Python implementation of the Barone-Adesi And Whaley model for the valuation of American options and their greeks.

BAW.py

"""
BAW.PY

Implements the Barone-Adesi And Whaley model for the valuation of American options and their greeks. 
"""

import numpy as _np
import cmath as _cm

# Option Styles
_AMERICAN = 'American'
_EUROPEAN = 'European'
# Option Types 
_CALL = 'Call'
_PUT = 'Put'
# Output Types 
_VALUE = 'Value'
_DELTA = 'Delta'
_GAMMA = 'Gamma'
_VEGA = 'Vega'
_THETA = 'Theta'

_dS = 0.001
_dT = 1 / 365
_dV = 0.00001

_ITERATION_MAX_ERROR = 0.001

def _standardNormalPDF(x): 
    val = (1 / (2 * _cm.pi)**0.5) * _np.exp(-1 * (x**2) / 2)
    return val
def _standardNormalCDF(X):     
    y = _np.abs(X) 
    
    if y > 37: 
        return 0
    else:
        Exponential = _np.exp(-1 * (y**2) / 2)
        
    if y < 7.07106781186547:
        SumA = 0.0352624965998911 * y + 0.700383064443688
        SumA = SumA * y + 6.37396220353165
        SumA = SumA * y + 33.912866078383
        SumA = SumA * y + 112.079291497871
        SumA = SumA * y + 221.213596169931
        SumA = SumA * y + 220.206867912376
        SumB = 0.0883883476483184 * y + 1.75566716318264
        SumB = SumB * y + 16.064177579207
        SumB = SumB * y + 86.7807322029461
        SumB = SumB * y + 296.564248779674
        SumB = SumB * y + 637.333633378831
        SumB = SumB * y + 793.826512519948
        SumB = SumB * y + 440.413735824752
        _standardNormalCDF = Exponential * SumA / SumB
    else:
        SumA = y + 0.65
        SumA = y + 4 / SumA
        SumA = y + 3 / SumA
        SumA = y + 2 / SumA
        SumA = y + 1 / SumA
        _standardNormalCDF = Exponential / (SumA * 2.506628274631)
    
    if X > 0:
         return 1 - _standardNormalCDF
    else: 
         return _standardNormalCDF
        
def _priceEuropeanOption(option_type_flag, S, X, T, r, b, v):
    '''
    Black-Scholes
    '''

    d1 = (_np.log(S / X) + (b + v**2 / 2) * T) / (v * (T)**0.5)
    d2 = d1 - v * (T)**0.5

    if option_type_flag == 'Call':
        bsp = S * _np.exp((b - r) * T) * _standardNormalCDF(d1) - X * _np.exp(-r * T) * _standardNormalCDF(d2)
    else:
        bsp = X * _np.exp(-r * T) * _standardNormalCDF(-d2) - S * _np.exp((b - r) * T) * _standardNormalCDF(-d1)
        
    return bsp

def _priceAmericanOption(option_type_flag, S, X, T, r, b, v):
    '''
    Barone-Adesi-Whaley
    '''
    
    if option_type_flag == 'Call':
        return _approximateAmericanCall(S, X, T, r, b, v)
    elif option_type_flag == 'Put':
        return _approximateAmericanPut(S, X, T, r, b, v)
def _approximateAmericanCall(S, X, T, r, b, v):
    '''
    Barone-Adesi And Whaley
    '''

    if b >= r:
        return _priceEuropeanOption('Call', S, X, T, r, b, v)
    else:
        Sk = _Kc(X, T, r, b, v)
        N = 2 * b / v**2                                           
        k = 2 * r / (v**2 * (1 - _np.exp(-1 * r * T)))
        d1 = (_np.log(Sk / X) + (b + (v**2) / 2) * T) / (v * (T**0.5))
        Q2 = (-1 * (N - 1) + ((N - 1)**2 + 4 * k))**0.5 / 2
        a2 = (Sk / Q2) * (1 - _np.exp((b - r) * T) * _standardNormalCDF(d1))
        if S < Sk:
            return _priceEuropeanOption('Call', S, X, T, r, b, v) + a2 * (S / Sk)**Q2
        else:
            return S - X
def _approximateAmericanPut(S, X, T, r, b, v):
    '''
    Barone-Adesi-Whaley
    '''

    Sk = _Kp(X, T, r, b, v)
    N = 2 * b / v**2
    k = 2 * r / (v**2 * (1 - _np.exp(-1 * r * T)))
    d1 = (_np.log(Sk / X) + (b + (v**2) / 2) * T) / (v * (T)**0.5)
    Q1 = (-1 * (N - 1) - (((N - 1)**2 + 4 * k))**0.5) / 2
    a1 = -1 * (Sk / Q1) * (1 - _np.exp((b - r) * T) * _standardNormalCDF(-1 * d1))

    if S > Sk:
        return _priceEuropeanOption('Put', S, X, T, r, b, v) + a1 * (S / Sk)**Q1
    else:
        return X - S
    
def _Kc(X, T, r, b, v):

