I'm trying to build a light for traffic simulation system I'm writing a light class which is represents a traffic signal like this

• period, i.e. the number of time steps between two changes from green to red
• green-period, i.e. the number of time steps where the signal is green. Like this pic:

  The image above is how you explained it's supposed to work.

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  The problem is in the first line of the method step, because time is always set to zero, and so the value "returned" (there's no return but I think you got it) will be always the same since the only "variable" always starts from the same value.

  def __init__(self, period, green_period):
      self.period = period
      self.green_period = green_period
      self.color = 'G'
      self.time = 0 # This variable will tell you which instant we're in
  

  Now that you have time as a variable attribute, you can re-write the step function this way.

  def step(self):
      self.time  = 1
      # If time is on green phase...
      if time%self.period < self.green_period: # [For example 9%7 < 3]
          # ...then turn it to green (doesn't matter if it's already green)
          self.color = 'G'
      else: # ...else, if the time isn't on green phase it must be on red phase
          self.color = 'R'
  

  This should work, I haven't tested it but I think it's very close to what you need.