Newbie: is a variable defined?

[Jeremy Yallop]

* Chad Netzer
|    The preferred way to test against None is with the "is" keyword, 
| presumably because something can equal None without being None 

That is a (fairly remote) possibility.  A more practical reason (IMO)
is that the test for identity is faster than the test for equality.

This is likely to be true in any language (consider comparing pointers
versus comparing strings in C, or `eq?' versus `equal?' in Scheme).

Anyway, here's Python (please excuse the long lines):

  >>> profile.run('[x for x in xrange(5000000) if x is None]')
           2 function calls in 7.300 CPU seconds
  
     Ordered by: standard name
  
     ncalls  tottime  percall  cumtime  percall filename:lineno(function)
          1    7.300    7.300    7.300    7.300 <string>:1(?)
          1    0.000    0.000    7.300    7.300 profile:0([x for x in xrange(5000000) if x is None])
          0    0.000             0.000          profile:0(profiler)
  
  
  >>> profile.run('[x for x in xrange(5000000) if x == None]')
           2 function calls in 9.210 CPU seconds
  
     Ordered by: standard name
  
     ncalls  tottime  percall  cumtime  percall filename:lineno(function)
          1    9.210    9.210    9.210    9.210 <string>:1(?)
          1    0.000    0.000    9.210    9.210 profile:0([x for x in xrange(5000000) if x == None])
          0    0.000             0.000          profile:0(profiler)

Jeremy.