added examples related to OOP lecture

examples/market.py

@@ -0,0 +1,64 @@
+"""
+Filename: market.py
+Reference: http://quant-econ.net/py/python_oop.html
+"""
+
+from __future__ import division
+from scipy.integrate import quad
+
+class Market:
+
+    def __init__(self, ad, bd, az, bz, tax):
+        """
+        Set up market parameters.  All parameters are scalars.  See
+        http://quant-econ.net/py/python_oop.html for interpretation.
+
+        """
+        self.ad, self.bd, self.az, self.bz, self.tax = ad, bd, az, bz, tax
+        if ad < az:
+            raise ValueError('Insufficient demand.')
+
+    def price(self):
+        "Return equilibrium price"
+        return  (self.ad - self.az + self.bz*self.tax)/(self.bd + self.bz) 
+
+    def quantity(self):
+        "Compute equilibrium quantity"
+        return  self.ad - self.bd * self.price()
+
+    def area1(self):
+        "Compute area under inverse demand function"
+        a, error = quad(lambda x: (self.ad/self.bd) - (1/self.bd)* x, 0, self.quantity())
+        return a
+
+    def consumer_surp(self):
+        "Compute consumer surplus"
+        return  self.area1() - self.price() * self.quantity()
+
+    def area2(self):
+        "Compute area above the supply curve.  Note that we exclude the tax."
+        a, error = quad(lambda x: -(self.az/self.bz) + (1/self.bz) * x, 0, self.quantity())  
+        return a
+
+    def producer_surp(self):
+        "Compute producer surplus"
+        return (self.price() - self.tax) * self.quantity() - self.area2()
+
+    def taxrev(self):
+        "Compute tax revenue"
+        return self.tax * self.quantity()
+
+    def inverse_demand(self,x):
+        "Compute inverse demand"
+        return self.ad/self.bd - (1/self.bd)* x
+
+    def inverse_supply(self,x):
+        "Compute inverse supply curve"
+        return -(self.az/self.bz) + (1/self.bz) * x + self.tax
+
+    def inverse_supply_no_tax(self,x):
+        "Compute inverse supply curve without tax"
+        return -(self.az/self.bz) + (1/self.bz) * x
+
+
+

examples/market_deadweight.py

@@ -0,0 +1,11 @@
+
+from market import Market
+
+def deadw(m):
+    "Computes deadweight loss for market m."
+    # == Create analogous market with no tax == #
+    m_no_tax = Market(m.ad, m.bd, m.az, m.bz, 0)   
+    # == Compare surplus, return difference == #
+    surp1 = m_no_tax.consumer_surp() + m_no_tax.producer_surp()  
+    surp2 = m.consumer_surp() + m.producer_surp() + m.taxrev()
+    return surp1 - surp2

examples/plot_market.py

@@ -0,0 +1,24 @@
+import matplotlib.pyplot as plt
+import numpy as np
+from market import Market
+
+# Baseline ad, bd, az, bz, tax
+baseline_params = 15, .5, -2, .5, 3
+m = Market(*baseline_params) 
+
+q_max = m.quantity() * 2
+q_grid = np.linspace(0.0, q_max, 100)
+pd = m.inverse_demand(q_grid)
+ps = m.inverse_supply(q_grid)
+psno = m.inverse_supply_no_tax(q_grid)    
+
+fig, ax = plt.subplots()
+ax.plot(q_grid, pd, lw=2, alpha=0.6, label='demand')
+ax.plot(q_grid, ps, lw=2, alpha=0.6, label='supply') 
+ax.plot(q_grid, psno, '--k', lw=2, alpha=0.6, label='supply without tax')
+ax.set_xlabel('quantity', fontsize=14)
+ax.set_xlim(0, q_max)
+ax.set_ylabel('price', fontsize=14)
+ax.legend(loc='lower right', frameon=False, fontsize=14)
+plt.show()
+

examples/solow.py

@@ -1,3 +1,7 @@
+"""
+Filename: solow.py
+Reference: http://quant-econ.net/py/python_oop.html
+"""
 from __future__ import division  # Omit for Python 3.x
 import numpy as np
 
@@ -11,22 +15,10 @@ class Solow:
     """
 
     def __init__(self, n, s, d, alpha, z, k):
-        """ Solow growth model with Cobb Douglas production function.
-
-        Parameters
-        ----------
-        n : scalar
-            population growth rate
-        s : scalar
-            savings rate
-        d : scalar
-            depreciation rate
-        alpha : scalar
-            capital's share of income
-        z : scalar
-            productivity parameter
-        k : scalar
-            current capital stock
+        """ 
+        Solow growth model with Cobb Douglas production function.  All
+        parameters are scalars.  See http://quant-econ.net/py/python_oop.html
+        for interpretation.
         """
         self.n, self.s, self.d, self.alpha, self.z = n, s, d, alpha, z
         self.k = k