Do I raise or return errors in Python?
Raise, return, and how to never fail silently in Python.
2021-02-09 05:34:48 -0500 -0500 February 9, 2021 4 min read
I hear this question a lot: “Do I raise or return this error in Python?”
The right answer will depend on the goals of your application logic. You want to ensure your Python code doesn’t fail silently, saving you and your teammates from having to hunt down deeply entrenched errors.
Here’s the difference between raise and return when handling failures in Python.
When to raise
The raise statement allows the programmer to force a specific exception to occur. (8.4 Raising Exceptions)
Use raise when you know you want a specific behavior, such as:
Raising an exception terminates the flow of your program, allowing the exception to bubble up the call stack. In the above example, this would let you explicitly handle TypeError later. If TypeError goes unhandled, code execution stops and you’ll get an unhandled exception message.
Raise is useful in cases where you want to define a certain behavior to occur. For example, you may choose to disallow certain words in a text field:
Raise takes an instance of an exception, or a derivative of the Exception class. Here are all of Python’s built-in exceptions. Raise can help you avoid writing functions that fail silently. For example, this code will not raise an exception if JAM doesn’t exist:
To cause the sandwich_or_bust() function to actually bust, add a raise :
Any time your code interacts with an external variable, module, or service, there is a possibility of failure. You can use raise in an if statement to help ensure those failures aren’t silent.
Raise in try and except
To handle a possible failure by taking an action if there is one, use a try … except statement.
This lets you buy_more_jam() before re-raising the exception. If you want to propagate a caught exception, use raise without arguments to avoid possible loss of the stack trace.
If you don’t know that the exception will be a ValueError , you can also use a bare except: or catch any derivative of the Exception class with except Exception: . Whenever possible, it’s better to raise and handle exceptions explicitly.
Use else for code to execute if the try does not raise an exception. For example:
You could also place the print line within the try block, however, this is less explicit.
When to return
When you use return in Python, you’re giving back a value. A function returns to the location it was called from.
While it’s more idiomatic to raise errors in Python, there may be occasions where you find return to be more applicable.
For example, if your Python code is interacting with other components that do not handle exception classes, you may want to return a message instead. Here’s an example using a try … except statement:
Note that when you return an Exception class object, you’ll get a representation of its associated value, usually the first item in its list of arguments. In the example above, this is the string explanation of the exception. In some cases, it may be a tuple with other information about the exception.
You may also use return to give a specific error object, such as with HttpResponseNotFound in Django. For example, you may want to return a 404 instead of a 403 for security reasons:
Using return can help you write appropriately noisy code when your function is expected to give back a certain value, and when interacting with outside elements.
The most important part
Silent failures create some of the most frustrating bugs to find and fix. You can help create a pleasant development experience for yourself and your team by using raise and return to ensure that errors are handled in your Python code.
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Victoria Drake
Victoria Drake is an experienced software engineer with a unique background in technical and executive leadership. She loves to help technology teams raise programming proficiency and streamline development processes. Victoria is an award-winning technical author and open source community mentor. She serves as core maintainer and co-author for the OWASP Web Security Testing Guide.