Allow subtracting assets if not existing before

Negative values were allowed before as well so it is only consequential
to allow subtracting from 0

pycardano/transaction.py

@@ -14,7 +14,7 @@
 
 from pycardano.address import Address
 from pycardano.certificate import Certificate
-from pycardano.exception import InvalidDataException, InvalidOperationException
+from pycardano.exception import InvalidDataException
 from pycardano.hash import (
     TRANSACTION_HASH_SIZE,
     AuxiliaryDataHash,
@@ -95,14 +95,7 @@ def __iadd__(self, other: Asset) -> Asset:
     def __sub__(self, other: Asset) -> Asset:
         new_asset = deepcopy(self)
         for n in other:
-            if n not in new_asset:
-                raise InvalidOperationException(
-                    f"Asset: {new_asset} does not have asset with name: {n}"
-                )
-            # According to ledger rule, the value of an asset could be negative, so we don't check the value here and
-            # will leave the check to users when necessary.
-            # https://github.com/input-output-hk/cardano-ledger/blob/master/eras/alonzo/test-suite/cddl-files/alonzo.cddl#L378
-            new_asset[n] -= other[n]
+            new_asset[n] = new_asset.get(n, 0) - other[n]
         return new_asset
 
     def __eq__(self, other):
@@ -135,9 +128,7 @@ def union(self, other: MultiAsset) -> MultiAsset:
     def __add__(self, other):
         new_multi_asset = deepcopy(self)
         for p in other:
-            if p not in new_multi_asset:
-                new_multi_asset[p] = Asset()
-            new_multi_asset[p] += other[p]
+            new_multi_asset[p] = new_multi_asset.get(p, Asset()) + other[p]
         return new_multi_asset
 
     def __iadd__(self, other):
@@ -148,11 +139,7 @@ def __iadd__(self, other):
     def __sub__(self, other: MultiAsset) -> MultiAsset:
         new_multi_asset = deepcopy(self)
         for p in other:
-            if p not in new_multi_asset:
-                raise InvalidOperationException(
-                    f"MultiAsset: {new_multi_asset} doesn't have policy: {p}"
-                )
-            new_multi_asset[p] -= other[p]
+            new_multi_asset[p] = new_multi_asset.get(p, Asset()) - other[p]
         return new_multi_asset
 
     def __eq__(self, other):