the corrige for EXO-class-sort-hash-nb.md

Author: parmentelat

notebooks/exos/basic/students.py

@@ -0,0 +1,65 @@
+class Student:
+    """
+    a class to demonstrate how to define
+    a custom class that can be used in a set or a dict
+    (and that can be sorted too, btw)
+
+    try and run it without the last 3 dunder methods to see what happens
+    """
+
+    def __init__(self, first_name, last_name, grade=None):
+        self.first_name = first_name
+        self.last_name = last_name
+        self.grade = grade
+
+    # pretty print
+    def __repr__(self):
+        return f"<{self.first_name} {self.last_name} ({self.grade})>"
+
+    # hash on first and last name
+    def __hash__(self):
+        return hash( (self.first_name, self.last_name) )
+
+    # the protocol for hashable objets require this to be defined too
+    def __eq__(self, other):
+        return self.first_name == other.first_name and self.last_name == other.last_name
+
+    # sort on grades
+    def __lt__(self, other):
+        return self.grade < other.grade
+
+#### usage
+
+
+print(10*'=', 'a set containing students')
+
+s1 = Student('John', 'Doe', 12)
+s2 = Student('Jane', 'Doe', 15)
+s3 = Student('Jean', 'Dupont', 11)
+s4 = Student('Marie', 'Martin', 14)
+
+# can use a set to store students
+students = {s1, s2, s3}
+
+print(f"{s1 in students=}")
+print(f"{s4 in students=}")
+
+s1.grade = 13
+print(f"after grade change {s1 in students=}")
+
+
+
+print(10*'=', 'dict works too')
+
+grades_by_students = {s: s.grade for s in students}
+print(f"{grades_by_students[s2]=}")
+
+# this one would not work if we had not defined __hash__ and __eq__
+s1bis = Student('John', 'Doe')
+print(f"{s1bis in students=} and {grades_by_students[s1bis]=}")
+
+
+print(10*'=', 'sorting')
+
+students = {s1, s2, s3, s4}
+print(f"{sorted(students)=}")