Exceptions¶

Long before the computing era, Benjamin Franklin once said: “If you fail to plan, you are planning to fail.” As you may have already noticed, when running computer programs, something unexpected always happens. Even when they are really simple, like the ones in these lessons: maybe a file is not there when you try to read it, or you type a string where a number was needed or simply you press <Enter> when you should have entered some text.

If there is a way for code to break, then somehow, somewhere, someone will break it. Using exceptions you can manage these problems in a responsible way: if a client asks you to capitalize an integer you can decline their bizarre suggestion. Once you learn how to do this, you will become an exceptional Python programmer.

An exception is an error that is raised by Python, when the execution of our code results in an unexpected outcome. Normally, an exception will have a type and a message. For example, when python prints such exceptions:

ZeroDivisionError: division by zero
TypeError: must be str, not int
IndexError: list index out of range

The ZeroDivisionError, TypeError and IndexError represent the error type and the text that follows the colon is the error message. This message usually describes the cause of the error in more detail and/or the expected vs. actual input.

The traceback¶

To illustrate the exceptions I am going to deliberately make mistakes in my code. The mere thought of this causes my neural nets to rebel, but this is for a good cause.

Let us first write some code to print Hello, world! three times:

for i in range(3)
    print("Hello, world!")

Then run:

macbook$ python3 mistakes.py
File "/Users/python-1o1/examples/mistakes.py", line 1
    for i in range(3)
                    ^
SyntaxError: invalid syntax

Instead of our welcoming, we see a problem: the last line displays the error type and also a description. A syntax error means you did not follow the rules for how to write valid Python code. You will encounter this a lot when you’re first learning Python. Above you will see some text that tells you where the problem occurred, this text is called a traceback. Here, we’re missing a colon at the end of the for loop, if we correct and run again everything works as expected:

Hello, world!
Hello, world!
Hello, world!

Common exceptions¶

Let us see a few more common exceptions before we learn how to raise and handle them. Open the x-files.txt and read it. This should be exciting to read.

with open("x-files.txt") as textfile:
    the_truth = textfile.read()
print(the_truth)

Why am I not surprised:

Traceback (most recent call last):
File "/Users/python-1o1/examples/mistakes.py", line 4, in <module>
    with open("x-files.txt") as textfile:
FileNotFoundError: [Errno 2] No such file or directory: 'x-files.txt'

Whenever we try to read a non-exitent file Python raises a FileNotFoundError. The truth must still be out there.

What if you try to add one, two and three but instead of the number 3 you use the word three:

>>> 1 + 2 + "three"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>>

How I laughed while typing that. Here you get a TypeError with a more helpful description. Python lets you know that you cannot add an integer and a string. This exception is very common, it occurs when you expect one type of data but receive another.

first_name = "Benjamin"
print(fist_name)

Traceback (most recent call last):
File "/Users/python-1o1/examples/mistakes.py", line 5, in <module>
    print(fist_name)
NameError: name 'fist_name' is not defined

The NameError exception is thrown whenever we try to use a variable that does not exist, or a function that was not defined. The error message is quite clear, you should be able to fix it easily. Most often there is a misspelled name or a missing import.

Handling exceptions¶

Python provides us with the try except construction to handle exceptions that might be raised by our code. The basic anatomy of the try except clause is as follows

try:
    # this code runs first
    ...
except:
    # runs when the code above fails
    ...

In plain English, the try except clause is basically saying: Try to do this, except if there’s an error, then do this instead.

There are a few options on what to do with the thrown exception from the try block. Let’s discuss them.

Catch specific exceptions¶

Python allows us to define which exception types we want to catch explictily. To do this, we need to specify the type to the except block.

a = 42
b = "ten"
try:
    sum = a + b
except TypeError:
    print("Cannot sum the variables, please pass numbers only.")

And when we run this code we get:

macbook$ python3 mistakes.py
Cannot sum the variables, please pass numbers only.

It is starting to look better, isn’t it? Now, we can actually log or print the exception itself:

a = 42
b = "ten"
try:
    sum = a + b
except TypeError as err:
    print(f"Cannot sum the variables, reason: {err}.")

macbook$ python3 mistakes.py
Cannot sum the variables, reason: unsupported operand type(s) for +: 'int' and 'str'.

Furthermore, we can catch multiple exception types in one clause if we want to handle them in the same way:

a = 42
b = 10
try:
    sum = a + b + c
except (TypeError, NameError) as err:
    print(f"Cannot sum the variables, reason: {err}.")

macbook$ python3 mistakes.py
Cannot sum the variables, reason: name 'c' is not defined.

Basically, we just need to pass a tuple containing the exception types that we need to handle.

Sometimes, we may need another except block with no specific exception type. The purpose of this is to catch all exception types. A typical use case would be when making a request to a third party API, we might not know all of the possible exception types, however, we still want to catch and handle them all.

a = 42
b = 10
c = 0
try:
    remainder = (a + b) % c
except (TypeError, NameError) as err:
    print(f"Cannot compute the result, reason: {err}.")
except Exception as err:
    print(f"A {type(err)} occurred, please inspect: {err}.")

macbook$ python3 mistakes.py
A <class 'ZeroDivisionError'> occurred, please inspect: integer division or modulo by zero.

In the case our code raises any exceptions other than TypeError and NameError, it will run last except block and print the message.

Notice that we used the Exception keyword to specify the generic error type that will match any exceptions.