Getting value of a variable without changing the expression in Python

[ansari . aafaq1994]

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?
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