One-line raise if

Question

I'm trying to condense raise if to one line. I had:

def hey(self, message):
    if not message:
        raise ValueError("message must be a string")

It works, but this code doesn't work:

def hey(self, message):
    raise ValueError("message must be a string") if not message

I get SyntaxError: invalid syntax. What do I do?

Answer 1

.... if predicate is invalid in Python. (Are you coming from Ruby?)

Use following:

if not message: raise ValueError("message must be a string")

UPDATE

To check whether the given message is string type, use isinstance:

>>> isinstance('aa', str) # OR  isinstance(.., basestring) in Python 2.x
True
>>> isinstance(11, str)
False
>>> isinstance('', str)
True

not message does not do what you want.

>>> not 'a string'
False
>>> not ''
True
>>> not [1]
False
>>> not []
True

---

if not message and message != '':
    raise ValueError("message is invalid: {!r}".format(message))

Answer 2

Old question, but here is another option that can give fairly condensed syntax without some of the drawbacks of assert (such as it disappearing when optimization flags are used):

def raiseif(cond, msg="", exc=AssertionError):
    if cond:
        raise exc(msg)

Applying to this specific question:

def hey(self, message):
    raiseif(
        not isinstance(message, str),
        msg="message must be a string",
        exc=ValueError
    )