    N = 2 * b / v**2
    m = 2 * r / v**2
    q2u = (-1 * (N - 1) + ((N - 1)**2 + 4 * m)**0.5) / 2
    su = X / (1 - 1 / q2u)
    h2 = -1 * (b * T + 2 * v * (T)**0.5) * X / (su - X)
    Si = X + (su - X) * (1 - _np.exp(h2))

    k = 2 * r / (v**2 * (1 - _np.exp(-1 * r * T)))
    d1 = (_np.log(Si / X) + (b + v**2 / 2) * T) / (v * (T)**0.5)
    Q2 = (-1 * (N - 1) + ((N - 1)**2 + 4 * k)**0.5) / 2
    LHS = Si - X
    RHS = _priceEuropeanOption('Call', Si, X, T, r, b, v) + (1 - _np.exp((b - r) * T) * _standardNormalCDF(d1)) * Si / Q2
    bi = _np.exp((b - r) * T) * _standardNormalCDF(d1) * (1 - 1 / Q2) + (1 - _np.exp((b - r) * T) * _standardNormalPDF(d1) / (v * (T)**0.5)) / Q2

    E = _ITERATION_MAX_ERROR
    
    while _np.abs(LHS - RHS) / X > E:
        Si = (X + RHS - bi * Si) / (1 - bi)
        d1 = (_np.log(Si / X) + (b + v**2 / 2) * T) / (v * (T)**0.5)
        LHS = Si - X
        RHS = _priceEuropeanOption('Call', Si, X, T, r, b, v) + (1 - _np.exp((b - r) * T) * _standardNormalCDF(d1)) * Si / Q2
        bi = _np.exp((b - r) * T) * _standardNormalCDF(d1) * (1 - 1 / Q2) + (1 - _np.exp((b - r) * T) * _standardNormalCDF(d1) / (v * (T)**0.5)) / Q2
    
    return Si
def _Kp(X, T, r, b, v):

    N = 2 * b / v**2
    m = 2 * r / v**2
    q1u = (-1 * (N - 1) - ((N - 1)**2 + 4 * m)**0.5) / 2
    su = X / (1 - 1 / q1u)
    h1 = (b * T - 2 * v * (T)**0.5) * X / (X - su)
    Si = su + (X - su) * _np.exp(h1)

    k = 2 * r / (v**2 * (1 - _np.exp(-1 * r * T)))
    d1 = (_np.log(Si / X) + (b + v**2 / 2) * T) / (v * (T)**0.5)
    Q1 = (-1 * (N - 1) - ((N - 1)**2 + 4 * k)**0.5) / 2
    LHS = X - Si
    RHS = _priceEuropeanOption( 'Put', Si, X, T, r, b, v) - (1 - _np.exp((b - r) * T) * _standardNormalCDF(-1 * d1)) * Si / Q1
    bi = -1 * _np.exp((b - r) * T) * _standardNormalCDF(-1 * d1) * (1 - 1 / Q1) - (1 + _np.exp((b - r) * T) * _standardNormalPDF(-d1) / (v * (T)**0.5)) / Q1
    
    E = _ITERATION_MAX_ERROR
    
    while _np.abs(LHS - RHS) / X > E:
        Si = (X - RHS + bi * Si) / (1 + bi)
        d1 = (_np.log(Si / X) + (b + v**2 / 2) * T) / (v * (T)**0.5)
        LHS = X - Si
        RHS = _priceEuropeanOption('Put', Si, X, T, r, b, v) - (1 - _np.exp((b - r) * T) * _standardNormalCDF(-1 * d1)) * Si / Q1
        bi = -_np.exp((b - r) * T) * _standardNormalCDF(-1 * d1) * (1 - 1 / Q1) - (1 + _np.exp((b - r) * T) * _standardNormalCDF(-1 * d1) / (v * (T)**0.5)) / Q1
        
    return Si

def _checkBadFlagInput(option_style_flag, output_flag, option_type_flag):

    styles = (_AMERICAN, _EUROPEAN)
    if option_style_flag not in styles:
        raise ValueError('Option Style must be one of %s' % (styles))
    outputs = (_VALUE, _DELTA, _GAMMA, _VEGA, _THETA)
    if output_flag not in outputs:
        raise ValueError('Output Type must be one of %s' % (outputs))
    types = (_CALL, _PUT)
    if option_type_flag not in types:
        raise ValueError('Option Type must be one of %s' % (types))

def _checkBadNumericInput(spot_price, strike_price, expiration_time_in_years, interest_rate_dec_pa, carry_rate_dec_pa, volatility_dec_pa):
    
