raise statement on a conditional expression

Question

How do I elegantly implement the "Samurai principle" (return victorious, or not at all) on my functions?

return <value> if <bool> else raise <exception>

Educative

What is that Samurai here? What do you think about adding some details? – Flagella

the Samurai Principle - Return victorious, or not at all. If a function fulfills its responsibilities, return the appropiate result object, and if it doesn't then throw an Exception. – Educative

Oh, I see. Maybe some link would be good, such as Samurai Principle – Flagella

Answer 1

Inline/ternary if is an expression, not a statement. Your attempt means "if bool, return value, else return the result of raise expression" - which is nonsense of course, because raise exception is itself a statement not an expression.

There's no way to do this inline, and you shouldn't want to. Do it explicitly:

if not bool:
    raise MyException
return value

Malacology

I think the question can actually be translated to: is there a way to raise an exception as an expression instead as a statement? To which the answer is yes, as @Theirs suggested. – Kasten

Answer 2

If you absolutely want to raise in an expression, you could do

def raiser(ex): raise ex

return <value> if <bool> else raiser(<exception>)

This "tries" to return the return value of raiser(), which would be None, if there was no unconditional raise in the function.

Theirs

I find this handy for the short-hand ternary when configuring a settings file that relies on environment variables. This way I can still have clean dict rows down the file, but also give helpful and more specific error messages when a specific setting is simply missing. Ex: {'f': os.environ.get('FOO') or raise_foobar(), ...} – Pledge

Answer 3

I like to do it with assertions, so you emphasize that that member must to be, like a contract.

>>> def foo(self):
...     assert self.value, "Not Found"
...     return self.value

Willetta

In such way you wouldn't raise a specific execption, would you? – Educative

I've also noticed that assert is, like in other languages, only reproduced when on debug so ig you find an error on runtime, would it assert it as well? – Educative

This is not very pythonic, and it is mentioned before if you run Python interpreter without debut option the asserts are ignored... P.D: -O : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x – Infundibuliform

what happens when self.value = 0. If I run >>> assert 0, it raises an error. I suppose you will have to alter your example. edited question – Stylus

Answer 4

There is a way to raise inside of a ternary, the trick is to use exec:

def raising_ternary(x):
    return x if x else exec("raise Exception('its just not true')")

As you can see calling it with True will do the first part of the ternary and calling it with False will raise the exception:

>>> def raising_ternary(x):
...     return x if x else exec("raise Exception('its just not true')")
... 
>>> raising_ternary(True)
True
>>> raising_ternary(False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in raising_ternary
  File "<string>", line 1, in <module>
Exception: its just not true
>>> 

Kinaesthesia

while I think this was genius, from a Pythonic point of view it also makes my eyes bleed..... – Prolocutor

haha i knew the stigma that comes with this type of suggestion but i couldnt resist at least posting it for other people to see :D – Kinaesthesia

exec() is dangerous and shouldn't be used this way. – Tauto

Answer 5

Well, you could test for the bool separately:

if expr: raise exception('foo')
return val

That way, you could test for expr earlier.

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