NonlinearIOSystem.__str__() should not fail for successfully constructed systems #329
Description
The __str__ method in InputOutputSystem assumes that the ninputs, noutputs and nstates have all been set to integers, but the constructors do not guarantee this. For example:
>> sys = ctl.NonlinearIOSystem(lambda t, x, u, params: x)
>> print(sys)
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-4-85c64df40808> in <module>()
----> 1 print(sys)
/home/arnold/pythonBox/control_dev/python-control-rabraker/control/iosys.py in __str__(self)
214 """String representation of an input/output system"""
215 str = "System: " + (self.name if self.name else "(none)") + "\n"
--> 216 str += "Inputs (%d): " % self.ninputs
217 for key in self.input_index: str += key + ", "
218 str += "\nOutputs (%d): " % self.noutputs
TypeError: %d format: a number is required, not NoneType
I would think that once a system is constructed, __str__() should always succeed. The simplest way to fix this is to replace %d with %s in the format string, so it can accept NoneType. I can submit a PR and add a regression test if this is the desired fix.
This behavior also means that when noutputs etc are not set, many of the overload operators give unhelpful errors
>>> sys = ctl.NonlinearIOSystem(lambda t, x, u, params: x)
>>> sys2 = sys * sys
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-27-ea1505c3dfb9> in <module>()
----> 1 sys2 = sys * sys
/home/arnold/pythonBox/control_dev/python-control-rabraker/control/iosys.py in __mul__(sys2, sys1)
252
253 # Return the series interconnection between the systems
--> 254 newsys = InterconnectedSystem((sys1, sys2))
255
256 # Set up the connecton map
/home/arnold/pythonBox/control_dev/python-control-rabraker/control/iosys.py in __init__(self, syslist, connections, inplist, outlist, inputs, outputs, states, params, dt, name)
949 raise TypeError("System '%s' must define number of inputs, "
950 "outputs, states in order to be connected" %
--> 951 sys)
952
953 # Keep track of the offsets into the states, inputs, outputs
/home/arnold/pythonBox/control_dev/python-control-rabraker/control/iosys.py in __str__(self)
214 """String representation of an input/output system"""
215 str = "System: " + (self.name if self.name else "(none)") + "\n"
--> 216 str += "Inputs (%d): " % self.ninputs
217 for key in self.input_index: str += key + ", "
218 str += "\nOutputs (%d): " % self.noutputs
TypeError: %d format: a number is required, not NoneType
As something of a side note/question/point for discussion: I'm having a hard time groking why ninputs, noutputs and nstates are allowed to be None. Is there a downside to making inputs etc a required rather than optional argument in the NonlinearIOSystem constructor? It seems that might make the class easier to use because then all the operators (__mul__() etc) would always work and not depend on how the user called the constructor.