The Basics of Exceptions in Python
This post was originally meant to be a small part of my discussion of the Go programming language (coming soon) as a way of drawing contrast between Python and Go, but turned out to be extensive enough to justify its own post.
What exactly needs to be done in that “handling” step typically depends on what went wrong, and so there are many types of errors. Programmers can check the type of an error to determine what went wrong and react accordingly. Errors can be thought of as objects and their types as classes that can be subclassed like any other class. But unlike other data, they follow an error-specific path through the code, short-circuitiing functions all the way up the stack until handled.
If that all sounded like gibberish to you, let’s walk through an example, starting with how you’d signal an error in your own code.
Let’s write a toy function to divide two numbers, aware of the fact that dividing a number by 0 isn’t permitted.
def divide(x, y): if y == 0: raise Exception("Can't divide by zero") return x / y
The interesting thing here is the
raise keyword, which causes the function to exit prematurely;
y is 0, the line
return x / y never executes.
Instead, we say that the function “errors”.
You can run this function in the Python REPL and see that it works fine most of the time but prints an error when the second argument is 0.
>>> divide(5, 10) 0.5 >>> divide(3, 0) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in divide Exception: Can't divide by zero
It’s important to understand that the exception isn’t returned from the function, it’s propagated via an entirely different pathway.
You can see this a few ways – one is by trying to assign the result of
divide to a variable.
>>> x = divide(3, 0) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in divide Exception: Can't divide by zero >>> x Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'x' is not defined
No value is returned and the variable is never set. The error short-circuits the function before it returns anything. Returning a value and raising an error are mutually exclusive: a function either returns something or it errors.
Errors, unlike return values, automatically rise through calling functions as well (that’s why we call it raising an error).
For example, if
divide is called within another function, the error will escalate through both layers and short-circuit the calling function as well.
def calculate_percent(numerator, denominator): raw_quotient = divide(numerator, denominator) return 100 * raw_quotient
>>> calculate_percent(10, 0) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in calculate_percent File "<stdin>", line 3, in divide Exception: Can't divide by zero
You can see what’s called the “stack trace” of the error as it bubbles up from its origin (the
raise line in
divide) up through
calculate_percent and finally into
<module> (which is actually the Python REPL).
Having a stack trace is extremely helpful;
divide could be called multiple times in our program, but with this knowledge we can quickly understand the exact provenance of the error and know where to look in order to fix it.
Functions that call
calculate_percent, should ideally be aware of the exceptions it might raise and handle them accordingly.
While errors aren’t returned per se, they can be “caught” in a variable through a mechanism called
def calculate_percent(numerator, denominator): try: raw_quotient = divide(numerator, denominator) except Exception as exc: print("Uh oh, ran into an error:", exc) return 0 return 100 * raw_quotient
>>> pct = calculate_percent(3, 0) Uh oh, ran into an error: Can't divide by zero >>> pct 0
Here, we “catch” the exception and prevent it from short-circuting this function.
If we run into an exception in the
try block, Python immeditely stops what it’s doing and switches execution over to the
except block instead of propagating the error further.
raw_quotient still isn’t defined (because
divide didn’t return a value), we as the programmer get a chance to take appropriate actions to rescue2 the program.
In this case, we might decide that in the rare case where we encounter an error in
divide, it’s safe to return 0 as long as we print a warning3.
The function returns a value and it’s stored in the
pct variable, which wouldn’t have happened if we’d let the exception go unhandled.
except blocks will catch errors in the corresponding
try block no matter how many functions deep they were originally raised.
Here, the exception is actually coming from the
divide function, a layer down from
calculate_percent, but it still gets trapped here.
We could also catch the error in any function that calls
calculate_percent, or even more levels up.
Unfortunately, our code has a flaw: it will also catch any type of exception in Python because of the
except Exception bit.
Any error that is a type of
Exception will trigger it – which is pretty much all errors!
It would be better to be more specific.
Luckily there is a more granular type of error in Python to signal dividing by zero, the
We’ll update our
divide function to use it4.
def divide(x, y): if y == 0: raise ZeroDivisionError("Can't divide by zero") return x / y
>>> divide(3, 0) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in divide ZeroDivisionError: Can't divide by zero
Notice that our error message now shows the precise type of error:
Rather than catching all exceptions, we can update our calling function to catch only that one.
def calculate_percent(numerator, denominator): try: raw_quotient = divide(numerator, denominator) except ZeroDivisionError as exc: print("Uh oh, ran into a divide-by-zero error:", exc) return 0 return 100 * raw_quotient
>>> pct = calculate_percent(5, 3) >>> pct 166.66666666666669 >>> pct = calculate_percent(5, 0) Uh oh, ran into a divide-by-zero error: Can't divide by zero >>> pct 0
Other types of errors will still cause our function to short-circuit though.
