How could I import a class from a python script into another python script?

Question

I am trying to write a script contains some classes and save for example as model.py.

import numpy as np
from scipy import integrate

class Cosmology(object):
    def __init__(self, omega_m=0.3, omega_lam=0.7):
        # no quintessence, no radiation in this universe!
        self.omega_m = omega_m
        self.omega_lam = omega_lam
        self.omega_c = (1. - omega_m - omega_lam)
        #self.omega_r = 0

    def a(self, z):
        return 1./(1+z)

    def E(self, a):
        return (self.omega_m*a**(-3) + self.omega_c*a**(-2) + self.omega_lam)**0.5

    def __angKernel(self, x):
        return self.E(x**-1)**-1

    def Da(self, z, z_ref=0):
        if isinstance(z, np.ndarray):
            da = np.zeros_like(z)
            for i in range(len(da)):
                da[i] = self.Da(z[i], z_ref)
            return da
        else:
            if z < 0:
                raise ValueError(" z is negative")
            if z < z_ref:
                raise ValueError(" z should not not be smaller than the reference redshift")

            d = integrate.quad(self.__angKernel, z_ref+1, z+1,epsrel=1.e-6, epsabs=1.e-12)
            rk = (abs(self.omega_c))**0.5
            if (rk*d > 0.01):
                if self.omega_c > 0:
                    d = sinh(rk*d)/rk
                if self.omega_c < 0:
                    d = sin(rk*d)/rk
            return d/(1+z)

Then I want to call Cosmology class into another script, but I get the following error:

>>>from model import Cosmology as cosmo
>>>print cosmo.a(1.)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method a() must be called with Cosmology instance as first argument (got int instance instead)

I don't quite understand what the problem is!! Any tips??

put @classmethod on top of the def a. (I assume you don't need self at all inside of the method) — Kenji Noguchi
– Kenji Noguchi, Commented May 25, 2014 at 21:20
Your problem isn't the import; you're trying to call an instance method on the class. — jonrsharpe
– jonrsharpe, Commented May 25, 2014 at 21:20

tbrisker · Accepted Answer · 2014-05-25 21:22:44Z

2

You are trying to call an instance method from a class. In order to use the a() method, you need to create an instance of the Cosmology class:

>>>from model import Cosmology
>>>cosmo = Cosmology()
>>>cosmo.a(1.)
0.5

Or, if you want a() to be a class method, you need to decorate it with the @classmethod decorator - see here for more details.

answered May 25, 2014 at 21:22

tbrisker

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