diff --git a/map2map/data/fields.py b/map2map/data/fields.py
index e023375..6ed0750 100644
--- a/map2map/data/fields.py
+++ b/map2map/data/fields.py
@@ -68,14 +68,14 @@ class FieldDataset(Dataset):
         if in_norms is not None:
             assert len(in_patterns) == len(in_norms), \
                     'numbers of input normalization functions and fields do not match'
-            in_norms = [import_attr(norm, norms.__name__, callback_at)
+            in_norms = [import_attr(norm, norms, callback_at=callback_at)
                         for norm in in_norms]
         self.in_norms = in_norms
 
         if tgt_norms is not None:
             assert len(tgt_patterns) == len(tgt_norms), \
                     'numbers of target normalization functions and fields do not match'
-            tgt_norms = [import_attr(norm, norms.__name__, callback_at)
+            tgt_norms = [import_attr(norm, norms, callback_at=callback_at)
                          for norm in tgt_norms]
         self.tgt_norms = tgt_norms
 
diff --git a/map2map/test.py b/map2map/test.py
index 0e26521..841294b 100644
--- a/map2map/test.py
+++ b/map2map/test.py
@@ -40,9 +40,9 @@ def test(args):
 
     in_chan, out_chan = test_dataset.in_chan, test_dataset.tgt_chan
 
-    model = import_attr(args.model, models.__name__, args.callback_at)
+    model = import_attr(args.model, models, callback_at=args.callback_at)
     model = model(sum(in_chan), sum(out_chan), scale_factor=args.scale_factor)
-    criterion = import_attr(args.criterion, torch.nn.__name__, args.callback_at)
+    criterion = import_attr(args.criterion, torch.nn, callback_at=args.callback_at)
     criterion = criterion()
 
     device = torch.device('cpu')
diff --git a/map2map/train.py b/map2map/train.py
index e88a0bb..04cd923 100644
--- a/map2map/train.py
+++ b/map2map/train.py
@@ -119,18 +119,19 @@ def gpu_worker(local_rank, node, args):
 
     args.in_chan, args.out_chan = train_dataset.in_chan, train_dataset.tgt_chan
 
-    model = import_attr(args.model, models.__name__, args.callback_at)
+    model = import_attr(args.model, models, callback_at=args.callback_at)
     model = model(sum(args.in_chan), sum(args.out_chan),
                   scale_factor=args.scale_factor)
     model.to(device)
     model = DistributedDataParallel(model, device_ids=[device],
             process_group=dist.new_group())
 
-    criterion = import_attr(args.criterion, nn.__name__, args.callback_at)
+    criterion = import_attr(args.criterion, nn, models,
+                            callback_at=args.callback_at)
     criterion = criterion()
     criterion.to(device)
 
-    optimizer = import_attr(args.optimizer, optim.__name__, args.callback_at)
+    optimizer = import_attr(args.optimizer, optim, callback_at=args.callback_at)
     optimizer = optimizer(
         model.parameters(),
         lr=args.lr,
diff --git a/map2map/utils/imp.py b/map2map/utils/imp.py
index 41efc09..b79a77b 100644
--- a/map2map/utils/imp.py
+++ b/map2map/utils/imp.py
@@ -2,37 +2,52 @@ import os
 import importlib
 
 
-def import_attr(name, pkg, callback_at=None):
+def import_attr(name, *pkgs, callback_at=None):
     """Import attribute. Try package first and then callback directory.
 
     To use a callback, `name` must contain a module, formatted as 'mod.attr'.
 
     Examples
     --------
-    >>> import_attr('attr', 'pkg1.pkg2')
+    >>> import_attr('attr', pkg1.pkg2)
 
     tries to import attr from pkg1.pkg2.
 
-    >>> import_attr('mod.attr', 'pkg1.pkg2', 'path/to/cb_dir')
+    >>> import_attr('mod.attr', pkg1.pkg2, pkg3, callback_at='path/to/cb_dir')
 
-    first tries to import attr from pkg1.pkg2.mod, then from
-    'path/to/cb_dir/mod.py'.
+    first tries to import attr from pkg1.pkg2.mod, then from pkg3.mod, finally
+    from 'path/to/cb_dir/mod.py'.
     """
     if name.count('.') == 0:
         attr = name
 
-        return getattr(importlib.import_module(pkg), attr)
+        errors = []
+
+        for pkg in pkgs:
+            try:
+                return getattr(importlib.import_module(pkg.__name__), attr)
+            except (ModuleNotFoundError, AttributeError) as e:
+                errors.append(e)
+
+        raise Exception(errors)
     else:
         mod, attr = name.rsplit('.', 1)
 
-        try:
-            return getattr(importlib.import_module(pkg + '.' + mod), attr)
-        except (ModuleNotFoundError, AttributeError):
-            if callback_at is None:
-                raise
+        errors = []
+
+        for pkg in pkgs:
+            try:
+                return getattr(
+                    importlib.import_module(pkg.__name__ + '.' + mod), attr)
+            except (ModuleNotFoundError, AttributeError):
+                errors.append(e)
+
+        if callback_at is None:
+            raise Exception(errors)
 
         callback_at = os.path.join(callback_at, mod + '.py')
-        assert os.path.isfile(callback_at), 'callback file not found'
+        if not os.path.isfile(callback_at):
+            raise FileNotFoundError('callback file not found')
 
         spec = importlib.util.spec_from_file_location(mod, callback_at)
         module = importlib.util.module_from_spec(spec)