Python "ValueError: setting an array element with a sequence." I don't see which part is not working

Question

The goal with the following code is to plug the function "ddW" into odeint to find W for a given X (later on). I'm using the print function to make sure functions run).

import matplotlib.pyplot as plt
import numpy as np
from scipy.integrate import odeint
import pint
u = pint.UnitRegistry()

R = 8.31446261815324  * u.J/(u.mol*u.K)
Rgas = R.to            (u.atm*u.L/(u.mol*u.K))

P0  = 10    * u.atm
T   = 400   * u.K
v0  = 2     * u.L/u.min
α   = 0.02  * u.kg**-1

k   = 1.4   * u.L**2/(u.mol*u.kg*u.min)

FA0 = 0.5*P0*v0/(Rgas*T) #Assuming I.G.

def ddW(param,w):                               #param: [X,P]
    X       = param[0]
    P       = u.Quantity(param[1]).magnitude    * u.atm
    W       = u.Quantity(w).magnitude           * u.kg

    d       = np.zeros(2)
    d[0]    = k*FA0/(v0**2*P0)  * P*W*(1-X)     #dX/dW
    d[1]    = -α/2 *P0**2       * (1-X)/P       #dP/dW
    return d

param0 = [0,P0]
Wrange = np.linspace(0,100) *u.kg

#PBR     = odeint(ddW,param0,Wrange)


#plt.plot(Wrange,PBR)

print(ddW(param0,0))

Traceback (most recent call last):

  File "<ipython-input-1-83022bc3b5da>", line 1, in <module>
    runfile(REDACTED, wdir=REDACTED)

  File "C:\Users\Spencer\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 705, in runfile
    execfile(filename, namespace)

  File "C:\Users\Spencer\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 102, in execfile
    exec(compile(f.read(), filename, 'exec'), namespace)

  File "REDACTED", line 43, in <module>
    print(ddW(param0,0))

  File "REDACTED", line 33, in ddW
    d[1]    = -α/2 *P0**2       * (1-X)/P       #dP/dW

ValueError: setting an array element with a sequence.

I previously had the same error message for what is now line 32 (defining d[0]) until I defined W in line 29. What is bothering me is that none of the inputs for d[1] (as far as I can tell) have size >1, so it should fit.

Answer 1

Assuming your fix for line 32 (defining d[0]) was to replace w with W where you took the magnitude of w, you need to do the same for the quantity you are trying to put into d[1].

The right side of = for line 32 is a dimensionless Quantity object, whereas the right side of = for line 33 is a Quantity object with dimension standard_atmosphere / kilogram which can't be put into array d as is because it needs to be magnitude only so it can be converted to type float.

Try this if you want to keep your array d as type float:

val     = -a/2 *P0**2       * (1-X)/P       #dP/dW
d[1]    = val.magnitude