self._validate_tz(value)
         if self.min and self.min > value:
             value = max(value, self.min)
@@ -72,6 +74,8 @@ def _validate_value(self, proposal):
     def _validate_min(self, proposal):
         """Enforce min <= value <= max"""
         min = proposal["value"]
+        if min is None:
+            return min
         min = self._validate_tz(min)
         if self.max and min > self.max:
             raise TraitError("Setting min > max")
@@ -83,6 +87,8 @@ def _validate_min(self, proposal):
     def _validate_max(self, proposal):
         """Enforce min <= value <= max"""
         max = proposal["value"]
+        if max is None:
+            return max
         max = self._validate_tz(max)
         if self.min and max < self.min:
             raise TraitError("setting max < min")