Allow subtracting assets if not existing before (#317)

* Allow subtracting assets if not existing before

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

* Fix testcases

Author: nielstron

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):

test/pycardano/test_transaction.py

@@ -311,8 +311,12 @@ def test_multi_asset_subtraction():
         }
     )
 
-    with pytest.raises(InvalidOperationException):
-        a - b
+    assert a - b == MultiAsset.from_primitive(
+        {
+            b"1" * SCRIPT_HASH_SIZE: {b"Token1": -9, b"Token2": -18},
+            b"2" * SCRIPT_HASH_SIZE: {b"Token1": -1, b"Token2": -2},
+        }
+    )
 
 
 def test_asset_comparison():
@@ -427,11 +431,25 @@ def test_values():
         [101, {b"1" * SCRIPT_HASH_SIZE: {b"Token1": 1, b"Token2": 2}}]
     )
 
-    with pytest.raises(InvalidOperationException):
-        a - c
+    assert a - c == Value.from_primitive(
+        [
+            -10,
+            {
+                b"1" * SCRIPT_HASH_SIZE: {b"Token1": -10, b"Token2": -20},
+                b"2" * SCRIPT_HASH_SIZE: {b"Token1": -11, b"Token2": -22},
+            },
+        ]
+    )
 
-    with pytest.raises(InvalidOperationException):
-        b - c
+    assert b - c == Value.from_primitive(
+        [
+            0,
+            {
+                b"1" * SCRIPT_HASH_SIZE: {b"Token1": 0, b"Token2": 0},
+                b"2" * SCRIPT_HASH_SIZE: {b"Token1": -11, b"Token2": -22},
+            },
+        ]
+    )
 
 
 def test_inline_datum_serdes():