Merge pull request #135 from miohtama/feat/better-abi-errors

When ABI call output decoding fails, give more information what happened

web3/contract.py

@@ -9,11 +9,19 @@
 )
 from eth_abi.exceptions import (
     EncodingError,
+    DecodingError,
+)
+
+from web3.exceptions import (
+    BadFunctionCallOutput,
 )
 
 from web3.utils.encoding import (
     encode_hex,
 )
+from web3.utils.exception import (
+    raise_from,
+)
 from web3.utils.formatting import (
     add_0x_prefix,
     remove_0x_prefix,
@@ -653,7 +661,21 @@ def call_contract_function(contract,
     function_abi = contract._find_matching_fn_abi(function_name, args, kwargs)
 
     output_types = get_abi_output_types(function_abi)
-    output_data = decode_abi(output_types, return_data)
+
+    try:
+        output_data = decode_abi(output_types, return_data)
+    except DecodingError as e:
+        # Provide a more helpful error message than the one provided by
+        # eth-abi-utils
+        msg = (
+            "Could not decode contract function call {} return data {} for "
+            "output_types {}".format(
+                function_name,
+                return_data,
+                output_types
+            )
+        )
+        raise_from(BadFunctionCallOutput(msg), e)
 
     normalized_data = [
         normalize_return_type(data_type, data_value)

web3/exceptions.py

@@ -23,3 +23,10 @@ def __init__(self, result):
         else:
             message = "Invalid JSON RPC response: " + json.dumps(result)
         Exception.__init__(self, message)
+
+
+class BadFunctionCallOutput(Exception):
+    """We failed to decode ABI output.
+
+    Most likely ABI mismatch.
+    """

web3/utils/exception.py

@@ -0,0 +1,7 @@
+import sys
+
+# raise MyException() from original_exception compatibility
+if sys.version_info.major == 2:
+    from .exception_py2 import raise_from  # noqa: F401
+else:
+    from .exception_py3 import raise_from  # noqa: F401

web3/utils/exception_py2.py

@@ -0,0 +1,2 @@
+def raise_from(my_exception, other_exception):
+    raise my_exception, None, sys.exc_info()[2]  # noqa: W602, E999

web3/utils/exception_py3.py

@@ -0,0 +1,2 @@
+def raise_from(my_exception, other_exception):
+    raise my_exception from other_exception