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is and == (is not other_thread)

Remco Gerlich scarblac at pino.selwerd.nl
Thu Mar 1 08:04:30 EST 2001 
Grant Griffin <not.this at seebelow.org> wrote in comp.lang.python:
> I've had a few of my own little questions about "is" and "==", vis-a-vis
> strings:
> 
> 1) Are two strings that are equal in the "==" sense always guaranteed to
> be equal in the "is" sense?

No. Only when they have been interned. No guarantees about interning are
given, but you can use intern() yourself, I think string literals are
automatically interned, and maybe short strings as well. But no, you can't
in general use 'is' for string comparison.

> 2) Is "is" faster than "==" for strings?  Of hand, it would seem so. 
> That's why I'm tempted to use it for string equality.  (OTOH, if strings
> can be "==" equal without being "is" equal, than I guess that's wildly
> false economy. <wink>)

'is' may be slightly faster, but can't be used in general.

> 3) Internally, does "==" for strings always begin with an "is"
> operation?

'==' starts with an 'is' test for any argument.

Simple test:

class A:
   def __cmp__(self, other):
      return -1 # Compares different to anything

print A() == A() # Prints 0, two different instances
x = A()
print x == x # Prints 1, the 'is' test is used and __cmp__ is skipped


> (btw,-nobody-ever-answers-every-part-of-my-silly-multipart-questions,
>    -so-i-dare-you-folks-to-buck-the-trend-<wink>)-ly y'rs,

Oops. It was an accident, honest! I hadn't read this yet!

-- 
Remco Gerlich
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