On 08/10/2013 03:09 PM, Chris Angelico wrote:
On Sat, Aug 10, 2013 at 10:48 PM, Gary Herron
<gary.her...@islandtraining.com> wrote:
This is an oversimplification, but generally useful for all beginner (and
most advanced) programmers:
Don't use "is" for comparisons. Use "==".
It 20 years of programming Python, I've *needed* to use "is" ... only once
or twice.
Hrm, I wouldn't make it that hard a rule. Both comparisons have their
place. As has been mentioned earlier in this thread, checking if
something is None is spelled "if something is None". Checking if it
equals zero is spelled "if it == 0", which is a quite different check.
The other common check that uses 'is' is with an argument default
where absolutely anything could be passed:
_notpassed = object()
def frob(appendage, device=_notpassed):
"""Use some appendage to frob some device, or None to frob nothing.
Omit device to frob whatever is currently held in that appendage"""
if device is _notpassed:
device = ... # whatever you need
if device is not None:
# frob the device
But granted, equality comparisons are a LOT more common than identity
comparisons.
ChrisA
Everything you say is true, and even reasonable for those who know
what's up.
But for each of your examples, using "==" is equivalent to using "is".
Each of
if something == None
if device == _not passed
if device != None
would all work as expected. In none of those cases is "is" actually
needed.
Given that, and the implementation dependent oddities, I still believe
that it is *highly* misleading to teach a beginner about "is".
Here's a challenge: What is the simplest non-contrived example where an
"is" comparison is *required*. Where substitution of an "==" comparison
would cause the program to fail or be significantly less efficient?
(I'm not including the nearly immeasurably small timing difference
between v==None and v is None.)
Gary Herron
--
http://mail.python.org/mailman/listinfo/python-list
