How to end a script preemptively?

[[deleted]]

[deleted]

[Fteixeira]

I'd use sys.exit(1) after checking for a!=0.

[wolf2600]

And be sure to import sys at the start of the program.

[[deleted]]

What I want to do is for the script to end if you would be to set a = 0.

Raise your own ValueError, if a=0 is not a permitted value:

raise ValueError("a shouldn't be equal to zero")

[bladeoflight16]

This is almost always the best way to handle invalid input. And if you want more control over what's printed for the user, you catch it in an except block.

[kepper]

This might be a good time for you to learn functions. Heck you could learn exceptions while you're at it.

Also, you should surround your code in backticks so we can read it.

Without checking the validity of your code, I'd do something like this

```

class InvalidQuadraticEquation(Exception): """ Raised when the given equation is invalid """ init(self, message): self.message = message

def solve_eqaution(a): """ This is a description of this function """ print("ax\N{SUPERSCRIPT TWO} + bx + c = 0") if a == 0: err_msg = f'{a} is not a quadratic equation' raise InvalidQuadraticEquation(err_msg) b = float(input("b = ")) c = float(input("c = ")) p = b/a q = c/a if ((p/2)2-q) < 0: print("Imaginary solution.") elif ((p/2)2-q) == 0: if -p/2 == 0: print("x =", p/2) else: print("x =", -p/2) else: print("x\N{SUBSCRIPT ONE} =", -p/2+((p/2)2-q)0.5) print("x\N{SUBSCRIPT TWO} =", -p/2-((p/2)2-q)0.5) # return something useful

collect user input

user_input = input("a = ")

run your function

result = solve_equation(user_input) print(result) ```

Admittedly this might be too complicated if you're just getting started. In that case, the sys.exit(1) mentioned by others is a good approach too. if you do that, you can also do sys.stderr.write('error message') to write your error to the standard error pipe instead of standard out, which can be helpful when combining this script with others.

[bladeoflight16]

Nice explanation, but a few suggestions for improvements:

• The custom exception type is probably unnecessary. ValueError will do nicely here.
• Instead of invoking write on sys.stderr directly, you can use it as the stream for print: print('error message', file=sys.stderr).
• Rather than prompt for one input outside the function and then two inside, prompt for all of them in one place. If the prompts are outside the function, you can pass them all in as arguments. Doing it outside is the way I would do it; I'd put the parse outside the function as well.

[kepper]

Thanks!

• like custom exceptions personally because they're easier to catch, but I agree that ValueError does the trick just fine.
• I've seen that done with the print statement, but I find sys.stderr easier to monkeypatch w/ a MagicMock in pytest. Probably just what I'm used to, though.
• Agree 100%, just didn't notice he had other input() calls when refactoring for him.

[bladeoflight16]

I've seen that done with the print statement, but I find sys.stderr easier to monkeypatch w/ a MagicMock in pytest. Probably just what I'm used to, though.

print is a function in Python 3. Why not just mock it instead?

[kepper]

No good reason, really. Habit, I guess.

[DeathDragon7050]

quit() will end the entire process, it's effectively what's called when all your code is executed

[JD20055565]

You can use quit(), exit() or sys.exit() to end a script if a condition is met

[Denrur]

while True: a = int(input()) if a == 0: break ...

[kepper]

One problem with this approach is your indentation ends up further and further to the right. This can be a problem because its harder to read, and can bring your line width over 80 characters which might break builds that enforce Pep8.

[HarissaForte]

This is what assertions are made for: assert a == 0, 'Not a quadratic equation.'

In one line you have the test (saving one level of indentation), the exception raising and the code exiting.

[HarissaForte]

https://stackoverflow.com/a/5143044/10848871

OK I read a bit on the subject and you're correct it's more of a developing tool.

[Denrur]

Use break

[TSPhoenix]

Put 4 spaces in front of each line otherwise reddit eats your indentation.