basic withdraw and upgrade

rowers/credits.py

@@ -0,0 +1,26 @@
+from rowers.models import Rower
+
+class InsufficientCreditError(Exception):
+    """Raised when trying to subtract more than available"""
+    pass
+
+def upgrade(amount, rower):
+    if rower.eurocredits > amount:
+        return rower.eurocredits
+    else:
+        rower.eurocredits = amount
+        rower.save()
+        return rower.eurocredits
+    return rower.eurocredits
+
+def withdraw(amount, rower):
+    if rower.eurocredits < amount:
+        rower.eurocredits = 0
+        rower.save()
+        raise InsufficientCreditError
+    else:
+        rower.eurocredits = rower.eurocredits - amount
+        rower.save()
+        return rower.eurocredits
+
+    return rower.eurocredits