True of False

Gary Herron gherron at islandtraining.com
Thu Sep 27 19:38:39 CEST 2007


Richard Thomas wrote:
> On 27/09/2007, Casey <Caseyweb at gmail.com> wrote:
>   
>> On Sep 27, 12:48 pm, "Simon Brunning" <si... at brunningonline.net>
>> wrote:
>>     
>>> On 9/27/07, kou... at hotmail.com <kou... at hotmail.com> wrote:
>>>
>>>       
>>>> I tried writing a true and false If statement and didn't get
>>>> anything?  I read some previous posts, but I must be missing
>>>> something.  I just tried something easy:
>>>>         
>>>> a = ["a", "b", "c", "d", "e", "f"]
>>>>         
>>>> if "c" in a == True:
>>>>      Print "Yes"
>>>>         
>>>> When I run this, it runs, but nothing prints.  What am I doing wrong?
>>>>         
>>> Just use
>>>
>>> if "c" in a:
>>>
>>> and all will be well. The True object isn't the only truthy value in
>>> Python - see <http://docs.python.org/lib/truth.html>.
>>>       
>> I would recommend the OP try this:
>>
>> run the (I)python shell and try the following:
>>
>>     
>>>>> a = [x for x in "abcdefg"]
>>>>> a
>>>>>           
>> ['a','b','c','d','e','f','g']
>>     
>>>>> "c" in a
>>>>>           
>> True
>>     
>>>>> "c" in a == True
>>>>>           
>> False
>>     
>>>>> ("c" in a) == True
>>>>>           
>> True
>>
>> The reason your conditional failed is that it was interpreted as "c"
>> in (a == True) which is False.
>> the "==" operator binds at a higher precedence level than the "in"
>> operator, just as multiplication
>> binds higher than addition
>>
>>     
>
> Actually it evaluates '("c" in a) and (a == True)'. You can check like so:
>
> import dis
> a = list("abcdef")
> dis.dis(lambda: "c" in a == True)
>
> And just follow the bytecode operations.
>   
Yikes.  So I did that and you're correct.   I've always looked at
chained comparisons with mild suspicion.  Now I guess that suspicion is
justified.  Interpreting
    a<b<c
as
    a<b and b<c
make perfect sense to me, but interpreting
    "c" in a == True
as
    ("c" in a) and (a == True)
is not at all natural.

Gary Herron

> -- Richard.
>
>   
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>>     




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