# How to use Python in CompuCell3D¶

The most convenient way to start Python scripting in CC3D is by learning Twedit++. With just few clicks you will be able to create a template of working CC3D simulation which then you can customize to fit your needs. Additionally, each CC3D installation includes examples of simple simulations that demonstrate usage of most important CC3D features and studying these will give you a lot of insight into how to build Python scripts in CC3D.

Hint: Twedit++ has CC3D Python Menu which greatly simplifies Python coding in CC3D. Make sure to familiarize yourself with this conveninent tool.

Every CC3D simulation that uses Python consists of the, so called, main Python script. The structure of this script is fairly “rigid” (templated) which implies that, unless you know exactly what you are doing, you should make changes in this script only in few distinct places, leaving the rest of the template untouched. The goal of the main Python script is to setup a CC3D simulation and make sure that all modules are initialized in the correct order. Typically, the only place where you, as a user, will modify this script is towards the end of the script where you register your extension modules (steppables and plugins).

Another task of main Python script is to load CC3DML file which contains initial description of cellular behaviors. You may ask, why we need CC3DML file when we are using Python. Wasn’t the goal of Python to replace CC3DML? There are two answers to this question short and long. The short answer is that CC3DML provides the description of INITIAL cell behaviors and we will modify those behaviors as simulation runs using Python. But we still need a starting point for our simulation and this is precisely what CC3DML file provides. If you, however, dislike XML, and would rather not use separate file you can easily convert CC3DML into equivalent Python function – all you have to do is to use Twedit++ context menu. We will come back to this topic later. For now, let’s assume that we will still load CC3DML along with main Python script.

Let us start with simple example. We assume that you have already read “Introduction to CompuCell3D” manual and know how to use Twedit++ Simulation Wizard to create simple CC3D simulation. For completeness, however, we include here basic steps that you need to follow to generate simulation code using Twedit++.

To invoke the simulation wizard to create a simulation, we click CC3DProject->New CC3D Project in the menu bar. In the initial screen we specify the name of the model (cellsorting), its storage directory - C:\CC3DProjects and whether we will store the model as pure CC3DML, Python and CC3DML or pure Python. Here we will use Python and CC3DML.

Remark: Simulation code for cellsorting will be generated in C:\CC3DProjects\cellsorting. On Linux/OSX/Unix systems it will be generated in <your home directory>/CC3DProjects/cellsorting

On the next page of the Wizard we specify GGH global parameters, including cell-lattice dimensions, the cell fluctuation amplitude, the duration of the simulation in Monte-Carlo steps and the initial cell-lattice configuration. In this example, we specify a 100x100x1 cell-lattice, i.e., a 2D model, a fluctuation amplitude of 10, a simulation duration of 10000 MCS and a pixel-copy range of 2. BlobInitializer initializes the simulation with a disk of cells of specified size.

On the next Wizard page we name the cell types in the model. We will use two cells types: Condensing (more cohesive) and NonCondensing (less cohesive). CC3D by default includes a special generalized-cell type Medium with unconstrained volume which fills otherwise unspecified space in the cell-lattice.

We skip the Chemical Field page of the Wizard and move to the Cell Behaviors and Properties page. Here we select the biological behaviors we will include in our model. Objects in CC3D have no properties or behaviors unless we specify them explicitly. Since cell sorting depends on differential adhesion between cells, we select the Contact Adhesion module from the Adhesion section and give the cells a defined volume using the Volume Constraint module.

We skip the next page related to Python scripting, after which Twedit++-CC3D generates the draft simulation code. Double clicking on cellsorting.cc3d opens both the CC3DML (cellsorting.xml) and Python scripts for the model.

The structure of generated CC3D simulation code is stored in .cc3d file (C:\\CC3DProjects\\cellsorting):

<Simulation version="3.6.2">

<XMLScript Type="XMLScript">Simulation/cellsorting.xml</XMLScript>

<PythonScript Type="PythonScript">Simulation/cellsorting.py</PythonScript>

<Resource Type="Python">Simulation/cellsortingSteppables.py</Resource>

</Simulation>


Cellsorting.cc3d stores names of the files that actually implement the simulation, and most importantly it tells you that both cellsorting.xml, cellsorting.py and cellsortingSteppables.py are part of the same simulation. CompuCell3D analyzes .cc3d file and when it sees <PythonScript> tag it knows that users will be using Python scripting. In such situation CompuCell3D opens Python script specified in .cc3d file (here cellsorting.py) and if user specified CC3DML script using <XMLScript> tag it loads this CC3DML file as well. In other words, .cc3d file is used to link Python simulation files together in an unbigous way. It also creates “root directory” for simulation so that in the Python or XML code modelers can refer to file resources using partial paths i.e. if you store additional files in the Simulation directory you can refer to them via Simulation\your_file_name instead of typing full path e.g. C:\CC3DProjects\cellsorting\Simulation\your\_file\_name . For more discussion on this topic please see CompuCell Manual.

