This behavior is actually correct: all LTI systems (including the TransferFunction class) have a timebase dt. The timebase can any of the following (as described here):

dt = None: no timebase specified
dt = 0: continuous time system
dt > 0: discrete time system with sampling period ‘dt’
dt = True: discrete time with unspecified sampling period

If you create system without a timebase (the default case) then it will automatically be converted to the appropriate timebase when it is operated upon.

The following code demonstrates the behavior:

tf1 = control.TransferFunction([1,1],[1,2,3])       # unspecified timebase
tf2 = control.TransferFunction([1,1],[1,2,3], 0)    # continuous time
tf3 = control.TransferFunction([1,1],[1,2,3], True) # discrete time, unspecified sampling
tf4 = control.TransferFunction([1,1],[1,2,3], 1)    # discrete time, with sample time

tf1*tf2             # OK; convert tf1 to continuous time
tf1*tf3             # OK; convert tf1 to discrete time
tf1*tf4             # OK; convert tf1 to discrete time time
tf2*tf3             # Error; incompatible timebases
tf2*tf4             # Error; incompatible timebases
tf3*tf4             # OK; convert tf3 to fixed sample time

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