Units in Python - CalcTree

In CalcTree, you can easily use units across your calculations. Here, we explain how to reference a quantity inside Python, perform operations on it, and reference the quantities defined in Python elsewhere on the CalcTree page. 
When using formulae on a CalcTree page, you can use units of measurement to define quantities. For example, in a math parameter:
﻿
Using units in a math parameter
﻿﻿distance
:1.00 m​
﻿
﻿
Let's start by referencing this quantity inside a Python source.
Referencing a quantity in PythonYou can reference an existing quantity from your calculations inside Python. For example, the parameter defined above, can be referenced in Python as _page_distance, which will be of quantity type: 
﻿
Referencing the distance parameter in Python
﻿﻿check_type
:<class 'pint.Quantity'>​
﻿
﻿
you can get the magnitude and the unit of a quantity using the magnitude and unit methods:
﻿
Accessing magnitude and unit of a quantity
﻿﻿distance_magnitude
:1.00​
﻿
﻿﻿distance_unit
:meter​
﻿
﻿
Defining a new quantity in Python For defining a quantity in Python, you can use the ct.quantity method. For example to define a quantity of 1.5 seconds: 
﻿
Defining a quantity in Python
﻿
You can also define a quantity of 1.0 magnitude of a unit, using the ct.units method: 
﻿
Defining a quantity of one magnitude of a unit
﻿﻿one_second
:1.00 s​
﻿
﻿
Operations on quantitiesYou can multiply or divide quantities, which will result in a derived quantity. For example:
﻿
Creating a derived quantity
﻿﻿speed
:0.80 m / s​
﻿
﻿
You can also add or subtract quantities of the same type. The necessary unit conversions will happen automatically. For example: 
﻿
Adding two compatible quantities
﻿﻿total_distance
:1.3048 m​
﻿
﻿
Note that you can't use math.sqrt() on a quantity. you can use the power operator instead:
﻿
﻿
﻿
use the ** operator instead of math.sqrt()
﻿
Built-in quality control with unitsIf a quantity of incompatible type is tried in a Python addition or subtraction operation, CalcTree will prevent the calculation from proceeding and shows the relevant error. For example this line will inforce a quantity of Length for the parameter distance when received inside Python:
﻿
Enforce a quantity type of Length to be passed to Python
thus, sending a quantity other than length as the distance will not be accepted: 
﻿
Quality control with units
if you have a Python script that assumes certain quantities for its inputs, you can add quality control to it using the above capability. 
﻿
Unit conversionThe conversion of units will be done automatically inside Python, for example when adding and subtracting compatible quantities. If you want to explicitly convert a quantity into a desired unit, you can use the .to() method inside Python, or the to keyword in Math parameters on the page:
﻿
﻿
unit conversion inside Python 
﻿﻿inch to mm
:25.40 mm​
﻿
﻿
unit conversion in math parameters
﻿
Automatic prefix selectionWhen the magnitude of a quantity becomes too small or too large, CalcTree automatically picks an appropriate unit prefix, so that the magnitude becomes a more readable number, for example: 
﻿
﻿
you can override this behavior by using the to conversion keyword mentioned in the previous section:
﻿
﻿
Underlying library used for unitsCalcTree uses a Python library called pint to handle units. You can access pint documentation here. 
﻿
﻿
﻿
﻿
Still have more questions?Join the conversation on Slack below!
﻿
﻿
﻿
﻿
﻿