When making a polar plot the units used for input data are radians, however the axes produced for the plot are in degrees. This seems unnecessarily confusing. A short illustrative example is provided below:

import matplotlib.pyplot as plt
theta = [1.5, 3, 10, 90, 180]
r = range(1,6)
ax = plt.subplot(111,polar=True)
ax.scatter(theta,r)
plt.show()

This extends to the placing of axis labels etc, i.e. to move the r axis labels to the vertical grid line you use degrees:

ax.set_rgrids(r, angle = 90)

As mentioned to @jenshnielsen