Pythonic way of returning None if input itself is None?

Question

What's the best way to write a null-check in Python? Try-Except blocks, if blocks...?

Given a function that returns None if the input is None, a valid input, what's the best approach to deal with this situation? Take this function for example:

def urlify(path):
    if path is None:
        return None

    return "<a href=\"{0!s}\">{0!s}</a>".format(path)

You could do return path and "<a href=\"{0!s}\">{0!s}</a>".format(path) — vaultah
– vaultah, Commented Dec 12, 2016 at 11:08
I'd avoid using str.format() to generate HTML instead of a proper sanitizing template engine. Too many security holes to leave open. — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Dec 12, 2016 at 11:15
@IgnacioVazquez-Abrams Will do, this was a quick example for a Jinja2 extension filter — Alxe
– Alxe, Commented Dec 12, 2016 at 11:17

Moses Koledoye · Accepted Answer · 2016-12-12 11:15:01Z

4

You could use a ternary operator with the return statement to return None if path is None or return something else otherwise.

def urlify(path):
    return path if path is None else "<a href=\"{0!s}\">{0!s}</a>".format(path)

However considering functions return None by default, you can simply do:

def urlify(path):
    if path is not None:
        return "<a href=\"{0!s}\">{0!s}</a>".format(path)

Or as vaultah suggested in the comments, you could also short-circuit using and.

answered Dec 12, 2016 at 11:15

Moses Koledoye

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3 Comments

Alxe Over a year ago

I like the first suggestion in cases the return value is brief, otherwise it ends up taking too much horizontal space. The latter is the one I like the most, but is not as obvious that it returns None. I don't really understand short-circuiting using and, I can't comment on his @vaultah suggested.

Brian Rodriguez Over a year ago

I personally prefer the short-circuit and, but these choices allow empty strings/False/etc to be passed as well. I suppose it depends on how flexible OP'd like their function to be.

Brian Rodriguez Over a year ago

@Alxe (a and b) is (a if not a else b)

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