On Oct 14, 2016 9:14 AM, "Gustavo Carneiro" <gjcarne...@gmail.com> wrote: > > Sorry if I missed the boat, but only just now saw this PEP. > > Glancing through the PEP, I don't see mentioned anywhere the SQL alternative of having a coalesce() function: https://www.postgresql.org/docs/9.6/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL > > In Python, something like this: > > def coalesce(*args): > for arg in args: > if arg is not None: > return arg > return None > > Just drop it into builtins, and voila. No need for lengthy discussions about which operator to use because IMHO it needs no operator. > > Sure, it's not as sexy as a fancy new operator, nor as headline grabbing, but it is pretty useful.
That function is for a different purpose. It selects the first non-null value from a flat collection. The coalesce operators are for traveling down a reference tree, and shortcutting out without an exception if the path ends. For example: return x?.a?.b?.c instead of: if x is None: return None if x.a is None: return None if x.a.b is None: return None return x.a.b.c You can use try-catch, but you might catch an irrelevant exception. try: return x.a.b.c except AttributeError: return None If `x` is an int, `x.a` will throw an AttributeError even though `x` is not None. A function for the above case is: def coalesce(obj, *names): for name in names: if obj is None: return None obj = getattr(obj, name) return obj return coalesce(x, 'a', 'b', 'c') See this section for some examples: https://www.python.org/dev/peps/pep-0505/#behavior-in-other-languages (The PEP might need more simple examples. The Motivating Examples are full chunks of code from real libraries, so they're full of distractions.)
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