异常处理后返回的DataFrame为NoneType的问题求助

在读取Excel文件时，捕获FileNotFoundError并打印错误信息后递归调用read_wb()，但第二次输入正确文件名找到文件时，返回的df仍为NoneType，后续函数使用该df时触发AttributeError: 'NoneType' object has no attribute 'loc'。

相关代码 #

原read_wb函数 #

def read_wb():
    try:
        wb_name = input("What is the name of your workbook (add .xlsx): ")
        df = pd.read_excel(wb_name)
        return df
    except FileNotFoundError:
        print("Ooops... It looks like this file is spelled wrong or it's not in the same folder of the script. Try again.")
        read_wb()

报错调用片段 #

def main():
    df = read_wb()
--> df = modify_df(df)
    df = return_ws(df)
    repeat = input("Do want to run another file ('y' or 'n'): ").lower()

完整报错栈 #

AttributeError                            Traceback (most recent call last)
c:\Users\vn54jdq\Downloads\WFS Review Folders\Hams Filter Project.ipynb Cell 20 in <cell line: 146>()
    145         main()
    146 if __name__ == "__main__":
--> 147     main()

c:\Users\vn54jdq\Downloads\WFS Review Folders\Hams Filter Project.ipynb Cell 20 in main()
    139 def main():
    140     df = read_wb()
--> 141     df = modify_df(df)
    142     df = return_ws(df)
    143     repeat = input("Do want to run another file ('y' or 'n'): ").lower()

c:\Users\vn54jdq\Downloads\WFS Review Folders\Hams Filter Project.ipynb Cell 20 in modify_df(df)
     18 tic = time.perf_counter()
     19 ## This first part of the script goes through the list product_cat and product_cat_type. 
     20 ## If found, then it will mark a 'n' in the Chemical (Y/N) columns so it can be counted as an exclusion.
     21 ## Then it searches for product types that are in the key flags that we are searching for like chemical...etc
---&gt; 23 df.loc[df[df['Product Category'].str.contains(r'(?:\b(?=\w)|(?!\w))(?:{})(?:\b(?<=\w)|(?<!\w))'.format('|'.join(sorted(map(re.escape, product_cat), key=len, reverse=True))), case=False, regex=True).fillna(False)].index, "Chemical (Y/N)"] = 'n' 
     24 df.loc[df[df['Product Type'].str.contains(r'(?:\b(?=\w)|(?!\w))(?:{})(?:\b(?<=\w)|(?<!\w))'.format('|'.join(sorted(map(re.escape, values_flattened), key=len, reverse=True))), case=False, regex=True).fillna(False)].index, "Chemical (Y/N)"] = 'n'
     25 df.loc[df[df['Product Type'].str.contains(r'(?:\b(?=\w)|(?!\w))(?:{})(?:\b(?<=\w)|(?<!\w))'.format('|'.join(sorted(map(re.escape, chem_values_flattened), key=len, reverse=True))), case=False, regex=True).fillna(False)].index, "Chemical (Y/N)"] = 'y'

AttributeError: 'NoneType' object has no attribute 'loc'

问题原因 #

异常处理块中递归调用read_wb()时，没有返回该递归调用的结果。第一次调用read_wb()触发异常后进入递归，内层的read_wb()成功读取文件并返回df，但外层的read_wb()函数没有将这个结果返回，最终导致外层函数默认返回None。

解决方案 #

方案1：修复递归调用的返回值 #

修改异常块，返回递归调用的read_wb()结果：

def read_wb():
    try:
        wb_name = input("What is the name of your workbook (add .xlsx): ")
        df = pd.read_excel(wb_name)
        return df
    except FileNotFoundError:
        print("Ooops... It looks like this file is spelled wrong or it's not in the same folder of the script. Try again.")
        return read_wb()  # 新增return，返回递归调用的结果

方案2：用循环替代递归（更推荐） #

递归多次重试可能导致栈溢出，改用循环实现更稳健：

def read_wb():
    while True:
        try:
            wb_name = input("What is the name of your workbook (add .xlsx): ")
            df = pd.read_excel(wb_name)
            return df
        except FileNotFoundError:
            print("Ooops... It looks like this file is spelled wrong or it's not in the same folder of the script. Try again.")

验证效果 #

修改后，无论第一次输入错误多少次，只要最后输入正确文件名，函数都会正确返回读取到的DataFrame，后续modify_df(df)调用不会再触发AttributeError。

内容的提问来源于stack exchange，提问作者Kevin Rodgers Jr.