# Establish a variable inside a driver expression

I'm quite new to python and drivers. I'm writing a rather complicated scripted expression inside the driver-area and I need to repeatedly use a long number sequence. Would it be possible to first store this number inside a new variable and then refer to that variable instead of the number?

The driver namespace

Can add functions and constants to the driver namespace

import bpy

def func(a, b, c):
return a * b * c

m = 9.4443299912

bpy.app.driver_namespace["f"] = func
bpy.app.driver_namespace["m"] = m


Run the script above, (copy & paste into text editor hit Run Script button,) to make the new members available to the driver namespace. To make it autorun when file is loaded, give the text block a py extension and check the Register checkbox on RHS of text editor header

Can now consider m in a driver expression to be a named constant like pi and e.

Use in a driver expression like f(m, m, m) Can be typed directly into a field by preceding with a hash # character. Once the expression is a driver (purple) on editing expression there is no longer a need for #.

There are a number of functions and properties etc already defined to the namespace, a sample of python console output.

>>> for k, v in bpy.app.driver_namespace.items():
...     k, v
...
('asin', <built-in function asin>)
('asinh', <built-in function asinh>)
('atan', <built-in function atan>)
...
('pi', 3.141592653589793)
('e', 2.718281828459045)
...
('f', <function func at 0x7f8f18d34c80>)
('m', 9.4443299912)
('frame', 1.0)


Where frame is the context.scene.frame_current.

EDIT: would like to see a "sub expression" driver variable type where you could do this for scalars, or using other defined variables. Fitting everything into the scripted expression can be a PITA.

• Thanks, this seems like a good answer. I would like to have just one clarification due to my lack of experience: would you add the first part of your answer directly to the expression-field of the scripted expression, or do I need to establish it in a separate python file? – Antti Mar 21 '18 at 15:42
• @Antti edited answer, possibly the easiest solution is to make the file autorun on load. – batFINGER Mar 21 '18 at 15:57
• If you prefer not to use a separate file, consider (lambda m: m * m * m)(9.4443299912). – wchargin Mar 21 '18 at 20:21