Steven D'Aprano wrote:
Gary Herron wrote:
Robert Kern wrote:
...
Use "is" when you really need to compare by object identity and not
value.
But that definition is the *source* of the trouble. It is *completely*
meaningless to newbies. Until one has experience in programming in
general and experience in Python in particular, the difference between
"object identity" and "value" is a mystery.
Then teach them the difference, rather than give them bogus advice.
So in order to lead newbies away from this *very* common trap they often
fall into, it is still a valid rule to say
Newbies: Never use "is" to compare immutable types.
Look in the standard library, and you will see dozens of cases of
first-quality code breaking your "valid" rule.
Your rule is not valid. A better rule might be:
Never use "is" to compare equality.
Or even:
Never use "is" unless you know the difference between identity and equality.
Or even:
Only use "is" on Tuesdays.
At least that last rule is occasionally right (in the same way a stopped
clock is right twice a day), while your rule is *always* wrong. It is never
correct to avoid using "is" when you need to compare for identity.
of even better
Newbies: Never use "is" to compare anything.
Worse and worse! Now you're actively teaching newbies to write buggy code!
Nonsense. Show me "newbie" level code that's buggy with "==" but
correct with "is".
However, I do like your restatement of the rule this way:
Never use "is" unless you know the difference between identity and
equality.
That warns newbies away from the usual pitfall, and (perhaps) won't
offend those
who seem to forget what "newbie" means.
Gary Herron
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