Dealing with layers in Python PCell

Comments

• Matthias

  October 2021

  You cannot access layers from the application from a PCell. Instead, the PCell will give you a layout object which is not the application's layout.

  PCell layer parameters are given as LayerInfo structures. But the PCell helper class will provide the layer index as a service. This happens in the following way: declare a parameter with a name - e.g. "wheretogo". If the PCell helper class will see a parameter declared as Layer, is will provide the corresponding layer index in an attribute called like the parameter name plus "_layer". In the example that will be "wheretogo_layer".

  Here is the code from the Python PCell sample:

      # parameter declaration as "Layer" with name "l" in __init__:
      self.param("l", self.TypeLayer, "Layer", default = pya.LayerInfo(1, 0))
  ...
      # in "produce_impl" "self.l_layer" is the layer index
      self.cell().shapes(self.l_layer).insert(pya.Polygon(...))
  

  You can achieve the same by obtaining the layer index from the PCell's context layout:

      # in "produce_impl" you can turn self.l (a LayerInfo object) into a layer index:
      layer_index = self.layout.layer(self.l)
  

  Matthias
• ravn

  October 2021

  Thank you for your reply @Matthias. Unfortunately, I am not sure I understand. You wrote I cannot access layers from the PCell, but I can get a list of all layer indexes. Is it only that I cannot assign to them? The thing I want to do is to create a "fully parametric" class of PCell's. I have code that looks like this:

  class VParser:
      def __init__(self, filename):
          g = {"sqrt": math.sqrt}
          self.l = {}
          with open(filename) as file:
              exec(file.read(), g, self.l)
  

  and filename is a python executable where I want to store variables, e.g. a = 5. In produce_implI then want to

       exec("x = "+self.w_str, {}, vp.l)
       w_dbu = vp.l['x']/self.layout.dbu
  

  where now vp.l is a VParser object.

  With this construction, we can have other things than numbers setting e.g. sizes of objects. I can for example do

  self.param("w_str", self.TypeString, "Width", default = "w")
  

  to have the width of something be w which is defined in a file. I can even do

  self.param("w_str", self.TypeString, "Width", default = "w+a")
  

  if I want that.

  Now, the issue is that I would like to be able to choose also the layer as a parameter in the external file. But I cannot understand how to do that. Perhaps it is not possible?
• Matthias

  October 2021

  Honestly, this is weird, but I guess I cannot convince you not to do that. I feels like delegating an architecture problem to a configuration. I rarely have seen such things doing good. In my experience such a scheme just adds obfuscation at the virtual benefit of some apparent flexibility. And I'm not talking about the error handling which you have to do or the inherent security holes you generate with "eval" on an input string.

  But well ...

  Anyway, if you want to introduce layers, treat them as strings - e.g "1/0" for layer 1, datatype 0. You can turn this string into a layer index you use:

  layer_string = ...  # e.g. "1/0"
  layer_index = self.layout.layer(pya.LayerInfo.from_string(layer_info))
  

  Matthias