Why do you allow an array index to start at 0?

Although I agree that this is not ideal, zero is not null.

Why wouldn't you want a user_id being 0?

When possible, you should avoid having any userid that could be mistaken for a non-userid number - it prevents a lot of silly programming errors from going unnoticed or turning into bigger problems, and has very little downside. http://www.phabricator.com/docs/phabricator/article/Things_Y...

Perhaps some implementation might consider 0 to be a 'NULL' value, assume it's null data and thus ignore it.

NULL != 0.

That never stopped JavaScript, Python and friends from giving them equal truthiness.

Ehm, Python doesn't have a NULL:

  >>> NULL == 0
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'NULL' is not defined

Even None, which is as close to NULL you are going to get in Python is not 0.

  >>> None == 0
  False

This is probably what the parent meant:

    >>> 'truthy' if None else 'falsy'
    'falsy'
    >>> 'truthy' if False  else 'falsy'
    'falsy'
    >>> 'truthy' if 0 else 'falsy'
    'falsy'

> That never stopped JavaScript, Python and friends from giving them equal truthiness.

Implicit casts -- including to booleans -- are dangerous, but if you try to write APIs to try to avoid every bad thing that might happen with naive use of implicit casting in every language that might be used to consume your API, you won't be able to do anything at all.

"truthiness" is true/false and NOT NULL! 0 != null null != false

In [1]: bool(0)

Out[1]: False

Idiomatic Python may say "if not user_id:" and erroneously skip this.

There is no `NULL` in Python. It is `None`.

Idiomatic test for `None` is:

   if something is None:
      # ...

It would be incorrect to use `if not something:` here.

I believe it has to do with Unix-like OSes always assigning the UID 0 to root.

This doesn't make any sense unless for some reason you are creating unix users based on the users in your application and using the application user ID as the unix user ID which would be silly.

The convention of 0 as NULL is more likely, as UID 0 doesn't really make sense in this context.