Fix fee too low

User can potentially provide signing keys that are not required by the finalized transaction.
For example, if a transaction include a minting script and signing key for the script, but the minted value is 0, the signature of the minting script isn't required.
This commit will auto detect such cases and skip the signing of this key.

pycardano/transaction.py

@@ -38,13 +38,13 @@
 from pycardano.serialization import (
     ArrayCBORSerializable,
     CBORSerializable,
+    DictBase,
     DictCBORSerializable,
     MapCBORSerializable,
     Primitive,
     default_encoder,
-    list_hook,
-    DictBase,
     limit_primitive_type,
+    list_hook,
 )
 from pycardano.types import typechecked
 from pycardano.witness import TransactionWitnessSet

pycardano/txbuilder.py

@@ -928,12 +928,16 @@ def _build_tx_body(self) -> TransactionBody:
         )
         return tx_body
 
-    def _build_fake_vkey_witnesses(self) -> List[VerificationKeyWitness]:
+    def _build_required_vkeys(self) -> Set[VerificationKeyHash]:
         vkey_hashes = self._input_vkey_hashes()
         vkey_hashes.update(self._required_signer_vkey_hashes())
         vkey_hashes.update(self._native_scripts_vkey_hashes())
         vkey_hashes.update(self._certificate_vkey_hashes())
         vkey_hashes.update(self._withdrawal_vkey_hashes())
+        return vkey_hashes
+
+    def _build_fake_vkey_witnesses(self) -> List[VerificationKeyWitness]:
+        vkey_hashes = self._build_required_vkeys()
 
         witness_count = self.witness_override or len(vkey_hashes)
 
@@ -1441,6 +1445,7 @@ def build_and_sign(
         auto_validity_start_offset: Optional[int] = None,
         auto_ttl_offset: Optional[int] = None,
         auto_required_signers: Optional[bool] = None,
+        force_skeys: Optional[bool] = False,
     ) -> Transaction:
         """Build a transaction body from all constraints set through the builder and sign the transaction with
         provided signing keys.
@@ -1462,6 +1467,10 @@ def build_and_sign(
             auto_required_signers (Optional[bool]): Automatically add all pubkeyhashes of transaction inputs
                 and the given signers to required signatories (default only for Smart Contract transactions).
                 Manually set required signers will always take precedence.
+            force_skeys (Optional[bool]): Whether to force the use of signing keys for signing the transaction.
+                Default is False, which means that provided signing keys will only be used to sign the transaction if
+                they are actually required by the transaction. This is useful to reduce tx fees by not including
+                unnecessary signatures. If set to True, all provided signing keys will be used to sign the transaction.
 
         Returns:
             Transaction: A signed transaction.
@@ -1483,7 +1492,15 @@ def build_and_sign(
         witness_set = self.build_witness_set(True)
         witness_set.vkey_witnesses = []
 
+        required_vkeys = self._build_required_vkeys()
+
         for signing_key in set(signing_keys):
+            vkey_hash = signing_key.to_verification_key().hash()
+            if not force_skeys and vkey_hash not in required_vkeys:
+                logger.warn(
+                    f"Verification key hash {vkey_hash} is not required for this tx."
+                )
+                continue
             signature = signing_key.sign(tx_body.hash())
             witness_set.vkey_witnesses.append(
                 VerificationKeyWitness(signing_key.to_verification_key(), signature)