python - 3D surface not transparent inspite of setting alpha

Question

I am trying to create a 3D surface with transparency. When I try the following code below, I expect to get two semi-transparent faces of a cube. However, both the faces are opaque inspite of supplying the alpha=0.5 argument. Any pointer on why this is happening and how to fix it ? I am using Python 3.3 (IPython notebook with the QT backend)and Matplotlib 1.3.1.

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
import mpl_toolkits.mplot3d as mp3d

bot = [(0, 0, 0),
       (1, 0, 0),
       (1, 1, 0),
       (0, 1, 0),
       ]

top = [(0, 0, 1),
       (1, 0, 1),
       (1, 1, 1),
       (0, 1, 1),
       ]

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
face1 = mp3d.art3d.Poly3DCollection([bot], alpha=0.5, linewidth=1)
face2 = mp3d.art3d.Poly3DCollection([top], alpha=0.5, linewidth=1)

ax.add_collection3d(face1)
ax.add_collection3d(face2)

Answer 1

Based on David Zwicker's input, I was able to get transparency working by setting the facecolor directly as a 4-tuple with alpha.

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
import mpl_toolkits.mplot3d as mp3d

bot = [(0, 0, 0),
       (1, 0, 0),
       (1, 1, 0),
       (0, 1, 0),
       ]

top = [(0, 0, 1),
       (1, 0, 1),
       (1, 1, 1),
       (0, 1, 1),
       ]

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
face1 = mp3d.art3d.Poly3DCollection([bot], alpha=0.5, linewidth=1)
face2 = mp3d.art3d.Poly3DCollection([top], alpha=0.5, linewidth=1)

# This is the key step to get transparency working
alpha = 0.5
face1.set_facecolor((0, 0, 1, alpha))
face2.set_facecolor((0, 0, 1, alpha))

ax.add_collection3d(face1)
ax.add_collection3d(face2)