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Quick Start

Every output below comes from running the code with principle-viz 0.10.0.

Solve and draw a market

Demand is P = 10 - Q and supply is P = 2 + Q. Line.from_inverse(intercept, slope) builds a line from its inverse form P = intercept + slope * Q.

from principle_viz.core.equilibrium import solve_equilibrium
from principle_viz.core.line import Line
from principle_viz.plot.figure import MarketFigure

demand = Line.from_inverse(10.0, -1.0)  # P = 10 - Q
supply = Line.from_inverse(2.0, 1.0)  # P = 2 + Q
eq = solve_equilibrium(demand, supply)
print(eq)

fig = MarketFigure(x_max=12, y_max=12, title="Basic Equilibrium")
fig.add_curves(demand, supply, q_max=10)
fig.add_equilibrium(eq)
fig.finalize()
fig.save("basic_equilibrium.png")
fig.close()

Output:

EquilibriumResult(q_star=4.0, p_star=6.0, is_valid_market=True, notes=())

Basic equilibrium figure

Compare a tax

compare_tax_scenario solves the market before and after the tax. This one is a per-unit tax of 1 on producers.

from principle_viz.core.line import Line
from principle_viz.policy.analysis import compare_tax_scenario
from principle_viz.policy.tax import TaxOn, TaxScenario, TaxType

demand = Line.from_inverse(10.0, -1.0)
supply = Line.from_inverse(2.0, 1.0)

scenario = TaxScenario(tax_type=TaxType.PER_UNIT_TAX, amount=1.0, tax_on=TaxOn.PRODUCER)
result = compare_tax_scenario(demand, supply, scenario)

print(f"before tax: Q = {result.baseline_equilibrium.q_star}, P = {result.baseline_equilibrium.p_star}")
print(f"after tax:  Q = {result.post_tax.q_star}")
print(f"buyers pay {result.post_tax.consumer_price}, sellers keep {result.post_tax.producer_price}")
print(f"tax revenue = {result.post_tax.tax_revenue}")

Output:

before tax: Q = 4.0, P = 6.0
after tax:  Q = 3.5
buyers pay 6.5, sellers keep 5.5
tax revenue = 3.5

TaxType also offers FIXED_TAX and AD_VALOREM_TAX; TaxOn is CONSUMER or PRODUCER. To draw the tax on a figure, use MarketFigure.add_tax_transform.

From the command line

The same market, solved without writing Python:

principle-viz equilibrium \
  --demand-intercept 10 --demand-slope -1 \
  --supply-intercept 2 --supply-slope 1

{
  "q_star": 4.0,
  "p_star": 6.0,
  "is_valid_market": true,
  "notes": []
}

The tax scenario above is principle-viz tax with --tax-type per_unit --amount 1 --tax-on producer; it prints the baseline and post-tax equilibrium as JSON.

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