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from typing import List

class Student:

def __init__(self, name: str, grades: List[int] = []): # This is bad!

self.name = name

self.grades = grades

def take_exam(self, result):

self.grades.append(result)

bob = Student("Bob")

rolf = Student("Rolf")

bob.take_exam(90)

print(bob.grades)

print(rolf.grades) # Whaaaaaat

# The function parameters evaluate when the function is defined, not when it runs.

# That means that self.grades is a name for the list that was evaluated when the function was defined.

# We're then modifying it in take_exam

# But all calls to the __init__ method have the same list (because parameters are only evaluated once!)

# So all students have the same list

# Avoid it by not having mutable parameters. Instead, do what we did in prior lectures:

from typing import List

class Student:

def __init__(self, name: str, grades: List[int] = None):

self.name = name

self.grades = grades or [] # New list created if one isn't passed

def take_exam(self, result):

self.grades.append(result)

bob = Student("Bob")

rolf = Student("Rolf")

bob.take_exam(90)

print(bob.grades)

print(rolf.grades) # Now it's empty.