How to replace all non-NaN entries of a dataframe with 1 and all NaN with 0

You can take the return value of df.notnull(), which is False where the DataFrame contains NaN and True otherwise and cast it to integer, giving you 0 where the DataFrame is NaN and 1 otherwise:

newdf = df.notnull().astype('int')

If you really want to write into your original DataFrame, this will work:

df.loc[~df.isnull()] = 1  # not nan
df.loc[df.isnull()] = 0   # nan