Exceptions

An exception is an error that occurs during the execution of a program. When an exception occurs, the program stops and an error message is displayed. You have likely seen some exceptions already in the previous lessons. For example, if you try to divide a number by zero, you will get a ZeroDivisionError exception. Try it out in the editor below:

Exception example

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Common Exception Types

There are several built-in exceptions in Python. Here are some of the most common ones:

IndentationError

Occurs when there is a problem with the indentation of your code.

# IndentationError: unexpected indent
x = 5
y = 6
 z = x + y

# IndentationError: expected an indented block
if x > 5:
print("x is greater than 5")

ValueError

A ValueError occurs when a function receives a bad argument. For example, The Int function can convert a string to an integer, but only if the string contains a valid integer.

y = int("24") # y = 24
x = int("hello") # ValueError

TypeError

A TypeError occurs when an operation is performed between two incompatible types, but it can also occur when calling a function with the wrong number of arguments.

# TypeError: can only concatenate str (not "int") to str
x = "hello" + 5

# TypeError: too many arguments
int(1,2,3)

NameError

A NameError occurs when you use a variable that doesn't exist.

print(x) # NameError

IndexError

An IndexError occurs when you try to get the index of a sequence that is out of range.

x = [1, 2, 3]
x[5] # IndexError

KeyError

A KeyError occurs when you try to get a value in a dictionary using a key that doesn't exist.

x = {"a": 1, "b": 2}
x["c"] # KeyError

Handling Exceptions

Often you will want your program to continue to run in spite of an exception.

The try and except are used to handle exceptions. Which means doing something when an exception occurs.

try:
    print(x)
except:
    print("x is not defined")

In the example above, we try to print the variable x, but it doesn't exist, so we get a NameError. However, since the print statement is inside a try block, the program does not crash and instead prints "x is not defined".

In this example, we did not specify which exception to handle. This means that the except block will handle any type of exception. It is however best practice to specify which exception to handle.

try:
    print(x)
except NameError:
    print("x is not defined")

try blocks function like if statements, in that you can have additional except blocks to handle different exceptions, and an else block if no exception occurs, and a finally block always executes when the try block is finished.

try:
    print(x)
    5/0
except NameError:
    print("x is not defined")
except ZeroDivisionError:
    print("You can't divide by zero")
except:
    print("Something else went wrong")
else:
    print("Nothing went wrong")
finally:
    print("The 'try except' is finished")

Getting the exception

You may want to do something with the exception object, like printing the error message. You can use as keyword, to assign the exception object to a variable. Then you can access the exception object's properties and methods, such as its message.

try:
    print(x)
except NameError as e:
    print(e.message)

Exercise

Add error handling to the functions below. When an error occurs, return the error type.

Exercise

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