User's Guide


This package contains classes that call Python functions. The classes can be used to send data to Python functions, and to obtain data from Python functions. This allows for example to use Python to communicate with web services, with hardware, or to do other computations inside a Python module.

The code has been tested with Python 3.8.10 on Linux 64 bit and Windows 64 bit.

Software configuration to use classes from this package

To use classes from this package, a Python 3.8 runtime environment must be installed. Also, the system environment variable PYTHONPATH may need to be set in order for Python to find the modules that contain the functions. These modules are stored in the directory Buildings/Resources/Python-Sources. In addition, an environment variable (LD_LIBRARY_PATH on Linux and PATH on Windows) may need to be set in order for a simulation environment to find the dynamically linked libraries.

The table below gives hints if there are problems running models that use Python code.

System Settings
Linux 64 bit

If the examples do not translate or simulate, enter on a console the commands

export PYTHONPATH=${PYTHONPATH}:"Path_To_Buildings_Library"/Resources/Python-Sources
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:"Path_To_Buildings_Library"/Resources/Library/linux64

and restart the Modelica environment.

If this does not help, make sure is installed on your system. You can check this with


On Ubuntu 20.04, this library can be installed with

sudo apt-get install libpython3.8-dev
Windows with Anaconda

On Windows with Ananconda, prior to starting Dymola, you may need to set the PYTHONPATH using

set PYTHONPATH=E:\modelica-buildings\Buildings\Resources\Python-Sources;C:\ProgramData\Anaconda3\lib

For Dymola, if the Python examples hang during simulation, you may want to run dymosim.exe, which is the program generated by Dymola when translating a model, from a DOS prompt to see additional error messages.


Because the Python libraries link to compiled C code, Dymola needs to be configured to generate code for 64-bit. This can be done by entering on the Dymola command line the assignment


Type of Python functions

Two different types of Python functions are supported: Functions that do not need to pass Python objects between one invocation to another, and functions that need to pass a Python object from one invocation to another. For the first case, a Python function may be

def returnTwiceTheInput(xR):
    return 2.*xR

For the second case, a Python function may be

def incrementAndReturnACounter(i, obj):
    if obj == None:
        # Initialize the Python object
        obj = {'counter': i}
        # Use the python object
        obj['counter'] = obj['counter'] + i
    return [i, obj]

For the first case, set in the function the input argument passPythonObject = false, and for the second case, set passPythonObject = true. The second case allows for example to build up a Python data structure (or to instantiate a Python object), and do computations on this object at each function invocation. For example, a Model Predictive Control algorithm or a machine learning algorithm, implemented in Python, could be fed with data at each time step. It could then store this data and use the current and its historical data to feed its algorithm. Based on this algorithm, it could output a control signal for use in another Modelica model.

Number of values to read to Python and write from Python

The parameters nDblWri (or nIntWri or nStrWri) and nDblRea (or nIntRea) declare how many double (integer or string) values should be written to, or read from, the Python function. These values can be zero, in which case the Python function receives no arguments for this data type, or it must return a list with zero elements. However, because Modelica does not allow arrays with zero elements, the arrays dblWri and dblRea, respectively, must contain exactly one element if nDblWri=0 or nDblRea=0. In this situation, dblWri is a dummy argument that will not be written to Python, and dblRea contains a number that must not be used in any model.

Arguments of the Python function

The arguments of the python functions are, in this order, floats, integers and strings (and the Python object if passPythonObject = true). If there is only one element of each data type, then a single value is passed. If there are multiple elements of each data type, then they are stored in a list. If there is no value of a data type (such as if nDblWri=0), then the argument is not present. Thus, if a data type is not present, then the function will not receive an empty list of this data type. If there are no arguments at all, then the function takes no arguments (except if passPythonObject = true, in which case the only argument is the Python object).

The table below shows the list of arguments for various combinations where no, one or two double values, integers and strings are passed as an argument to a Python function.

Returns values of the Python function

The Python function must return their values in the following order:

  1. If the function returns one or multiple double values, then the first return value must be a double (if nDblRea=1) or a list of doubles (if nDblRea > 1).
  2. If the function returns one or multiple integer values, then the next return value must be an integer (if nIntRea=1) or a list of integers (if nIntRea > 1).
  3. If nDblRea = nIntRea = 0, then the return values of the function, if any, are ignored.

The table below shows valid return types for various combinations where no, one or two double values and integer values are returned.

Pure Modelica functions (functions without side effects)

The functions that exchange data with Python are implemented as pure Modelica functions. Pure functions always return the same value if called repeatedly. If these functions are used to call hardware sensors or web services, they need to be called from a when-equation.

See the Modelica language specification for an explanation of pure and impure functions.


Various examples are provided, and for each of these, the Python functions are stored in the directory Buildings/Resources/Python-Sources.

The examples Buildings.Utilities.IO.Python_3_8.Functions.Examples.Exchange and Buildings.Utilities.IO.Python_3_8.Functions.Examples.ExchangeWithPassPythonObject contains various calls to different Python functions without and with memory.

The example Buildings.Utilities.IO.Python_3_8.Examples.KalmanFilter shows how to implement in a Modelica block a call to a Python function. This Python function stores its memory on disk between invocations (which, in general, is not recommended).

The example Buildings.Utilities.IO.Python_3_8.Examples.SimpleRoom shows a similiar example. However, rather than using a file to store the room temperature and energy between invocations, the function returns an object with this information, and receives this object again in the next invocation.

Implementation notes

String values cannot be returned from a Python function. The reason is that Dymola 2013 FD01 generates a compile time error if a Modelica function returns (Real[nR], Integer[nI], String). This will be fixed in Dymola 2014. (Support request #14983.)

Extends from Modelica.Icons.Information (Icon for general information packages).