Getting value of a variable without changing the expression in Python

[ansari.aafaq1994 at gmail.com]

Lets say i have a expression:
flowrate=(veloctiy*area)
i can find the flowrate with this expression
but if i dont wanna change the expression and want to find the velocity by giving it value of area and flowrate ,how do i do this in python?
is there any library or package or module for this?