feat: add custom fee limit example (#900)

Signed-off-by: Akshat Kumar <akshat230405@gmail.com>
Signed-off-by: Akshat8510 <akshat230405@gmail.com>

Author: Akshat8510

CHANGELOG.md

@@ -7,7 +7,8 @@ This changelog is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.
 ## [Unreleased]
 
 ### Added
-
+- Add example demonstrating usage of `CustomFeeLimit` in `examples/transaction/custom_fee_limit.py`
+  
 ### Changed
 
 - Removed duplicate import of transaction_pb2 in transaction.py

examples/transaction/custom_fee_limit.py

@@ -0,0 +1,144 @@
+"""
+Example: Using CustomFeeLimit with a revenue-generating topic.
+
+- Creates a topic that charges a fixed custom fee per message.
+- Submits a message with a CustomFeeLimit specifying how much the payer is
+  willing to pay in custom fees for that message.
+"""
+
+import os
+import sys
+from dotenv import load_dotenv
+
+from hiero_sdk_python import (
+    Client,
+    AccountId,
+    PrivateKey,
+    Hbar,
+    Network,
+    TopicCreateTransaction,
+    TopicMessageSubmitTransaction,
+    CustomFixedFee,
+)
+from hiero_sdk_python.transaction.custom_fee_limit import CustomFeeLimit
+
+
+def setup_client() -> tuple[Client, AccountId]:
+    """Initialize client and operator from .env file."""
+    load_dotenv()
+
+    if "OPERATOR_ID" not in os.environ or "OPERATOR_KEY" not in os.environ:
+        print("Environment variables OPERATOR_ID or OPERATOR_KEY are missing.")
+        sys.exit(1)
+
+    try:
+        operator_id = AccountId.from_string(os.environ["OPERATOR_ID"])
+        operator_key = PrivateKey.from_string(os.environ["OPERATOR_KEY"])
+    except Exception as e:  # noqa: BLE001
+        print(f"Failed to parse OPERATOR_ID or OPERATOR_KEY: {e}")
+        sys.exit(1)
+
+    network_name = os.environ.get("NETWORK", "testnet")
+
+    try:
+        client = Client(Network(network_name))
+    except Exception as e:
+        print(f"Failed to create client for network '{network_name}': {e}")
+        sys.exit(1)
+
+    client.set_operator(operator_id, operator_key)
+    print(f"Operator set: {operator_id}")
+
+    return client, operator_id
+
+
+def create_revenue_generating_topic(client: Client, operator_id: AccountId):
+    """
+    Create a topic that charges a fixed custom fee per message.
+
+    The topic charges 1 HBAR (in tinybars) to the operator account for every message.
+    """
+    print("\nCreating a topic with a fixed custom fee per message...")
+
+    # Charge 1 HBAR to the operator for every message
+    custom_fee = CustomFixedFee(
+        amount=Hbar(1).to_tinybars(),
+        fee_collector_account_id=operator_id,
+    )
+
+    try:
+        topic_tx = TopicCreateTransaction()
+        topic_tx.set_custom_fees([custom_fee])
+
+        # execute() returns the receipt
+        topic_receipt = topic_tx.execute(client)
+
+        topic_id = topic_receipt.topic_id
+        print(f"Topic created successfully: {topic_id}")
+        print("This topic charges a fixed fee of 1 HBAR per message.")
+
+        return topic_id
+    except Exception as e:   # noqa: BLE001
+        print(f"Failed to create topic: {e}")
+        return None
+
+
+def submit_message_with_custom_fee_limit(
+    client: Client, topic_id, operator_id: AccountId
+) -> None:
+    """
+    Submit a message to the topic with a CustomFeeLimit applied.
+
+    The CustomFeeLimit caps the total custom fees the payer is willing to pay
+    for this message at 2 HBAR.
+    """
+    print("\nSubmitting a message with a CustomFeeLimit...")
+
+    # We are willing to pay up to 2 HBAR in custom fees for this message
+    limit_fee = CustomFixedFee(
+        amount=Hbar(2).to_tinybars(),
+        fee_collector_account_id=operator_id,
+    )
+
+    fee_limit = CustomFeeLimit()
+    fee_limit.set_payer_id(operator_id)
+    fee_limit.add_custom_fee(limit_fee)
+
+    print(
+        f"Setting fee limit: max {limit_fee.amount} tinybars "
+        f"in custom fees for payer {operator_id}"
+    )
+
+    try:
+        submit_tx = TopicMessageSubmitTransaction()
+        submit_tx.set_topic_id(topic_id)
+        submit_tx.set_message("Hello Hedera with Fee Limits!")
+
+        # Ensure the base transaction fee is high enough to cover processing
+        submit_tx.transaction_fee = Hbar(5).to_tinybars()
+
+        # Attach the custom fee limit to the transaction
+        submit_tx.set_custom_fee_limits([fee_limit])
+
+        submit_receipt = submit_tx.execute(client)
+
+        print("Message submitted successfully!")
+        print(f"Transaction status: {submit_receipt.status}")
+    except Exception as e:    # noqa: BLE001
+        print(f"Transaction failed: {e}")
+
+
+def main() -> None:
+    client, operator_id = setup_client()
+
+    topic_id = create_revenue_generating_topic(client, operator_id)
+    if topic_id is None:
+        return
+
+    submit_message_with_custom_fee_limit(client, topic_id, operator_id)
+
+    print("\nExample complete.")
+
+
+if __name__ == "__main__":
+    main()