I was wondering if somebody could tell me the motivation behind the current convention for default initialization of LTI systems to have dt=None? Using dt=None means an unspecified time base, allowing the system to be interpreted either as cont or discrete time. (I couldn't find any discussion of this but please let me know if I missed it).

A system with an unspecified dt seems to be an unusual object:

• printing out its transfer function shows 's' as the complex variable, suggesting it is a cont-time system
• but isdtime(sys) evaluates to true (but not if strict=True is specified) -- I thought this was a bug at first
• cont and discrete time systems have substantially different behavior, which seems to be at odds with the notion that an object should know how to handle itself

Have you thought about using a default of dt=0 (continuous time), and leave the dt=None case be a special case, to be instantiated only when needed? (Issue#419 made me think of this and is related)