Add MetadataAtVersionNotFound error

async_substrate_interface/async_substrate.py

@@ -39,6 +39,7 @@
     ExtrinsicNotFound,
     BlockNotFound,
     MaxRetriesExceeded,
+    MetadataAtVersionNotFound,
 )
 from async_substrate_interface.protocols import Keypair
 from async_substrate_interface.types import (
@@ -817,10 +818,7 @@ async def _load_registry_at_block(
                 "Client error: Execution failed: Other: Exported method Metadata_metadata_at_version is not found"
                 in e.args
             ):
-                raise SubstrateRequestException(
-                    "You are attempting to call a block too old for this version of async-substrate-interface. Please"
-                    " instead use legacy py-substrate-interface for these very old blocks."
-                )
+                raise MetadataAtVersionNotFound
             else:
                 raise e
         metadata_option_hex_str = metadata_rpc_result["result"]

async_substrate_interface/errors.py

@@ -12,6 +12,16 @@ class MaxRetriesExceeded(SubstrateRequestException):
     pass
 
 
+class MetadataAtVersionNotFound(SubstrateRequestException):
+    def __init__(self):
+        message = (
+            "Exported method Metadata_metadata_at_version is not found. This indicates the block is quite old, and is"
+            "not supported by async-substrate-interface. If you need this, we recommend using the legacy "
+            "substrate-interface (https://github.com/JAMdotTech/py-polkadot-sdk)."
+        )
+        super().__init__(message)
+
+
 class StorageFunctionNotFound(ValueError):
     pass
 

async_substrate_interface/sync_substrate.py

@@ -21,6 +21,7 @@
     SubstrateRequestException,
     BlockNotFound,
     MaxRetriesExceeded,
+    MetadataAtVersionNotFound,
 )
 from async_substrate_interface.protocols import Keypair
 from async_substrate_interface.types import (
@@ -617,11 +618,20 @@ def _get_current_block_hash(
     def _load_registry_at_block(self, block_hash: Optional[str]) -> MetadataV15:
         # Should be called for any block that fails decoding.
         # Possibly the metadata was different.
-        metadata_rpc_result = self.rpc_request(
-            "state_call",
-            ["Metadata_metadata_at_version", self.metadata_version_hex],
-            block_hash=block_hash,
-        )
+        try:
+            metadata_rpc_result = self.rpc_request(
+                "state_call",
+                ["Metadata_metadata_at_version", self.metadata_version_hex],
+                block_hash=block_hash,
+            )
+        except SubstrateRequestException as e:
+            if (
+                "Client error: Execution failed: Other: Exported method Metadata_metadata_at_version is not found"
+                in e.args
+            ):
+                raise MetadataAtVersionNotFound
+            else:
+                raise e
         metadata_option_hex_str = metadata_rpc_result["result"]
         metadata_option_bytes = bytes.fromhex(metadata_option_hex_str[2:])
         metadata = MetadataV15.decode_from_metadata_option(metadata_option_bytes)