Pandas update multiple columns at once

I'm trying to update a couple fields at once - I have two data sources and I'm trying to reconcile them. I know I could do some ugly merging and then delete columns, but was expecting this code below to work:

df = pd.DataFrame([['A','B','C',np.nan,np.nan,np.nan],
                  ['D','E','F',np.nan,np.nan,np.nan],[np.nan,np.nan,np.nan,'a','b','d'],
                  [np.nan,np.nan,np.nan,'d','e','f']], columns = ['Col1','Col2','Col3','col1_v2','col2_v2','col3_v2'])

print df

 Col1 Col2 Col3 col1_v2 col2_v2 col3_v2
0    A    B    C     NaN     NaN     NaN
1    D    E    F     NaN     NaN     NaN
2  NaN  NaN  NaN       a       b       d
3  NaN  NaN  NaN       d       e       f

#update 
df.loc[df['Col1'].isnull(),['Col1','Col2', 'Col3']] = df[['col1_v2','col2_v2','col3_v2']]

print df

 Col1 Col2 Col3 col1_v2 col2_v2 col3_v2
0    A    B    C     NaN     NaN     NaN
1    D    E    F     NaN     NaN     NaN
2  NaN  NaN  NaN       a       b       d
3  NaN  NaN  NaN       d       e       f

My desired output would be:

 Col1 Col2 Col3 col1_v2 col2_v2 col3_v2
0    A    B    C     NaN     NaN     NaN
1    D    E    F     NaN     NaN     NaN
2    a    b    c       a       b       d
3    d    e    f       d       e       f

I'm betting it has to do with updating/setting on a slice, but I always use .loc to update values, just not on multiple columns at once.

I feel like there's an easy way to do this that I'm just missing, any thoughts/suggestions would be welcome!

Edit to reflect solution below Thanks for the comment on the indexes. However, I have a question about this as it relates to series. If I wanted to update an individual series in a similar manner, I could do something like this:

df.loc[df['Col1'].isnull(),['Col1']] = df['col1_v2']

print df

  Col1 Col2 Col3 col1_v2 col2_v2 col3_v2
0    A    B    C     NaN     NaN     NaN
1    D    E    F     NaN     NaN     NaN
2    a  NaN  NaN       a       b       d
3    d  NaN  NaN       d       e       f

Note that I didn't account for the indexes here, I filtered to a 2x1 series and set that equal to a 4x1 series, yet it handled it correctly. Thoughts? I'm trying to understand the functionality a bit better of something I've used for a while, but I guess don't have a full grasp of the underlying mechanism/rule

Solution

you want to replace

print df.loc[df['Col1'].isnull(),['Col1','Col2', 'Col3']]

  Col1 Col2 Col3
2  NaN  NaN  NaN
3  NaN  NaN  NaN

With:

replace_with_this = df.loc[df['Col1'].isnull(),['col1_v2','col2_v2', 'col3_v2']]
print replace_with_this

  col1_v2 col2_v2 col3_v2
2       a       b       d
3       d       e       f

Seems reasonable. However, when you do the assignment, you need to account for index alignment, which includes columns.

So, this should work:

df.loc[df['Col1'].isnull(),['Col1','Col2', 'Col3']] = replace_with_this.values

print df

  Col1 Col2 Col3 col1_v2 col2_v2 col3_v2
0    A    B    C     NaN     NaN     NaN
1    D    E    F     NaN     NaN     NaN
2    a    b    d       a       b       d
3    d    e    f       d       e       f

I accounted for columns by using .values at the end. This stripped the column information from the replace_with_this dataframe and just used the values in the appropriate positions.