Add set_backend() utility.

keras/utils/backend_utils.py

@@ -1,6 +1,10 @@
+import copy
+import importlib
+import os
 import sys
 
 from keras import backend as backend_module
+from keras.api_export import keras_export
 from keras.backend.common import global_state
 
 
@@ -82,3 +86,41 @@ def __getattr__(self, name):
             from keras import backend as numpy_backend
 
             return getattr(numpy_backend, name)
+
+
+@keras_export("keras.config.set_backend")
+def set_backend(backend):
+    """Reload the backend (and the Keras package).
+
+    Example:
+
+    ```python
+    keras.config.set_backend("jax")
+    ```
+
+    Note that this will **NOT** convert the type of any already
+    instantiated objects, except for the `keras` module itself.
+    Thus, any layers / tensors / etc. already created will no
+    longer be usable without errors. It is strongly recommended **not**
+    to keep around **any** Keras-originated objects instances created
+    before calling `set_backend()`.
+    """
+    os.environ["KERAS_BACKEND"] = backend
+    # Clear module cache.
+    loaded_modules = [
+        key for key in sys.modules.keys() if key.startswith("keras")
+    ]
+    for key in loaded_modules:
+        del sys.modules[key]
+    # Reimport Keras with the new backend (set via KERAS_BACKEND).
+    import keras
+
+    # Finally: refresh all imported Keras submodules.
+    globs = copy.copy(globals())
+    for key, value in globs.items():
+        if value.__class__ == keras.__class__:
+            if str(value).startswith("<module 'keras."):
+                module_name = str(value)
+                module_name = module_name[module_name.find("'") + 1 :]
+                module_name = module_name[: module_name.find("'")]
+                globals()[key] = importlib.import_module(module_name)