list comprehension problem

[Diez B. Roggisch]

mk wrote:

> Hello everyone,
> 
> print hosts
> hosts = [ s.strip() for s in hosts if s is not '' and s is not None and
> s is not '\n' ]
> print hosts
> 
> ['9.156.44.227\n', '9.156.46.34 \n', '\n']
> ['9.156.44.227', '9.156.46.34', '']
> 
> Why does the hosts list after list comprehension still contain '' in
> last position?
> 
> I checked that:
> 
> print hosts
> hosts = [ s.strip() for s in hosts if s != '' and s != None and s != '\n'
> ] print hosts
> 
> ..works as expected:
> 
> ['9.156.44.227\n', '9.156.46.34 \n', '\n']
> ['9.156.44.227', '9.156.46.34']
> 
> 
> Are there two '\n' strings in the interpreter's memory or smth so the
> identity check "s is not '\n'" does not work as expected?
> 
> This is weird. I expected that at all times there is only one '\n'
> string in Python's cache or whatever that all labels meant by the
> programmer as '\n' string actually point to. Is that wrong assumption?

Yes. Never use "is" unless you know 100% that you are talking about the same
object, not just equality.

Diez