Re: True of False

[Bruno Desthuilliers]

[EMAIL PROTECTED] a écrit :
> 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?

See other answers for the details. Anyway, since '"c" in a' is already a 
boolean expression, testing the result of the evaluation of this 
expression against a boolean is a pure waste of time. The usual idiom - 
and this is definitively not Python-specific - is:

if "c" in a:
   print "Yes"

Also, in Python (and in some other languages too), there's a notion of 
"something" vs "nothing" - where, in a boolean context, "something" 
evals to true and "nothing" to false. wrt/ Python, the empty string, an 
empty container (list, tuple, dict, set etc), numerical zero's (int or 
float), and of course the None object all eval to false, and most other 
objects eval to true.
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Re: True of False

[Marc Christiansen]

Casey <[EMAIL PROTECTED]> wrote:
> 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.

That was my first thought, too. But watch:
  >>> a = list("abcde")
  >>> a
  ['a', 'b', 'c', 'd', 'e']
  >>> "c" in (a == True)
  Traceback (most recent call last):
File "", line 1, in 
  TypeError: argument of type 'bool' is not iterable

Then it dawned on me (is this the right phrase?): It's the same situation
as e.g. x < y >= 1, which means the same as "x < y and y >= 1" (except
that y is only evaluated once). So '"c" in a == True' gets evaluated as
'"c" in a and a == True'.

> the "==" operator binds at a higher precedence level than the "in"
> operator, just as multiplication binds higher than addition

Operator precedence plays no role in this case. It is a case of
'chained' comparisons.

Hope that clears it up
Marc
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Re: True of False

[Marc 'BlackJack' Rintsch]

On Thu, 27 Sep 2007 17:06:30 +, Duncan Booth wrote:

> Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote:
> 
>> In [268]: 'c' in a == True
>> Out[268]: False
>> 
>> In [269]: ('c' in a) == True
>> Out[269]: True
>> 
>> In [270]: 'c' in (a == True)
>> ---
>>  
>> Traceback (most recent call
>>last) 
>> 
>> /home/bj/ in ()
>> 
>>: argument of type 'bool' is not iterable
>> 
>> 
>> What's going on there?
> 
> See http://docs.python.org/ref/comparisons.html
> 
>> Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent
>> to x < y and y <= z, except that y is evaluated only once (but in both
>> cases z is not evaluated at all when x < y is found to be false). 
> 
> In exactly the same way:
> 
>'c' in a == True
> 
> is equivalent to:
> 
>'c' in a and a == True
> 
> which is False.

Aaah *enlightenment*, I'm using this for range checks like in the docs,
but it wasn't obvious to me in this case.  Thanks.

Ciao,
Marc 'BlackJack' Rintsch
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Re: True of False

[Erik Jones]

On Sep 27, 2007, at 12:29 PM, Erik Jones wrote:

>
> On Sep 27, 2007, at 11:47 AM, Marc 'BlackJack' Rintsch wrote:
>
>> On Thu, 27 Sep 2007 09:33:34 -0700, koutoo 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?
>>
>> Wow that's odd:
>>
>> In [265]: a = list('abcdef')
>>
>> In [266]: a
>> Out[266]: ['a', 'b', 'c', 'd', 'e', 'f']
>>
>> In [267]: 'c' in a
>> Out[267]: True
>>
>> In [268]: 'c' in a == True
>> Out[268]: False
>>
>> In [269]: ('c' in a) == True
>> Out[269]: True
>>
>> In [270]: 'c' in (a == True)
>> - 
>> -
>> -
>>  Traceback (most recent
>> call last)
>>
>> /home/bj/ in ()
>>
>> : argument of type 'bool' is not  
>> iterable
>>
>>
>> What's going on there?
>
> That is weird.  Given 270, what's happening in 268.
>
> Erik Jones

Cool, Richard Thomas answered this one for me.

Erik Jones

Software Developer | Emma®
[EMAIL PROTECTED]
800.595.4401 or 615.292.5888
615.292.0777 (fax)

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Re: True of False

[Gary Herron]

Richard Thomas wrote:
> On 27/09/2007, Casey <[EMAIL PROTECTED]> wrote:
>   
>> On Sep 27, 12:48 pm, "Simon Brunning" <[EMAIL PROTECTED]>
>> wrote:
>> 
>>> On 9/27/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> 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 .
>>>   
>> 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 -- Richard.
>
>   
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>> 

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Re: True of False

[Erik Jones]

On Sep 27, 2007, at 11:47 AM, Marc 'BlackJack' Rintsch wrote:

> On Thu, 27 Sep 2007 09:33:34 -0700, koutoo 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?
>
> Wow that's odd:
>
> In [265]: a = list('abcdef')
>
> In [266]: a
> Out[266]: ['a', 'b', 'c', 'd', 'e', 'f']
>
> In [267]: 'c' in a
> Out[267]: True
>
> In [268]: 'c' in a == True
> Out[268]: False
>
> In [269]: ('c' in a) == True
> Out[269]: True
>
> In [270]: 'c' in (a == True)
> -- 
> -
>  Traceback (most recent  
> call last)
>
> /home/bj/ in ()
>
> : argument of type 'bool' is not iterable
>
>
> What's going on there?

