With Matematica this integral gives me 3. wolfram alpha
With integrate.tplquad I get -3.
This is Matematica code.
Integrate[1-x, {(0,2),(0,3-1.5*x),(0,6-3*x -2*y)}]
I can't see what I am doing wrong with ntegrate.tplquad
f = lambda x,y,z: 1-x
x1, x2 = 0,2
y1, y2 = lambda x : 0 , lambda x:3-1.5*x
z1, z2 = lambda x,y:0, lambda x,y: 6 -3*x -2*y
print(integrate.tplquad(f,x1,x2, y1, y2, z1, z2)[0])
-3.0
Double-check the tplquad docstring; you'll see that the signature of the function f is f(z, y, x). So it looks like your integrand should be:
f = lambda x, y, z: 1 - z