Tracebacks

We talked about how to read the traceback from a Python program exception earlier in the day, but let’s talk about what we can do about it.

Uncaught Exceptions Exit Our Program

Let’s make a file called exceptions.py:

# exceptions.py

# this will throw an exception!
int("a")

print("End of the program.")

And run it:

(env) $ python exceptions.py

Traceback (most recent call last):
  File "exceptions.py", line 2, in <module>
    int("a")
ValueError: invalid literal for int() with base 10: 'a'

We’ll see that “Reached end of the program” was never printed out.

If we’re running our Python code from a file, an uncaught exception will quit the program. Notice our program never reaches “End of the program.”

Using try and except to catch Exceptions

In order to prevent our program from exiting, we’ll need to catch the Exception with a try except block. The anatomy of a try except block:

try:
    <code to try>
except ExceptionClass:
    <code to run if an exception happens>

In order to write a try except block, we’ll need to know the class name of the Exception we’d like to catch. Luckily, the name is printed right there, in the traceback!

Let’s update exceptions.py

# exceptions.py

try:
    int("a")
except ValueError:
    print("Oops, couldn't convert that value into an int!")

print("Reached end of the program.")

And the output:

(env) $ python exceptions.py
Oops, couldn't convert that value into an int!
Reached end of the program.

You want to catch Exceptions that are as specific as possible.

Using as to Access The Exception

You can optionally assign a label to the exception, and the exception will be assigned to the variable you specified, so you can look at it’s message, or examine it in other ways, with except <ExceptionClass> as <variable_name>.

Using this syntax, variable_name can be anything. In this case, I picked error, but you’ll commonly see e used for this purpose in Python programs.

Let’s update exceptions.py one more time:

# exceptions.py

try:
    int("a")
except ValueError as error:
    print(f"Something went wrong. Message: {error}")

print("Reached end of the program.")

If we run this code, we’ll see:

(env) $ python exceptions.py
Something went wrong. Message: invalid literal for int() with base 10: 'a'
Reached end of the program.

This example just scratches the surface. We’ll cover Exceptions in much more detail in the next section.

Anatomy of a Traceback

When running our code from Python files, we’ll need to look at your tracebacks a little more carefully.

Remember, to understand tracebacks, read them from bottom to top.

When we ran our first example, we saw:

Traceback (most recent call last):
  File "exceptions.py", line 2, in <module>
    int("a")
ValueError: invalid literal for int() with base 10: 'a'

If we start reading from the bottom up, we’ll notice a lot of useful information.

  1. First, we’ll see the exception that was thrown, along with its class ValueError.
  2. Next, we’ll see the code that caused the Exception.
  3. One line up from that, we’ll see the path and the file the exception originated in, as well as the line number to look on.