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In [1]:

import pandas as pd

In [21]:

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

In [22]:

df

Out[22]:

	A	B	C
0	-0.031630	-1.867797	1.264133
1	-0.135349	-0.957986	-0.054713
2	0.337048	-0.383348	-1.991538
3	1.009888	-0.779377	-1.481212
4	-0.962852	0.129088	-0.010233
5	-0.640262	0.848540	1.025306

In [33]:

df['D']=0.0

In [34]:

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

In [35]:

df['key']=key
df

Out[35]:

	A	B	C	D	key
0	-0.031630	-1.867797	1.264133	0	1
1	-0.135349	-0.957986	-0.054713	0	1
2	0.337048	-0.383348	-1.991538	0	1
3	1.009888	-0.779377	-1.481212	0	2
4	-0.962852	0.129088	-0.010233	0	2
5	-0.640262	0.848540	1.025306	0	2

this doesn't work:

In [36]:

df[df.key==1]['D'] = 1
df

Out[36]:

	A	B	C	D	key
0	-0.031630	-1.867797	1.264133	0	1
1	-0.135349	-0.957986	-0.054713	0	1
2	0.337048	-0.383348	-1.991538	0	1
3	1.009888	-0.779377	-1.481212	0	2
4	-0.962852	0.129088	-0.010233	0	2
5	-0.640262	0.848540	1.025306	0	2

this does: (why only this way?)

In [39]:

df.D[df.key==1]=3.4
df

Out[39]:

	A	B	C	D	key
0	-0.031630	-1.867797	1.264133	3.4	1
1	-0.135349	-0.957986	-0.054713	3.4	1
2	0.337048	-0.383348	-1.991538	3.4	1
3	1.009888	-0.779377	-1.481212	0.0	2
4	-0.962852	0.129088	-0.010233	0.0	2
5	-0.640262	0.848540	1.025306	0.0	2

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