A kind of 'delay_step' setting in synapse could cause an error

[Dong-Jiahuan]

When creating a brain simulation network, neuron group A and neuron group B form a synapse connection whose delay_step is set to 1 and then neuron group A and neuron group B form a synapse connection whose delay_step is set to None.

self.A2B = bp.synapses.Exponential(self.A, self.B, 
                                      bp.conn.FixedProb(0.02),
                                       output=bp.synouts.COBA(E=0.), g_max=we,
                                       tau=5., 
                                       method=method,
                                       delay_step=1)
self.A2C = bp.synapses.Exponential(self.A, self.C, 
                                      bp.conn.FixedProb(0.02),
                                       output=bp.synouts.COBA(E=0.), g_max=we,
                                       tau=5., 
                                       method=method,
                                       delay_step=None)

During building synapse connection between A and C, __init__ will call method register_delay and the first element of value corresponds to neuron group A's identifier in global_delay_data will be replaced by None. The element replaced by None is an instance of bm.LengthDelay before building synapse connection between A and C.

In this case, when self.A2B updating, it will call method get_delay_data and an error 'NoneType' object is not callable will be given.

[chaoming0625]

This is indeed a bug. Thanks for your report.