    if spot_price <= 0:
        raise ValueError('Spot Price must be > 0')
    if strike_price <= 0:
        raise ValueError('Strike Price must be > 0')
    if expiration_time_in_years <= 0:
        raise ValueError('Time until Expiration must be > 0')
    if interest_rate_dec_pa <= 0 or interest_rate_dec_pa >= 1:
        raise ValueError('Interest rate in annualized decimal format must be > 0 and < 1.00')
    if carry_rate_dec_pa <= 0 or carry_rate_dec_pa >= 1:
        raise ValueError('Carry Rate in annualized decimal format must be > 0 and < 1.00')
    if volatility_dec_pa <= 0 or volatility_dec_pa >= 10.00:
        raise ValueError('Volatility in annualized decimal format must be > 0 and < 10.00 ')

def getValue(option_style_flag, output_flag, option_type_flag, spot_price, strike_price, expiration_time_in_years, interest_rate_dec_pa, carry_rate_dec_pa, volatility_dec_pa):
    '''Returns the value of a financial option according to the specified flag and numeric inputs,

    Keyword arguments:
    option_style_flag -- specifies the style of option to be valued. Must be contained in ('American', 'European')
    output_flag -- specifies the option characteristic value to be calculated. 
                    For price, give 'Price'. 
                    For an option greek, give one of ('Delta', 'Gamma', 'Vega', 'Theta').
    option_type_flag -- specifies the type of option to be valued. Must be contained in ('Call', 'Put')
    spot_price -- Spot price of the underlying asset. Must be > 0.
    strike_price -- Strike price of the option. Must be > 0.
    expiration_time_in_years -- Time until Expiration. Must be > 0. 
    interest_rate_dec_pa -- Interest rate in annualized decimal format. Must be > 0 and < 1.00.
    carry_rate_dec_pa -- Carry rate in annualized decimal format. Must be > 0 and < 1.00.
    volatility_dec_pa -- Volatility in annualized decimal format. Must be > 0 and < 10.00
    '''

    S = spot_price
    X = strike_price 
    T = expiration_time_in_years
    r = interest_rate_dec_pa
    b = carry_rate_dec_pa
    v = volatility_dec_pa

    _checkBadFlagInput(option_style_flag, output_flag, option_type_flag)
    _checkBadNumericInput(S, X, T, r, b, v)

    if option_style_flag == _AMERICAN:
    
        if output_flag == _VALUE: 
            return _priceAmericanOption(option_type_flag, S, X, T, r, b, v)
        elif output_flag == _DELTA: 
            return (_priceAmericanOption(option_type_flag, S + _dS, X, T, r, b, v) - _priceAmericanOption(option_type_flag, S - _dS, X, T, r, b, v)) / (2 * _dS)
        elif output_flag == _GAMMA: 
            return (_priceAmericanOption(option_type_flag, S + _dS, X, T, r, b, v) - 2 * _priceAmericanOption(option_type_flag, S, X, T, r, b, v) + _priceAmericanOption(option_type_flag, S - _dS, X, T, r, b, v)) / _dS**2
        elif output_flag == _VEGA:
            return (_priceAmericanOption(option_type_flag, S + _dS, X, T, r, b, v + _dV) - _priceAmericanOption(option_type_flag, S + _dS, X, T, r, b, v - _dV)) / 2
        elif output_flag == _THETA:
            return _priceAmericanOption(option_type_flag, S + _dS, X, T - _dT, r, b, v) - _priceAmericanOption(option_type_flag, S + _dS, X, T, r, b, v)
            
    elif option_style_flag == _EUROPEAN:

        # TODO implement Greeks for european options
        if output_flag == _VALUE: 
            return _priceEuropeanOption(option_type_flag, S, X, T, r, b, v)

At line 247, should the option_style_flag == _European instead of "_American"?

Yes, that is correct - I just fixed it. I've really only used the script for American options, so I hadn't run into the issue. Good catch!

Hey I have a couple of novice questions if you wouldn't mind helping me out ––

• What is the carry rate?
• What is "dec pa"?
• How is volatility calculated?
• Which bond interest rate should be used? A year, a month, etc?

At line 104:
Q2 = (-1 * (N - 1) + ((N - 1)**2 + 4 * k))**0.5 / 2
Should it be:
Q2 = (-1 * (N - 1) + ((N - 1)**2 + 4 * k)**0.5) / 2

The square root operation should only apply to ((N - 1)**2 + 4 * k), right?

on line 167: When you first assign the slope, you appropriately use both the univariate CDF and PDF functions, as seen in the original paper.

on line 176: However, when you update the slope in proceeding iterations, you use the CDF both times.

This leads to substantial error in the final valuation of the American option. The second density function on line 176 should be the univariate normal PDF.