[MRG] get_backend compatibility with None entries (#525)

* backend None compatibility with test

* reference PR in RELEASES.md

* Update RELEASES.md

---------

Co-authored-by: Rémi Flamary <remi.flamary@gmail.com>

Author: eloitanguy

RELEASES.md

@@ -1,5 +1,13 @@
 # Releases
 
+## 0.9.2dev
+
+#### New features
++ Tweaked `get_backend` to ignore `None` inputs (PR # 525)
+
+#### Closed issues
+
+
 ## 0.9.1
 *August 2023*
 

ot/backend.py

@@ -157,11 +157,15 @@ def _check_args_backend(backend, args):
 def get_backend(*args):
     """Returns the proper backend for a list of input arrays
 
+        Accepts None entries in the arguments, and ignores them
+
         Also raises TypeError if all arrays are not from the same backend
     """
+    args = [arg for arg in args if arg is not None]  # exclude None entries
+
     # check that some arrays given
     if not len(args) > 0:
-        raise ValueError(" The function takes at least one parameter")
+        raise ValueError(" The function takes at least one (non-None) parameter")
 
     for backend in _BACKENDS:
         if _check_args_backend(backend, args):

test/test_backend.py

@@ -3,6 +3,7 @@
 # Author: Remi Flamary <remi.flamary@polytechnique.edu>
 #         Nicolas Courty <ncourty@irisa.fr>
 #
+#
 # License: MIT License
 
 import ot
@@ -753,3 +754,11 @@ def fun(a, b, d):
             [dl_dw, dl_db] = tape.gradient(manipulated_loss, [w, b])
             assert nx.allclose(dl_dw, w)
             assert nx.allclose(dl_db, b)
+
+
+def test_get_backend_none():
+    a, b = np.zeros((2, 3)), None
+    nx = get_backend(a, b)
+    assert str(nx) == 'numpy'
+    with pytest.raises(ValueError):
+        get_backend(None, None)