A small example package
Project description
MDM Py
This package is a Python
implementation of Marginal Distribution Models (MDMs), which can be used in Discrete Choice Modelling.
Install
This package should eventually be uploaded onto PyPI when the package is more ready. In that case:
pip install mdmpy
should work.
How to use
Simplest Case
Gradient Descent
In the simplest case, we will use the Multinomial Logit (MNL) model, which is used as a default. Assuming numpy
, scipy
and pandas
are installed, we generate choice data assuming a random utility model:
from string import ascii_uppercase as letters
import pandas as pd
import scipy.stats as stats
import numpy as np
NUM_INDIV = 57
NUM_CHOICES = 3
NUM_ATTR = 4
np.random.seed(2019)
X = np.random.random((NUM_ATTR,NUM_INDIV*NUM_CHOICES))
true_beta = np.random.random(NUM_ATTR)
V = np.dot(true_beta.T,X)
V = np.reshape(V,(NUM_INDIV,NUM_CHOICES))
eps = stats.gumbel_r.rvs(size=NUM_INDIV*NUM_CHOICES)
eps = np.reshape(eps,(NUM_INDIV,NUM_CHOICES))
U = V+eps
highest_util = np.argmax(U,1)
df = pd.DataFrame(X.T)
df['choice'] = [1 if idx==x else 0 for idx in highest_util for x in range(NUM_CHOICES)]
df['individual'] = [indiv for indiv in range(NUM_INDIV) for _ in range(NUM_CHOICES)]
df['altvar'] = [altlvl for _ in range(NUM_INDIV) for altlvl in letters[:NUM_CHOICES]]
With this package, we will assume that df
is the dataframe which is simply given to us. Instead of having the code itself find out how many individuals, choices and coefficients or attributes there are, we will simply feed them into the class. To perform a gradient descent with this class, we will use the grad_desc
method.
import mdmpy
# In a typical case one would load df before this line
mdm = mdmpy.MDM(df,4,3,[0,1,2,3])
np.random.seed(4)
init_beta = np.random.random(4)
grad_beta = mdm.grad_desc(init_beta)
print(grad_beta)
# expected output [0.30238122 0.07955214 0.86779824 0.50951981]
Solver
The MDM
class acts as a wrapper and adds the necessary pyomo
variables and sets to model the problem, but requires a solver. IPOPT, an interior point solver, is recommended. If you have such a solver, it can be called. Assuming IPOPT is being used:
import mdmpy
ipopt_exec_path = /path/to/ipopt
mdm = mdmpy.MDM(df,4,3,[0,1,2,3])
mdm.model_init()
mdm.model_solve("ipopt",ipopt_exec_path)
print([mdm.m.beta[idx].value for idx in mdm.m.beta])
# expected output [0.30238834989235025, 0.07953888508425154, 0.8678050334295714, 0.5095096796373667]
Todo
- Add documentation.
- Add more type hints, especially those involving Python builtins
- Add tests.
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