如何让类中的_str____）方法在python中返回此特定值

我有两个班，学生班和一个班。基本上，我可以招收学生，让他们上课。UniClass的

\uuuu str\uuuu

需要返回一个精确的值，我不知道如何获取它。以下是我目前的代码：

class Student():
    """ ein Student """

    def __init__ (self, name, imt_name, semester):
        self.name = name
        self.imt_name = imt_name
        self.semester = semester

    def __str__(self):
        return "{} {}{}{} in Semester {}".format(self.name, "[", self.imt_name, "]", self.semester)


class UniClass:


    def __init__(self, name):
        self.name = name
        self.students = set()

    def enroll_student(self, student):
            self.students.add(student)

    def __str__(self):
        a = set()

        if len(self.students) == 0:
            return "set()"
        else:
            for student in self.students:
               a.add(str(student))

        return str(a)

这些是我的主张：

# Student str
student_horst = Student("Horst", "horst99", 20)
assert str(student_horst) == "Horst [horst99] in Semester 20"
# UniClass str
programming_class = UniClass("Programmieren")
assert str(programming_class) == "set()"

programming_class.enroll_student(student_horst)
assert str(programming_class) == "{Horst [horst99] in Semester 20}"

student_horst = Student("Horst2", "horst100", 20)
student_horst2 = Student("Horst2", "horst100", 20)

programming_class.enroll_student(student_horst2)
assert str(programming_class) == "{Horst [horst99] in Semester 20, Horst2 [horst100] in Semester 20}" \
   or str(programming_class) == "{Horst2 [horst100] in Semester 20, Horst [horst99] in Semester 20}"

当前返回值为：

set(['Horst [horst99] in Semester 20', 'Horst2 [horst100] in Semester 20'])

---

您可能正在使用Python 2运行代码，其中

集合的
str
是：

Python 2.7.16 (default, Apr  9 2019, 04:50:39) 
[GCC 8.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> str({1, 2})
'set([1, 2])'

而Python 3（至少在其最新版本中）返回：

Python 3.8.0 (default, Nov 21 2019, 17:20:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> str({1, 2})
'{1, 2}'

正如你所说的那样

检查您真正执行的版本（
import sys；print（sys.version）
），并使用较新的版本