Comparing None with built-in types using arithmetic operators?

Question

Python 2.7.2 (default, Jun 12 2011, 14:24:46) [MSC v.1500 64 bit (AMD64)] on win32
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>>> None > 0
False
>>> None == 0
False
>>> None < 0
True

• Is comparing None using arithmetic operators well defined for built-in types (integers in this case)?
• Is the difference between the first two and the third comparison part of language specification (Python's specification - you must be kidding :)) or is it CPython's implementation detail?

Answer 1

The only meaningful comparison you can really use with None is if obj is None: (or if obj is not None:).

Comparison between different types has been removed from Python 3 for good reasons - they were a common source of errors and lead to confusion. For example

>>> "3" < 4
False

In Python 3, you get a TypeError when comparing values of different types like str vs. int or anything vs. None.

>>> None < 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: NoneType() < int()

(I mean "comparing" in the sense of trying to determine which of two values is larger/smaller. Comparison for equality is OK - it will always return False if two object are of different types.)

I haven't found a reference in the docs for this, but in Learning Python, 4th edition, Mark Lutz writes on page 204:

[...] Comparisons of differently typed objects (e.g., a string and a list) work — the language defines a fixed ordering among different types, which is deterministic, if not aesthetically pleasing. That is, the ordering is based on the names of the types involved: all integers are less than all strings, for example, because "int" is less than "str".

Answer 2

Some interesting quotes from http://bytes.com/topic/python/answers/801701-why-none-0-a

In early Python, the decision was made that the comparison of any two objects was legal and would return a consistent result. So objects of different types will compare according to an ordering on their types (an implementation dependent, unspecified, but consistent ordering), and objects of the same type will be compared according to rules that make sense for that type.

Other implementations have the right to compare an integer and None differently, but on a specific implementation, the result will not change.

Python 3 will raise an exception on such comparisons.

and

The problem is the typical one; Python did not originally have a Boolean type, and the retrofit resulted in weird semantics. C has the same issue.