Stock
system dynamics, bptk, bptk-py, python, business prototyping
Stock
Stock Constructor
Stock(model, name)
Stock in a SD DSL model.
Stock.equation
property equation()
Returns the equation as originally set.
Returns
The equation, either a SD DSL Element or Operator.
A stock is what a system remembers. Its value at any instant cannot be read off the rest of the system: it is the accumulation of everything that has flowed in and out since the start, which is why it needs both an initial value and an equation.
Create one through the model rather than directly: model.stock("population").
Stock.initial_value
property initial_value
The value the stock holds at starttime, before any flow has acted on it. Set it to a number or to a Constant, which is what lets a scenario override it.
population = model.stock("population")
population.initial_value = 80000000.0Stock.equation
property equation
The net change per timestep, not the value — usually the inflows minus the outflows.
population.equation = births - deathsInherited from Element
Everything else — equation, function_string, plot() and the arrayed-element API — comes from Element, where it is documented once for all element types.