document the 's' and 'z' constants for TransferFunction

murrayrm · murrayrm · commit 8e019587c9b3 · 2019-05-24T22:52:15.000-07:00
diff --git a/control/xferfcn.py b/control/xferfcn.py
@@ -90,6 +90,14 @@ class TransferFunction(LTI):
     are combined.  If 'dt' is set to True, the system will be treated as a
     discrete time system with unspecified sampling time.
 
+    The TransferFunction class defines two constants ``s`` and ``z`` that
+    represent the differentiation and delay operators in continuous and
+    discrete time.  These can be used to create variables that allow algebraic
+    creation of transfer functions.  For example,
+
+    >>> s = TransferFunction.s
+    >>> G  = (s + 1)/(s**2 + 2*s + 1)
+
     """
     def __init__(self, *args):
         """TransferFunction(num, den[, dt])
