Various improvements to data_layer acquisition logic. (#1534)

* Initialise data layer only once.
* Give DeprecationWarning when users set _data_layer directly.
* Remove debug print statement.

backend/chainlit/data/__init__.py

@@ -7,26 +7,38 @@
 )
 
 _data_layer: Optional[BaseDataLayer] = None
+_data_layer_initialized = False
 
 
 def get_data_layer():
-    global _data_layer
-    print("Getting data layer", _data_layer)
-
-    if not _data_layer:
-        from chainlit.config import config
-
-        if config.code.data_layer:
-            # When @data_layer is configured, call it to get data layer.
-            _data_layer = config.code.data_layer()
-        elif api_key := os.environ.get("LITERAL_API_KEY"):
-            # When LITERAL_API_KEY is defined, use LiteralAI data layer
-            from .literalai import LiteralDataLayer
-
-            # support legacy LITERAL_SERVER variable as fallback
-            server = os.environ.get("LITERAL_API_URL") or os.environ.get(
-                "LITERAL_SERVER"
+    global _data_layer, _data_layer_initialized
+
+    if not _data_layer_initialized:
+        if _data_layer:
+            # Data layer manually set, warn user that this is deprecated.
+            import warnings
+
+            warnings.warn(
+                "Setting data layer manually is deprecated. Use @data_layer instead.",
+                DeprecationWarning,
             )
-            _data_layer = LiteralDataLayer(api_key=api_key, server=server)
+
+        else:
+            from chainlit.config import config
+
+            if config.code.data_layer:
+                # When @data_layer is configured, call it to get data layer.
+                _data_layer = config.code.data_layer()
+            elif api_key := os.environ.get("LITERAL_API_KEY"):
+                # When LITERAL_API_KEY is defined, use LiteralAI data layer
+                from .literalai import LiteralDataLayer
+
+                # support legacy LITERAL_SERVER variable as fallback
+                server = os.environ.get("LITERAL_API_URL") or os.environ.get(
+                    "LITERAL_SERVER"
+                )
+                _data_layer = LiteralDataLayer(api_key=api_key, server=server)
+
+        _data_layer_initialized = True
 
     return _data_layer