无法将数据类型设置为数据框

Question

我有一个来自xml的数据框。 现在，我正在尝试将列“ a-g”转换为数值，以便可以应用计算 正确的方法是什么？请帮助（（（

这是我最初获得数据框的方式：

import xml.etree.ElementTree as et 
import pandas as pd
xtree = et.parse("data.xml")
xroot = xtree.getroot() 
entity = xroot[0]

df_credit = pd.DataFrame(columns = df_cols)

for credit in (entity.findall('credit')):   

    a = credit.find('amount').text    

    if credit.find('change/accrued_interest') is not None: 
        b = credit.find('change/accrued_interest').text        
    else:
        b = '-'    
    if credit.find('change/accrued_fines_penalties') is not None: 
        c = credit.find('change/accrued_fines_penalties').text
    else:        
        c='-' 
   # and so on

   ser = pd.Series([primary_contract_no,primary_contract_date,a, b,c,*** ], index = df_cols)

df_credit= df_credit.append(ser, ignore_index=True)  


 Result = df_credit[columns_to_keep]
 Result.set_index(levelG, inplace=True,append = True)

  # The result is in attached picture

然后我虽然将“-”替换为“ 0”，这将使我将DF转换为float但有堆栈...