Python: from function to attribute

Technical Note #17

August 2016
By Grant MacKinnon

Most users are familiar with creating their own attributes within objects within an Aimsun model. These will typically be numbers or strings but it is also possible to add a column to an object where a value is returned based on some Python Code, allowing a value to be calculated on the fly.

A simple example is one that places a label letter on a turn to say whether it is Left, Through, Right or U-Turn.

Aimsun Python function attribute Showing the attribute in the network

Click to enlarge: Showing the attribute in the network

Firstly we could create this column using the Types Editor (by selecting the relevant object type from the Types window), then adding a new column of Column Type Function, in this case one that returns a string as a result.

Aimsu Python function attribute Showing the attribute in the network

Click to enlarge: Create the column using the Types Editor

The function needs to contain a Python function with signature of the form:

def eval_attr(object):

which returns a value with the result.

Here is the code we store inside the column

def determineTurningLetterForAngle(angle):
	dirString = ""
	if abs(angle)<30.0: dirString+="T" elif abs(angle)>150.0:< dirString+="U" elif angle>0.0:
	return dirString

def retrieveTurningLetterForTurning(turning):
	angle = turning.calcAngleSections()
	directionString = determineTurningLetterForAngle(angle)
	return directionString
def eval_attr(object):
	res = retrieveTurningLetterForTurning(object)
	return res

However we can create all this from a script which creates the column, and populates the code.
The code to set up the column directly from a Python script is as follows:

def main():
	turnLetterAttr = model.getColumn("GKTurning::TurnLetter")
	if turnLetterAttr == None:
		turningType = model.getType("GKTurning")
		turnLetterAttr = 
GKColumn.String, GKColumn.ePython)


PYTHON_CODE_STRING contains the definition of the function.

Download Python script for creating a turning classification label

More technical notes

Switching to Python 3

May 2021: Python 2 reached end-of-life status in January 2020. Tessa Hayman gives you some tips on getting your script Python 3-ready for optimum performance in Aimsun Next

Macro Function Components

March 2016: Marga Delgado explains how to use macro function components to produce extra outputs when executing an assignment.

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