I have a DataFrame that looks like this:

        data
a   b
1   1   0.12   0.23   0.3
2   1   0.52   0.63   0.7

and I want to find the minimum value for each level of a ignoring the b level, so as an output I'm looking for something like

a   min
1   0.1
2   0.5

The simpliest is use min with parameter level=0:

print (df.data.min(level=0).reset_index(name='min'))a  min
0  1  0.1
1  2  0.5

If need output as df and only one column df:

print (df.min(level=0))data
a      
1   0.1
2   0.5

Or groupby by first level with aggregating min:

print (df.groupby(level=0).data.min().reset_index(name='min'))a  min
0  1  0.1
1  2  0.5