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Python Exception Handling

Links: 108 Python Index

Exception Handling

try, except and finally

  • Exceptions are propagated through the call stack in unhandled.
    • For example let us consider a program where we have a function  A that calls function B, which in turn calls function C.
    • If an exception occurs in function C but is not handled in C, the exception passes to B and then to A.
    • If never handled, an error message is displayed and our program comes to a sudden unexpected halt.
  • We handle exceptions using try except block.
Almost every class inherits from the Exception class.

  • Catching multiple and specific exceptions 

    try:
   # do something
   pass

except ValueError:
   # handle ValueError exception
   pass

except (TypeError, ZeroDivisionError):
   # handle multiple exceptions
   # TypeError and ZeroDivisionError
   pass

except Exception as e:
   # handle all other exceptions
   pass

  • We can raise exceptions using the raise keyword.

    • Raising a general exception: raise Exception("some message")
    • Raising a specific exception: raise ValueError("Some thing is wrong")

  • finally clause is executed no matter what: 

    try:
   f = open("test.txt",encoding = 'utf-8')
   # perform file operations
finally:
   f.close()

  • Handling vs Raising exceptions:

    • attachments/Pasted image 20221121182647.jpg

Custom Exceptions

class GitHubApiException(Exception):
    def __init__(self, status_code):
        if status_code == 403:
            message = "Rate limit reached. Please wait a minute and try again."
        else:
            message = f"HTTP Status Code was: {status_code}."

        super().__init__(message)

  • Or we can have a custom message 

    class IncorrectValueError(Exception): 
    def __init__(self, value): 
        message = f"Got an incorrect value of {value}"
        super().__init__(message)

  • Or have multiple parameters and print them as a single message: 

    class CustomException(Exception):
    def __init__(self, ext_message: str, code: int) -> None:
        message = f"Some {ext_message} with {code}"
        super().__init__(message)

Assertions

Assertions are most useful in tests.
  • Using assertions
    • Assertions are used to raise an exception if a condition is not met.
import sys
assert ('linux' in sys.platform), "This code runs on Linux only."

# if the program is run on a different system then we will get the following error
Traceback (most recent call last):
  File "<input>", line 2, in <module>
AssertionError: This code runs on Linux only.
  • Handling assertions gracefully 
    def linux_interaction():
    assert ('linux' in sys.platform), "Function can only run on Linux systems."
    print('Doing something.')

try:
    linux_interaction()
except AssertionError as error:
    print(error)
    print('The linux_interaction() function was not executed')

# running it on windows system will give the following output
# Function can only run on Linux systems.
# The linux_interaction() function was not executed
What is the difference between raise and assert?
  • raise allows you to throw an exception at any time.
  • assert enables you to verify if a certain condition is met and throw an exception if it isn’t.
Don't use assertions as your only line of defence as they can be turned of using python -O main.py

Use exceptions

References

Last updated: 2022-11-21