deliberately Raise and throw an exception in python - python-3.x

Question

I have a use case where I need to raise an exception and again throw the exception. For this I'm using raise keyword in both try and catch. The code is as below
try:
username = input ("username:")
If Len(username) != 6:
raise Exception
except Exception:
raise Exception ("username is not valid")
Is this the right way to do like this? Is it compliant to coding standards to raise Exception in both try and except blocks?

Answer 1

I'm guessing this is a simplified version of your actual use case, in which case it is generally correct. A couple of notes:
You can use raise without anything after it to re-raise the same error.
try:
raise ValueError('message')
except ValueError:
run_something()
raise # will raise ValueError ('message')
Don't raise a general Exception and don't catch them either, be specific, otherwise this code will obscure other errors and will be difficult to debug. If nothing else suits you, make an exception of your own:
class MyException(Exception):
pass
Then you can use it:
raise MyException('my message')
In your use case, if I understood it correctly, all of this together would be:
class InvalidUsername(Exception):
pass
try:
username = input('username: ')
if len(username) > 6:
raise InvalidUsername('Username is too long')
if '!' in username:
raise InvalidUsername('Invalid character in username')
except InvalidUsername:
handle_invalid_user()
raise
Example in the console:
>>> try:
... username = input('username: ')
... if len(username) > 6:
... raise InvalidUsername('Username is too long')
... if '!' in username:
... raise InvalidUsername('Invalid character in username')
... except InvalidUsername:
... handle_invalid_user()
... raise
...
username: test1234
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
__main__.InvalidUsername: Username is too long
Or with an invalid character:
username: ofer!
Traceback (most recent call last):
File "<stdin>", line 6, in <module>
__main__.InvalidUsername: Invalid character in username

Answer 2

I was also looking to raise an exception when there is an error within the business logic from an external server and due to database connection.
I came across this arcticle to basically raise the exception especially when you are follwing an Controller-> Service -> Repository pattern.
https://flask.palletsprojects.com/en/2.1.x/errorhandling/
from flask import jsonify, request
class InvalidAPIUsage(Exception):
status_code = 400
def __init__(self, message, status_code=None, payload=None):
super().__init__()
self.message = message
if status_code is not None:
self.status_code = status_code
self.payload = payload
def to_dict(self):
rv = dict(self.payload or ())
rv['message'] = self.message
return rv
#app.errorhandler(InvalidAPIUsage)
def invalid_api_usage(e):
return jsonify(e.to_dict()), e.status_code
# an API app route for getting user information
# a correct request might be /api/user?user_id=420
#app.route("/api/user")
def user_api(user_id):
user_id = request.arg.get("user_id")
if not user_id:
raise InvalidAPIUsage("No user id provided!")
user = get_user(user_id=user_id)
if not user:
raise InvalidAPIUsage("No such user!", status_code=404)
return jsonify(user.to_dict())
[1]: https://flask.palletsprojects.com/en/2.1.x/errorhandling/
I hope this helps to developers who are developing enterprise level apps.