more explicit uint8ify, add comments

Author: barnjamin

pyteal/ast/abi/util.py

@@ -411,6 +411,7 @@ def type_spec_from_algosdk(t: Union[algosdk.abi.ABIType, str]) -> TypeSpec:
     )
 
     match t:
+        # Currently reference and transaction types are only strings
         case str():
             if algosdk.abi.is_abi_reference_type(t):
                 ref_dict: dict[str, TypeSpec] = {

pyteal/ast/itxn.py

@@ -235,6 +235,10 @@ def MethodCall(
         apps: List[Expr] = []
         assets: List[Expr] = []
 
+        # Reference types are encoded as a uint8 index into their respective arrays
+        def get_uint8(i):
+            return Suffix(Itob(Int(i)), Int(7))
+
         for idx, arg_ts in enumerate(arg_type_specs):
             arg = args[idx]
 
@@ -256,7 +260,7 @@ def MethodCall(
                         else:
                             raise TealTypeError(arg, Union[abi.Account, Expr])
 
-                        app_args.append(Suffix(Itob(Int(len(accts))), Int(7)))
+                        app_args.append(get_uint8(len(accts)))
 
                     case abi.ApplicationTypeSpec():
                         if isinstance(arg, Expr):
@@ -266,11 +270,12 @@ def MethodCall(
                         else:
                             raise TealTypeError(arg, Union[abi.Application, Expr])
 
-                        app_args.append(Suffix(Itob(Int(len(apps))), Int(7)))
+                        app_args.append(get_uint8(len(apps)))
 
                     # For assets, add to app_args prior to appending to assets array
                     case abi.AssetTypeSpec():
-                        app_args.append(Suffix(Itob(Int(len(assets))), Int(7)))
+                        app_args.append(get_uint8(len(assets)))
+
                         if isinstance(arg, Expr):
                             assets.append(arg)
                         elif isinstance(arg, abi.Asset):