KeyError: nan in dict

Question

I do as below

import numpy as np
from numpy import nan
df = pd.DataFrame({'a':[1, 2, 0, 1, np.nan, 2, 0]})
mapper = {2.0: 0.0, 1.0: 1.0 ,0.0: 2.0, nan : nan}
df['a'] = [ mapper[x] for x in df['a'] ]

and

KeyError: nan

I tried to change dtypes

df['a'] = df['a'].astype(object)

but again

KeyError: nan

what's wrong?

Answer 1

The problem is that nan is "not a number", and as such it equals no other number, not even another nan. You can read more about it here.

To demonstrate:

from numpy import nan
nan == nan
=> False

From this it must follow that nan is not in your dict, because it doesn't equal any of the keys.