Do I raise or return errors in Python?
Raise, return, and how to never fail silently in Python.python coding leadership
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
return when handling failures in Python.
raisestatement allows the programmer to force a specific exception to occur. (8.4 Raising Exceptions)
raise when you know you want a specific behavior, such as:
raise TypeError("Wanted strawberry, got grape.")
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:
if "raisins" in text_field: raise ValueError("That word is not allowed here")
Raise can help you avoid writing functions that fail silently. For example, this code will not raise an exception if
JAM doesn’t exist:
import os def sandwich_or_bust(bread: str) -> str: jam = os.getenv("JAM") return bread + str(jam) + bread s = sandwich_or_bust("\U0001F35E") print(s) # Prints "🍞None🍞" which is not very tasty.
To cause the
sandwich_or_bust() function to actually bust, add a
import os def sandwich_or_bust(bread: str) -> str: jam = os.getenv("JAM") if not jam: raise ValueError("There is no jam. Sad bread.") return bread + str(jam) + bread s = sandwich_or_bust("\U0001F35E") print(s) # ValueError: There is no jam. Sad bread.
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.
To handle a possible failure by taking an action if there is one, use a
try: s = sandwich_or_bust("\U0001F35E") print(s) except ValueError: buy_more_jam() raise
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.
else for code to execute if the
try does not raise an exception. For example:
try: s = sandwich_or_bust("\U0001F35E") print(s) except ValueError: buy_more_jam() raise else: print("Congratulations on your sandwich.")
You could also place the print line within the
try block, however, this is less explicit.
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
from typing import Union def share_sandwich(sandwich: int) -> Union[float, Exception]: try: bad_math = sandwich / 0 return bad_math except Exception as e: return e s = share_sandwich(1) print(s) # Prints "division by zero"
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:
if object.owner != request.user: return HttpResponseNotFound
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.
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
return to ensure that errors are handled in your Python code.
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