Fix to avoid a bad default value of a config option from causing trouble:

theano/configparser.py

@@ -194,7 +194,9 @@ def AddConfigVar(name, doc, configparam, root=config, in_c_key=True):
         setattr(root.__class__, sections[0], configparam)
         _config_var_list.append(configparam)
 
+
 class ConfigParam(object):
+
     def __init__(self, default, filter=None,  allow_override=True):
         """
         If allow_override is False, we can't change the value after the import of Theano.
@@ -207,9 +209,11 @@ class ConfigParam(object):
         # self.fullname  # set by AddConfigVar
         # self.doc       # set by AddConfigVar
 
-        # Check that default is a valid value
-        if self.filter:
-            self.filter(self.default)
+        # Note that we do not call `self.filter` on the default value: this
+        # will be done automatically in AddConfigVar, potentially with a
+        # more appropriate user-provided default value.
+        # Calling `filter` here may actually be harmful if the default value is
+        # invalid and causes a crash or has unwanted side effects.
 
     def __get__(self, *args):
         #print "GETTING PARAM", self.fullname, self, args
@@ -223,7 +227,7 @@ class ConfigParam(object):
         return self.val
 
     def __set__(self, cls, val):
-        if not self.allow_override and hasattr(self,'val'):
+        if not self.allow_override and hasattr(self, 'val'):
             raise Exception("Can't change the value of this config parameter after initialization!")
         #print "SETTING PARAM", self.fullname,(cls), val
         if self.filter:
@@ -231,7 +235,9 @@ class ConfigParam(object):
         else:
             self.val = val
 
-    deleter=None
+    # TODO What is this for?
+    deleter = None
+
 
 class EnumStr(ConfigParam):
     def __init__(self, default, *options, **kwargs):

theano/tests/test_config.py

+"""
+Test config options.
+"""
+
+import unittest
+from theano import config
+from theano.configparser import AddConfigVar, ConfigParam, THEANO_FLAGS_DICT
+
+class T_config(unittest.TestCase):
+
+    def test_invalid_default(self):
+        # Ensure an invalid default value found in the Theano code only causes
+        # a crash if it is not overridden by the user.
+        # TODO Note that currently, an invalid default value may still cause a
+        # crash if the user intends to override it inside the Python code with
+        # ``config.option = new_value``, instead of e.g. in the .theanorc. We
+        # may want to improve on this in the future.
+
+        def filter(val):
+            if val == 'invalid':
+                raise ValueError()
+            else:
+                return val
+
+        try:
+            # This should raise a ValueError because the default value is
+            # invalid.
+            AddConfigVar(
+                'T_config.test_invalid_default_a',
+                doc='unittest',
+                configparam=ConfigParam('invalid', filter=filter),
+                in_c_key=False)
+            assert False
+        except ValueError:
+            pass
+
+        try:
+            THEANO_FLAGS_DICT['T_config.test_invalid_default_b'] = 'ok'
+            # This should succeed since we defined a proper value, even
+            # though the default was invalid.
+            AddConfigVar(
+                    'T_config.test_invalid_default_b',
+                    doc='unittest',
+                    configparam=ConfigParam('invalid', filter=filter),
+                    in_c_key=False)
+        finally:
+            # Dicionary clean-up.
+            del THEANO_FLAGS_DICT['T_config.test_invalid_default_b']
+
+        # TODO We should remove these dummy options on test exit.