Let’s first look at a generated Python code:

File: C:\CC3DProjects\cellsorting\Simulation\cellsorting.py

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24  import sys from os import environ from os import getcwd import string sys.path.append(environ["PYTHON\_MODULE\_PATH"]) import CompuCellSetup sim, simthread = CompuCellSetup.getCoreSimulationObjects() CompuCellSetup.initializeSimulationObjects(sim, simthread) # Add Python steppables here steppableRegistry = CompuCellSetup.getSteppableRegistry() from cellsortingSteppables import cellsortingSteppable steppableInstance = cellsortingSteppable(sim, _frequency=1) steppableRegistry.registerSteppable(steppableInstance) CompuCellSetup.mainLoop(sim, simthread, steppableRegistry) 

The first line line provides access to standard functions and variables needed to manipulate the Python runtime environment. The next two lines (2, 3), import environ and getcwd housekeeping functions into the current namespace (i.e., current script) and are included in all our Python programs. In the next three lines,

import string

sys.path.append(environ["PYTHON\_MODULE\_PATH"])
import CompuCellSetup


we import the string module, which contains convenience functions for performing operations on strings of characters, set the search path for Python modules and import the CompuCellSetup module, which provides a set of convenience functions that simplify initialization of CompuCell3D simulations.

Next, we create and initialize the core CompuCell3D modules:

sim, simthread = CompuCellSetup.getCoreSimulationObjects()



We then create a steppable registry (a Python container that stores steppables, i.e., a list of all steppables that the Python code can access) and pass it to the function that runs the simulation. We also create and register cellsortingSteppable:

steppableRegistry=CompuCellSetup.getSteppableRegistry()

from cellsortingSteppables import cellsortingSteppable

steppableInstance=cellsortingSteppable(sim,_frequency=1)

steppableRegistry.registerSteppable(steppableInstance)



Once we open .cc3d file in CompuCell3D the simulation begins to run. When you look at he console output from this simulation it will look something like:

Figure 5 Printing cell ids using Python script

You may wonder where strings cell.id=1 come from but when you look at C:\CC3DProjects\cellsorting\Simulation\cellsortingSteppables.py file, it becomes obvious:

from PySteppables import *
import CompuCell
import sys

class cellsortingSteppable(SteppableBasePy):
def __init__(self, _simulator, _frequency=1):
SteppableBasePy.__init__(self, _simulator, _frequency)

def start(self):
# any code in the start function runs before MCS=0
pass

def step(self, mcs):
# type here the code that will run every _frequency MCS
for cell in self.cellList:
print "cell.id=", cell.id

def finish(self):
# Finish Function gets called after the last MCS
pass


Inside step function we have the following code snippet:

for cell in self.cellList:
print "cell.id=",cell.id


which prints to the screen id of every cell in the simulation. The step function is called every Monte Carlo Step (MCS) and therefore after completion of each MCS you see a list of all cell ids. In addition to step function you can see start and finish functions which have empty bodies. Start function is called after simulation have been initialized but before first MCS. Finish function is called immediately after last MCS. When writing Python extension modules you have flexibility to implement any combination of these 3 functions (start, step, finish).You can, of course, leave them unimplemented in which case they will have no effect on the simulation.

Let’s rephrase it again because this is the essence of Python scripting inside CC3D - each steppable will contain by default 3 functions:

1. start(self)
2. step(self,mcs)
3. finish(self)

Those 3 functions are imported , via inheritance, from SteppableBasePy (which in turn imports SteppablePy). The nice feature of inheritance is that once you import functions from base class you are free to redefine their content in the child class. We can redefine any combination of these functions. Had we not redefined e.g. finish functions then at the end simulation the implementation from SteppableBasePy of finish function would get called (which as you can see is an empty function).

\begin{align}\begin{aligned}(a + b)^2 = a^2 + 2ab + b^2\\(a - b)^2 = a^2 - 2ab + b^2\end{aligned}\end{align}
 [1] We have graphically edited screenshots of Wizard pages to save space.