added NotConstantError so get_constant_value can raise TypeError if

theano/tensor/basic.py

@@ -462,13 +462,27 @@ def _allclose(a, b, rtol=None, atol=None):
     return numpy.allclose(a, b, atol=atol_, rtol=rtol_)
 
 
+class NotConstantError(TypeError):
+    """
+    Raised by get_constant_value if called on something that is
+    not constant.
+    For now it is a TypeError, to maintain the old interface
+    that get_constant_value should raise a TypeError in this
+    situation. However, this is unsafe because get_constant_value
+    could inadvertently raise a TypeError if it has a bug.
+    So we should eventually make NotConstantError derive
+    from Exception directly, and modify all code that uses
+    get_constant_value to catch this more specific exception.
+    """
+    pass
+
 def get_constant_value(v):
     """return the constant scalar(0-D) value underlying variable `v`
 
     If v is the output of dimshuffles, fills, allocs, rebroadcasts, cast
     this function digs through them.
 
-    If `v` is not some view of constant data, then raise a TypeError.
+    If `v` is not some view of constant data, then raise a NotConstantError.
 
     :note: There may be another function similar to this one in the
         code, but I'm not sure where it is.
@@ -488,7 +502,7 @@ def get_constant_value(v):
             numpy.complex(data)  # works for all numeric scalars
             return data
         except Exception:
-            raise TypeError(
+            raise NotConstantError(
                 'v.data is non-numeric, non-scalar, or has more than one'
                 ' unique value', v)
     if v.owner: