More informative comment and error message for fetch_var()

python/paddle/v2/fluid/executor.py

@@ -152,8 +152,10 @@ def fetch_var(name, scope=None, return_numpy=True):
     """
     Fetch the value of the variable with the given name from the given scope
     Args:
-        name(str): name of the variable
-        scope(core.Scope|None): scope object.
+        name(str): name of the variable. Typically, only persistable variables
+            can be found in the scope used for running the program.
+        scope(core.Scope|None): scope object. It should be the scope where
+            you pass to Executor.run() when running your program.
             If None, global_scope() will be used.
         return_numpy(bool): whether convert the tensor to numpy.ndarray
     Returns:
@@ -165,7 +167,10 @@ def fetch_var(name, scope=None, return_numpy=True):
     assert isinstance(scope, core.Scope)
 
     var = global_scope().find_var(name)
-    assert var is not None, "Cannot find '%s' in scope." % name
+    assert var is not None, (
+        "Cannot find " + name + " in scope. Perhaps you need to make the"
+        " variable persistable by using var.persistable = True in your"
+        " program.")
     tensor = var.get_tensor()
     if return_numpy:
         tensor = as_numpy(tensor)