easy question on parsing python: "is not None"
in [Python]
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From: Carey Tilden on 5 Aug 2010 12:10 On Thu, Aug 5, 2010 at 8:42 AM, wheres pythonmonks <wherespythonmonks(a)gmail.com> wrote: > How does "x is not None" make any sense? "not x is None" does make sense. > > I can only surmise that in this context (preceding is) "not" is not a > unary right-associative operator, therefore: > > x is not None === IS_NOTEQ(X, None) > > Beside "not in" which seems to work similarly, is there other > syntactical sugar like this that I should be aware of? In addition to all the other fine responses, you also might want to take a look at the python grammar [1]. The relevant line is: comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not' Cheers, Carey [1] http://docs.python.org/reference/grammar.html
From: Chris Rebert on 5 Aug 2010 12:11 On Thu, Aug 5, 2010 at 8:56 AM, Roald de Vries <downaold(a)gmail.com> wrote: > On Aug 5, 2010, at 5:42 PM, wheres pythonmonks wrote: >> How does "x is not None" make any sense? Â "not x is None" does make sense. >> >> I can only surmise that in this context (preceding is) "not" is not a >> unary right-associative operator, therefore: >> >> x is not None === IS_NOTEQ(X, None) >> >> Beside "not in" which seems to work similarly, is there other >> syntactical sugar like this that I should be aware of? > > 'not None' first casts None to a bool, and then applies 'not', so 'x is not > None' means 'x is True'. Absolutely incorrect. Read the final paragraph of http://docs.python.org/reference/expressions.html#notin Cheers, Chris -- http://blog.rebertia.com
From: Chris Rebert on 5 Aug 2010 12:32 On Thu, Aug 5, 2010 at 8:42 AM, wheres pythonmonks <wherespythonmonks(a)gmail.com> wrote: > How does "x is not None" make any sense? Â "not x is None" does make sense. > > I can only surmise that in this context (preceding is) "not" is not a > unary right-associative operator, therefore: > > x is not None === IS_NOTEQ(X, None) > > Beside "not in" which seems to work similarly, is there other > syntactical sugar like this that I should be aware of? It's not quite the same type of sugar as that, but people sometimes forget that comparisons can be chained: if x < y <= z: Same as: if x < y and y <= z: # but y is only evaluated once Cheers, Chris
From: Roald de Vries on 5 Aug 2010 12:35 On Aug 5, 2010, at 6:11 PM, Chris Rebert wrote: > On Thu, Aug 5, 2010 at 8:56 AM, Roald de Vries <downaold(a)gmail.com> > wrote: >> On Aug 5, 2010, at 5:42 PM, wheres pythonmonks wrote: >>> How does "x is not None" make any sense? "not x is None" does >>> make sense. >>> >>> I can only surmise that in this context (preceding is) "not" is >>> not a >>> unary right-associative operator, therefore: >>> >>> x is not None === IS_NOTEQ(X, None) >>> >>> Beside "not in" which seems to work similarly, is there other >>> syntactical sugar like this that I should be aware of? >> >> 'not None' first casts None to a bool, and then applies 'not', so >> 'x is not >> None' means 'x is True'. > > Absolutely incorrect. Read the final paragraph of > http://docs.python.org/reference/expressions.html#notin Oops, sorry :$.
From: Dave Angel on 5 Aug 2010 13:03
Roald de Vries wrote: > <div class="moz-text-flowed" style="font-family: -moz-fixed">On Aug 5, > 2010, at 5:42 PM, wheres pythonmonks wrote: >> How does "x is not None" make any sense? "not x is None" does make >> sense. >> >> I can only surmise that in this context (preceding is) "not" is not a >> unary right-associative operator, therefore: >> >> x is not None === IS_NOTEQ(X, None) >> >> Beside "not in" which seems to work similarly, is there other >> syntactical sugar like this that I should be aware of? > > 'not None' first casts None to a bool, and then applies 'not', so 'x > is not None' means 'x is True'. > 'not x is None' is the same as 'not (x is None)' > > Cheers, Roald > Did you try it? Python 2.6.4 (r264:75706, Jan 22 2010, 16:41:54) [MSC v.1500 32 bit win32 Type "help", "copyright", "credits" or "license" for more informatio >>> 4 is not None True >>> True is not None True >>> False is not None True >>> None is not None False >>> Looks to me like x is not None is equivalent to not (x is None) (I get same results for 3.1) DaveA |