>>> del pct >>> pct = calculate_percent(5, 'abc') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in calculate_percent File "<stdin>", line 4, in divide TypeError: unsupported operand type(s) for /: 'int' and 'str' >>> pct Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'pct' is not defined.
Dividing an integer by a string is a
TypeError, which isn’t a
ZeroDivisionError, so our function fails just the same as if we hadn’t used
try/except at all.
To handle different types of errors, developers can chain multiple
except clauses with different exception types.
Python will check each one sequentially and enter the first
except block that the error matches against – or raise the error as usual if no blocks match.
def calculate_percent(numerator, denominator): try: raw_quotient = divide(numerator, denominator) except ZeroDivisionError: print("Division by 0 is not supported; returning 0") return 0 except TypeError: print("It looks like you entered data types that can't be divided; returning 0") return 0 except Exception as exc: print("Uh oh, an error I didn't anticipate:", exc) raise exc return 100 * raw_quotient
When an error arises in the
try block, Python first checks if it’s a
If so, it enters that block and runs the code inside, then exits the
try/except structure (it doesn’t check any of the following cases).
If not, it checks if the error is a
Again, it runs the code inside and then exits the
Last, it checks if the error is an
Exception, which is so general that it will catch basically anything else.
A few other things to note:
- Because of this sequential checking, it’s important to list exception types from most specific to most general.
If we’d listed
except Exceptionas the first case, even
TypeErrors would fall into that block and skip the others, since they are subtypes of
as excpart of the exception clause isn’t required. If it’s not present, errors of that type will still fall into the following block, but the error object won’t be stored in a variable for the developer to use. In the cases of
ZeroDivisionErrorerror, we omitted it, but we kept it in the general
Exceptioncase in order to print the error’s text.
Exceptionis the most general class of error5, so
except Exceptionwill catch anything. Catching all types of exception is helpful to demonstrate how exceptions are matched against different clauses, but rarely a good idea in real applications6.
- Errors can be “re-raised”, as we do here with
raise exc. You may want an error to still propagate up the stack, but only after you take some other actions. Here, we print a special message before re-raising the original error.
Let’s see our new code in action.
>>> pct = calculate_percent(5, 10) >>> pct 50.0
>>> pct = calculate_percent(5, 0) Division by 0 is not supported; returning 0 >>> pct 0
>>> pct = calculate_percent(5, 'abc') It looks like you entered data types that can't be divided; returning 0 >>> pct 0
Our code now gracefully handles “edge cases”.
Were we to encounter an unexpected error though, it would still surface via the
except Exception clause, making us aware of an additional edge case we should address in our code.
While this is a toy example, these are the tools used in real-world code to manage errors.
High quality libraries should raise meaningful exceptions for their calling code to handle.
Robust applications must anticipate a variety of possible error conditions and recover gracefully from them when possible.
try/except, we’re able to do that in Python.
I cover a few more advanced topics about exceptions in a separate post.
I can’t vouch for Java firsthand since I avoid it like the plague, but I did some research and it looks similar ↩︎
Interesting, in Ruby this block is actually called
To be clear, in production code, you would probably do something entirely different – maybe discard this input if it’s a data processing application, or return an error status if it’s a Rest API. But you’d use the
exceptblock to initiate that action. ↩︎
Experienced Python users will know that this is what happens by default if you try to divide a number by 0 in Python. We didn’t need to do a check to see if the denominator was 0 because Python actually raises this error for us when appropriate. But hey, this is a teaching example so I needed something simple. ↩︎
Not technically true, actually. There are certain errors in Python that aren’t a subclass of
Exception; it only covers “all built-in, non-system-exiting exceptions”. Generally you shouldn’t try to catch non-
Exceptionbased errors in your code because they’re things like
KeyboardInterrupt– things that aren’t meant to be suppressed in most applications. ↩︎
If an unforeseen error occurs in an application, you usually want to discover it and not allow it to pass silently through the system. That isn’t absolute though: in some cases it may be more important that the program doesn’t ever crash, so you might just log the error and move on. ↩︎