Appending to Pandas DataFrame with categorical columns

You have to keep the both data frames consistent. As you are converting the column a from first data frame as categorical, you need do the same for second data frame. You can do it as following-

import pandas as pd

df=pd.DataFrame([],columns=['a', 'b'])
df['a']=pd.Categorical([],[0, 1])

new_df=pd.DataFrame.from_dict({'a':[0,1,1,1,0,0],'b':[1,1,8,4,0,0]})
new_df['a'] = pd.Categorical(new_df['a'],[0, 1])

df.append(new_df, ignore_index=True)

Hope this helps.