feat: add global set_default_max_transaction_fee to Client

CHANGELOG.md

@@ -8,6 +8,7 @@ This changelog is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.
 
 ### Added
 - Add `__repr__` method to `TokenId` class for cleaner debugging output (#1653)
+- Add global `set_default_max_transaction_fee()` to Client. ([#2000](https://github.com/hiero-ledger/hiero-sdk-python/issues/2000))
 
 ### Src
 - Updated `AccountUpdateTransaction.set_key()` to accept generic `Key` objects (including `KeyList` and threshold keys), rather than strictly requiring a `PublicKey`.

src/hiero_sdk_python/client/client.py

@@ -73,6 +73,8 @@ def __init__(self, network: Network = None) -> None:
 
         self.logger: Logger = Logger(LogLevel.from_env(), "hiero_sdk_python")
 
+        self.default_max_transaction_fee: Hbar = None
+
     @classmethod
     def from_env(cls, network: Optional[NetworkName] = None) -> "Client":
         """
@@ -455,6 +457,29 @@ def set_max_backoff(self, max_backoff: Union[int, float]) -> "Client":
         self._max_backoff = float(max_backoff)
         return self
 
+    def set_default_max_transaction_fee(
+        self, default_max_transaction_fee: Hbar
+    ) -> "Client":
+        """
+        Set the maximum fee allowed per transaction.
+
+        Args:
+            default_max_transaction_fee (Hbar): Maximum fee allowed per transaction.
+
+        Returns:
+            Client: This client instance for fluent chaining.
+        """
+        if not isinstance(default_max_transaction_fee, Hbar):
+            raise TypeError(
+                f"default_max_transaction_fee must be of type Hbar, got {(type(default_max_transaction_fee).__name__)}"
+            )
+
+        if default_max_transaction_fee.to_tinybars() < 0:
+            raise ValueError("default_max_transaction_fee must be >= 0")
+
+        self.default_max_transaction_fee = default_max_transaction_fee
+        return self
+
     def update_network(self) -> "Client":
         """
         Refresh the network node list from the mirror node.