尝试:

df['float'] = df['strCondition'].str.extract(r'(\d+.\d+)').astype('float')

输出:

       DATE strCondition  float
0  4/3/2018          2.9    2.9
1  4/3/2018    3.1, text    3.1
2  4/3/2018     2.6 text    2.6
3  4/3/2018    text, 2.7    2.7