Refactor add_appointment cli code

common/tools.py

@@ -1,4 +1,5 @@
 import re
+from common.constants import LOCATOR_LEN_HEX
 
 
 def check_sha256_hex_format(value):
@@ -12,3 +13,15 @@ def check_sha256_hex_format(value):
         :mod:`bool`: Whether or not the value matches the format.
     """
     return isinstance(value, str) and re.match(r"^[0-9A-Fa-f]{64}$", value) is not None
+
+
+def compute_locator(tx_id):
+    """
+    Computes an appointment locator given a transaction id.
+    Args:
+        tx_id (:obj:`str`): the transaction id used to compute the locator.
+    Returns:
+       (:obj:`str`): The computed locator.
+    """
+
+    return tx_id[:LOCATOR_LEN_HEX]