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I want to represent the value of nil, a value specific to my application, and distinguish it from the None built in to Python. What is the most elegant way to do this? Note that nil is a unique constant value.

user3793111
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1 Answers1

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Use a sentinel object:

nil = object()

Now you can test for is nil or is not nil just as you can test for None.

Any code that uses this does, of course, have to import it from the module that defines it; it is not a built-in the way None is built-in.

Martijn Pieters
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  • If I make another constant in the same way, will that be distinct from `nil`? And is the best practice to simply leave nil in global scope in a file, if I want to expose it as much as a regular class (since I essentially want this to represent a class with only one object). – user3793111 Jul 01 '14 at 08:44
  • If you test with `is` the other value will be different. – Matthias Jul 01 '14 at 08:51
  • @Matthias For `object` instances, even `==` will consider them distinct. It's just not idiomatic to compare such "singletons" with `==` (also applies to `None`, `Ellipsis` and `NotImplemented`). –  Jul 01 '14 at 08:54
  • @delnan: Still learning after all those years ... – Matthias Jul 01 '14 at 09:13
  • @user3793111: yes, another constant will be entirely distinct. – Martijn Pieters Jul 01 '14 at 09:35