如何在函数Python中调用类

Question

I'm a complete amateur, and trying to work out how to write a function that takes a list of objects, and returns a list of the names of said objects (based on whether they pass if statement). This is the class I've written from help of tutorials:

class Student:  
passmark=50  

def __init__(self,name,mark):  
    self.name=name  
    self.mark=mark  

def passes(self):  
    return self.mark > Student.passmark 

So from now I'm assuming I make a list of objects, say:

students = []  

Though this list is just a brand new list, which was necessary sure but how would I link it to the class? From this point I want to find out which students have failed, and return them and also where I am confused:

def failed(list):  
    for student in Students:  
        if passmark > self.mark:  
            return list  

Is all I can muster, sorry I've just gotten to classes and calling classes is quite confusing for me. The above code doesn't reference the class at all, and I really am confused on how to do so. I've no syntax errors or anything, I think my logic is fatally flawed.

Answer 1

You want to take all the student from the students list. So use that in the for loop. Also, you correctly encapsulated the logic of pass/fail criteria in a method, so use that.

Here is the code I think will do want you want:

def failed(list_of_students):
   failed_students = []
   for student in list_of_students:  
       if not student.passes():  
           failed_students.append(student.name)  
   return failed_students

A more advanced way of doing it is by using list comprehension:

def failed(list_of_students):
    return [student for student in list_of_students if not student.passes()]

It is more pythonic, but my be harder to understand for a beginner with a C or Java background.

Answer 2

You can use a list comprehension like this:

def failed(list):
    return [student.name for student in students if not student.passes()]

Answer 3

Try this code. Using list comprehension to return results. It's a very powerful python tool.

class Student:  
    passmark = 50
    def __init__(self, name, mark):  
        self.name=name  
        self.mark=mark  

    def passes(self):  
        return self.mark > Student.passmark 

    def __repr__(self):
        return '{} {}'.format(self.name, self.mark)


def failed(students_list):  
    return [student for student in students_list if student.mark < Student.passmark]

Answer 4

Given a Student class like you defined:

class Student:  
    passmark=50  

    def __init__(self,name,mark):  
        self.name=name  
        self.mark=mark  

    def passes(self):  
        return self.mark > Student.passmark 

You could instantiate a list of students with:

students = [Student("John", 49), Student("Mary", 75)]

It looks like you are also trying to define a function that will return a list of all the failed students; you could do something like this:

def failed(student_list):
    return [x for x in student_list if not x.passes()]

Answer 5

mark_to_pass = 50

#Approach one
class Student:
    def __init__(self, student_name, student_mark):
        self.name = student_name
        self.mark = student_mark
        self.pass_mark = self.calculate_passing_mark(mark_to_pass)

    def calculate_passing_mark(self, mark_to_pass):
        if self.mark >= mark_to_pass:
            return True
        return False

if __name__ == '__main__':
    example_student = Student("Swanson", 75)
    print(example_student.pass_mark)

With this approach every time a student object is created it will tell create a field telling you that student has passed. When working with lists such as a list of student objects you need to add the student to your list. Example

students = []
students.append(example_student)

Now you can look through your student list by doing

for student in students:
   print(student.pass_mark) # or do some other logic passed on who passed or failed. Or even here you dont need to create pass_mark object you can just check if student.mark > pass_mark

Answer 6

I'm assuming that failed isn't a member function of the class Student. The below code should work for what you are trying to do.

class Student:
    passmark=50
    def __init__(self,name,mark):
        self.name=name
        self.mark=mark  

    def passes(self):  
         return self.mark > Student.passmark

students = [Student("tom",40),Student("joe",70)]

def failed(listofStudents):
    listofStudentsThatFail = []
    for student in listofStudents:
        if not student.passes():
            listofStudentsThatFail.append(student)
    return listofStudentsThatFail

for s in failed(students):
    print s.name

The ouput when you run the code is: tom