python class和try-exception问题

Question

提问在注释里

class StudentsDataException(Exception):
    pass

 #这里的pass是pass所有exception，下面代码为何还要raise exception?

class BadLine(StudentsDataException):
    def __init__(self, line_number, line_string):
        super().__init__(self)
        self.line_number = line_number
        self.line_string = line_string
这个class的具体作用是什么，能否解释一下每条代码的作用


class FileEmpty(StudentsDataException):
    def __init__(self):
        super().__init__(self)


from os import strerror


data = { }

file_name = input("Enter student's data filename: ")
line_number = 1
try:
    f = open(filename, "rt")
   
    lines = f.readlines()
    f.close()
    
    if len(lines) == 0:
        raise FileEmpty()
    
    for i in range(len(lines)):
        
        line = lines[i]
        
        columns = line.split()
       
        if len(columns) != 3:
            raise BadLine(i + 1, line)#能否解释一下(i+1,line)是什么情况
        
        student = columns[0] + ' ' + columns[1]
       
        try:
            points = float(columns[2])
        except ValueError:
            raise BadLine(i + 1, line)
        
        try:
            #print(data[student]),这里print根本没有显示任何内容,只是想看一下能print什么
            data[student] += points#想知道data[student]起始是不是0
        except KeyError:
            data[student] = points#keyerror的话没有映射对象，这里的“data[student] = points”代表什么意思
    
    for student in sorted(data.keys()):
        print(student,'\t', data[student])

except IOError as e:
    print("I/O error occurred: ", strerror(e.errno))
except BadLine as e:
    print("Bad line #" + str(e.line_number) + " in source file:" + e.line_string)
except FileEmpty:
    print("Source file empty")


"""
文件里的内容：
John    Smith    5
Anna    Boleyn    4.5
John    Smith    2
Anna    Boleyn    11
Andrew    Cox    1.5

最后的output是
Andrew Cox      1.5
Anna Boleyn      15.5
John Smith      7.0
"""

Answer 1

raise是抛出异常的意思

except IOError as e:
    print("I/O error occurred: ", strerror(e.errno))
except BadLine as e:
    print("Bad line #" + str(e.line_number) + " in source file:" + e.line_string)
except FileEmpty:
    print("Source file empty")

try except 才处理异常。

#定义一个异常StudentsDataException子类，类中有两个属性，一个代码所在行，一个异常信息
class BadLine(StudentsDataException):
    def __init__(self, line_number, line_string):
        super().__init__(self)
        self.line_number = line_number
        self.line_string = line_string