python - change index line dataframe -

df dataframe>

i have dataframe , want change names of line index please guide how can ?

df1=pd.concat(infg)  print df1          a18         a19         a20   peak  213.164273  319.549715  198.585854   peak  153.412569  349.485482  145.959831   peak  125.797093  358.592932  124.882848   peak  198.156800  327.608988  185.250738   peak  148.445582  351.232905  141.896721   peak  151.817385  346.848871  153.386892   

$ result below.

   a18         a19         a20   esp1  213.164273  319.549715  198.585854   esp2  153.412569  349.485482  145.959831   esp3  125.797093  358.592932  124.882848   esp4  198.156800  327.608988  185.250738   esp5   148.445582  351.232905  141.896721   esp6  151.817385  346.848871  153.386892   

df1.index = ['esp'+ str(i) in range(len(df1.index))]