[Python-ideas] Non-boolean return from contains (original) (raw)

Alex Gaynor alex.gaynor at gmail.com
Sun Jul 25 20:15:21 CEST 2010

Recently I've been wondering why contains casts all of it's returns to be boolean values. Specifically I'd like to propose that contains's return values be passed directly back as the result of the in operation. As a result I'd further propose the introduction of not_contains, which is the not in operator. The primary usecase for this is something like an expression recorder. For example in SQLAlchemy one can do:

User.id == 3, but not User.id in SQLList([1, 2, 3]), because it returns a bool always. not_contains is needed to be the analog of this, as it cannot be merely be a negation of contains when it's returning a non-bool result.

There should be no backwards compatibility issues to making contains return non-bools, unless there is code like:

x = y in foo assert type(x) is bool

However, for not_contains I'd propose the default implementation be:

def not_contains(self, val): x = val in self if type(x) is not bool: raise TypeError("%s returned a non-boolean value from contains and didn't provide an implementation of not_contains") return not x

This is not perfect (and it's at odds with the fact that ne doesn't return not self == other), but it seems to allow both the desired flexibility and backwards compatibility.

I'm not sure if this is something that'd be covered by the language moratorium, but if not I can try putting together a patch for this.

Alex

-- "I disapprove of what you say, but I will defend to the death your right to say it." -- Voltaire "The people's good is the highest law." -- Cicero "Code can always be simpler than you think, but never as simple as you want" -- Me

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