[Fix] backend_args should not be modified by get_file_backend (#897)

mmengine/fileio/io.py

@@ -91,8 +91,10 @@ def _get_file_backend(prefix: str, backend_args: dict):
     """
     # backend name has a higher priority
     if 'backend' in backend_args:
-        backend_name = backend_args.pop('backend')
-        backend = backends[backend_name](**backend_args)
+        # backend_args should not be modified
+        backend_args_bak = backend_args.copy()
+        backend_name = backend_args_bak.pop('backend')
+        backend = backends[backend_name](**backend_args_bak)
     else:
         backend = prefix_to_backends[prefix](**backend_args)
     return backend

tests/test_fileio/test_io.py

@@ -107,10 +107,13 @@ def test_get_file_backend():
     backend_args = {'backend': 'local'}
     backend = fileio.get_file_backend(uri=None, backend_args=backend_args)
     assert isinstance(backend, fileio.backends.LocalBackend)
+    # backend_args should not be modified
+    assert backend_args == {'backend': 'local'}
 
     backend_args = {'backend': 'petrel', 'enable_mc': True}
     backend = fileio.get_file_backend(uri=None, backend_args=backend_args)
     assert isinstance(backend, fileio.backends.PetrelBackend)
+    assert backend_args == {'backend': 'petrel', 'enable_mc': True}
 
     # backend name has a higher priority
     backend_args = {'backend': 'http'}