You should use sum:
Total = df['MyColumn'].sum()
print (Total)
319
Then you use loc with Series, in that case the index should be set as the same as the specific column you need to sum:
df.loc['Total'] = pd.Series(df['MyColumn'].sum(), index = ['MyColumn'])
print (df)
X MyColumn Y Z
0 A 84.0 13.0 69.0
1 B 76.0 77.0 127.0
2 C 28.0 69.0 16.0
3 D 28.0 28.0 31.0
4 E 19.0 20.0 85.0
5 F 84.0 193.0 70.0
Total NaN 319.0 NaN NaN
because if you pass scalar, the values of all rows will be filled:
df.loc['Total'] = df['MyColumn'].sum()
print (df)
X MyColumn Y Z
0 A 84 13.0 69.0
1 B 76 77.0 127.0
2 C 28 69.0 16.0
3 D 28 28.0 31.0
4 E 19 20.0 85.0
5 F 84 193.0 70.0
Total 319 319 319.0 319.0
Two other solutions are with at, and ix see the applications below:
df.at['Total', 'MyColumn'] = df['MyColumn'].sum()
print (df)
X MyColumn Y Z
0 A 84.0 13.0 69.0
1 B 76.0 77.0 127.0
2 C 28.0 69.0 16.0
3 D 28.0 28.0 31.0
4 E 19.0 20.0 85.0
5 F 84.0 193.0 70.0
Total NaN 319.0 NaN NaN
df.ix['Total', 'MyColumn'] = df['MyColumn'].sum()
print (df)
X MyColumn Y Z
0 A 84.0 13.0 69.0
1 B 76.0 77.0 127.0
2 C 28.0 69.0 16.0
3 D 28.0 28.0 31.0
4 E 19.0 20.0 85.0
5 F 84.0 193.0 70.0
Total NaN 319.0 NaN NaN
Note: Since Pandas v0.20, ix has been deprecated. Use loc or iloc instead.