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def divide(dividend, divisor):

if divisor == 0:

raise ZeroDivisionError("Divisor cannot be 0.")

return dividend / divisor

grades = [] # Imagine we have no grades yet

# average = divide(sum(grades) / len(grades)) # Error!

try:

average = divide(sum(grades), len(grades))

print(average)

except ZeroDivisionError as e:

print(e)

# Much friendlier error message because now we're dealing with it

# In a "students and grades" context, not purely in a mathematical context

# I.e. it doesn't make sense to put "There are no grades yet in your list"

# inside the `divide` function, because you could be dividing something

# that isn't grades, in another program.

print("There are no grades yet in your list.")

# -- Built-in errors --

# TypeError: something was the wrong type

# ValueError: something had the wrong value

# RuntimeError: most other things

# Full list of built-in errors: https://docs.python.org/3/library/exceptions.html

# -- Doing something if no error is raised --

grades = [90, 100, 85]

try:

average = divide(sum(grades), len(grades))

except ZeroDivisionError:

print("There are no grades yet in your list.")

else:

print(f"The average was {average}")

# -- Doing something no matter what --

# This is particularly useful when dealing with resources that you open and then must close

# The `finally` part always runs, so you could use it to close things down

# You can also use it to print something at the end of your try-block if you like.

students = [

{"name": "Bob", "grades": [75, 90]},

{"name": "Rolf", "grades": []},

{"name": "Jen", "grades": [100, 90]},

]

try:

for student in students:

name = student["name"]

grades = student["grades"]

average = divide(sum(grades), len(grades))

print(f"{name} averaged {average}.")

except ZeroDivisionError:

print(f"ERROR: {name} has no grades!")

else:

print("-- All student averages calculated --")

finally:

print("-- End of student average calculation --")