Why does assert np.nan == np.nan cause an error?

Question

If

assert 1 == 1

is fine, then why does:

assert np.nan == np.nan

cause an assertion error?

What's even more confusing, this is OK:

assert np.nan != np.nan

What's the best way to test for nan?

Answer 1

NaN has the property that it doesn't equal itself, you should use np.isnan to test NaN values, here np.isnan(np.nan) will yield True:

In[5]:
np.nan == np.nan

Out[5]: False

In[6]:
np.nan != np.nan

Out[6]: True

In[7]:
np.isnan(np.nan)

Out[7]: True