import pandas as pd

df = pd.DataFrame(randn(6,3),columns = list('ABC'))

df

df['D']=0.0

key = 3*[1]+3*[2]

df['key']=key
df
this doesn't work:
df[df.key==1]['D'] = 1
df
this does: (why only this way?)
df.D[df.key==1]=3.4
df