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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 --")