transaction: Make fee_amount() Optional.

Outgoing payments don't have a fee (PayPal takes their fee out on the
receiver's end).

paypal_rest/cliquery.py

@@ -140,7 +140,8 @@ def summarize_transaction(txn: Transaction, stream: TextIO) -> None:
         txn_amt = txn.amount()
         cart.append(CartItem(None, txn_name, None, 1, txn_amt, txn_amt))
     fee_amt = txn.fee_amount()
-    cart.append(CartItem(None, "PayPal Fee", None, 1, fee_amt, fee_amt))
+    if fee_amt is not None:
+        cart.append(CartItem(None, "PayPal Fee", None, 1, fee_amt, fee_amt))
     names = [
         item.name or item.description or item.code or "Unknown Item"
         for item in cart

paypal_rest/transaction.py

@@ -129,13 +129,21 @@ class Transaction(APIResponse):
                 except KeyError:
                     pass
 
+    def _fee_amount(txn_info: APIResponse) -> Optional[Amount]:  # type:ignore[misc]
+        try:
+            raw_fee = txn_info['fee_amount']
+        except KeyError:
+            return None
+        else:
+            return Amount.from_api(raw_fee)
+
     amount = _from_response(
         Amount.from_api,
         'transaction_info',
         'transaction_amount',
     )
     cart_items = _from_response(_cart_items, 'cart_info')
-    fee_amount = _from_response(Amount.from_api, 'transaction_info', 'fee_amount')
+    fee_amount = _from_response(_fee_amount, 'transaction_info')
     initiation_date = _from_response(
         parse_datetime,
         'transaction_info',

tests/test_transaction.py

@@ -166,6 +166,10 @@ def test_fee_amount_whole(number):
     txn = txn_mod.Transaction({'transaction_info': source})
     assert txn.fee_amount() == (int(number), 'JPY')
 
+def test_fee_amount_none():
+    txn = txn_mod.Transaction({'transaction_info': {}})
+    assert txn.fee_amount() is None
+
 def test_payer_email():
     email = 'test@example.net'
     source = payer_info(email_address=email)