Exceptions Consider the following simple program We have to travel - - PowerPoint PPT Presentation
Exceptions Consider the following simple program We have to travel 100 miles Ask the user for a speed to enter (in mph) Tell them the time, in hours, it will take to arrive denom = int(input("Enter a speed in mph: "))
We have to travel 100 miles Ask the user for a speed to enter (in mph) Tell them the time, in hours, it will take to arrive
denom = int(input("Enter a speed in mph: ")) print(100/denom, “ hours to travel 100 miles.”)
We have to travel 100 miles Ask the user for a speed to enter (in mph) Tell them the time, in hours, it will take to arrive
denom = int(input("Enter a speed in mph: ")) print(100/denom, “ hours to travel 100 miles.”)
This will crash if input is zero, though!
ZeroDivisionError: division by zero
When Python crashes, it always gives an error:
IndexError
Accessing an index out of bounds in a list
KeyError
Acessing a dictionary key that doesn’t exist
TypeError
Performing an illegal interaction between types (“2” + 2)
NameError
Referencing a variable that hasn’t been created yet
ValueError
Function takes in a value it can’t use: int(“cat”), for example
Typically, an error will crash the program…unless you catch it!
denom = 0 while denom <= 0: denom = int(input("Enter a speed in mph: ")) if (denom <= 0): print("Error: speed must be greater than zero! Try Again!") print(100/denom, " hours to travel 100 miles.")
This won’t crash!
Imagine you are writing the back end of a system
You are implementing the backend in Python, but you don’t have access to
the front end
You can’t prompt the user
You are writing a function that takes in two numbers:
distance – the distance to be traveled in MPH speed – the speed to travel in MPH
At this point, having a ZeroDivisionError is the correct action if the speed
is zero
Whoever is calling your function needs to call it correctly Not your job to change the input!
"a common mistake that people make when trying to design
something completely foolproof is to underestimate the ingenuity of complete fools“ – Douglas Adams, Mostly Harmless
If you are writing a function, you can’t change how people call the
function
But you can force the person using your function to use it correctly Raising and error informs a user they are using the function wrong Avoids “easy to miss” garbage-in garbage-out
The below is fine: The end user shouldn’t be calling your function like this. Are there any other ways the function travel_time shouldn’t be
called?
def travel_time(distance, speed): return distance / speed travel_time(5, 0) # throws a zero division error
What if the type is off?
This looks absurd, but this compiles and runs (albeit crashing due to a
type error)
What if distance or velocity is negative?
This wouldn’t make sense, so can we prevent it? As the function is written, it would return output, but is that output
useful?
travel_time("sheep", [2, 3, 5])
You can raise errors! This is called “defensive programming”
Making sure your code can’t be called in an incorrect way
def travel_time(distance, speed): if (distance < 0): raise ValueError("travel_time distance cannot be negative") if (speed < 0): raise ValueError("travel_time speed cannot be negative") return distance / speed travel_time(-5, 10)
If you call a function or perform an operation that could result in an
error, you can try to “catch” the error
You can “catch” the error in order to handle it
Format:
try: code that could throw an error except <KindOfError>: handling the exception
What if when you get user input, the user enters invalid input? Now, instead of crashing, we can catch the error, and just make the
user try again!
distance = 0; while distance == 0: try: distance = float(input("Enter a distance to travel in miles:")) except ValueError: print("You didn't enter a number. Please try again!")
https://storage.googleapis.com/cs1111/examples/exception/exce
ption-tracing1.py
There are actually 12, just change the last part of the url to: exception-tracing[x].py
Where x is a number 1-12