That is weird.  Given 270, what's happening in 268.

Erik Jones

Software Developer | Emma®
[EMAIL PROTECTED]
800.595.4401 or 615.292.5888
615.292.0777 (fax)

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Visit us online at http://www.myemma.com


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Re: True of False

[Casey]

On Sep 27, 1:12 pm, "Richard Thomas" <[EMAIL PROTECTED]> wrote:
> On 27/09/2007, Casey <[EMAIL PROTECTED]> wrote:
>
>
>
> > On Sep 27, 12:48 pm, "Simon Brunning" <[EMAIL PROTECTED]>
> > wrote:
> > > On 9/27/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> 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 .
>
> > 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.
>
> -- Richard.
>
> > --
> >http://mail.python.org/mailman/listinfo/python-list

Doh, I forgot about operator chaining here.  I'm so used to just
seeing a < b < c that I forget about arbitrary operator chaining and
think like a C++ programmer!

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Re: True of False

[Carsten Haese]

On Thu, 2007-09-27 at 16:47 +, Marc 'BlackJack' Rintsch wrote:
> On Thu, 27 Sep 2007 09:33:34 -0700, koutoo 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?
> 
> Wow that's odd:
> 
> In [265]: a = list('abcdef')
> 
> In [266]: a
> Out[266]: ['a', 'b', 'c', 'd', 'e', 'f']
> 
> In [267]: 'c' in a
> Out[267]: True
> 
> In [268]: 'c' in a == True
> Out[268]: False
> 
> In [269]: ('c' in a) == True
> Out[269]: True
> 
> In [270]: 'c' in (a == True)
> ---
>  Traceback (most recent call last)
> 
> /home/bj/ in ()
> 
> : argument of type 'bool' is not iterable
> 
> 
> What's going on there?

What's going on here is that both 'in' and '==' are comparison
operations, and Python allows you to chain comparisons. Just like "a < x
< b" is evaluated as "a < x and x < b", "'c' in a == True" is evaluated
as "'c' in a and a == True". Obviously, since a==True is false, the
chained comparison is False.

-- 
Carsten Haese
http://informixdb.sourceforge.net


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Re: True of False

[Richard Thomas]

On 27/09/2007, Casey <[EMAIL PROTECTED]> wrote:
> On Sep 27, 12:48 pm, "Simon Brunning" <[EMAIL PROTECTED]>
> wrote:
> > On 9/27/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> 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 .
>
> 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.

-- Richard.

> --
> http://mail.python.org/mailman/listinfo/python-list
>
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Re: True of False

[Steve Holden]

[EMAIL PROTECTED] 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?
> Thanks.

You are unnecessarily adding a comparison with True. The correct way to 
write that is

if "c" in a:
   print "yes"

Bu of course you haven't actually told us what you really did, because 
the code you represent has syntax errors.

 >>> a = ["a", "b", "c", "d", "e", "f"]
 >>> "c" in a
True
 >>> if "c" in a == True:
...print "found it"
...
 >>> if ("c" in a) == True:
... print "At last!"
...
At last!
 >>>
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Re: True of False

[Duncan Booth]

Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote:

> In [268]: 'c' in a == True
> Out[268]: False
> 
> In [269]: ('c' in a) == True
> Out[269]: True
> 
> In [270]: 'c' in (a == True)
> ---
>  
> Traceback (most recent call
>last) 
> 
> /home/bj/ in ()
> 
>: argument of type 'bool' is not iterable
> 
> 
> What's going on there?

See http://docs.python.org/ref/comparisons.html

> Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent
> to x < y and y <= z, except that y is evaluated only once (but in both
> cases z is not evaluated at all when x < y is found to be false). 

In exactly the same way:

   'c' in a == True

is equivalent to:

   'c' in a and a == True

which is False.


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Re: True of False

[Casey]

On Sep 27, 12:48 pm, "Simon Brunning" <[EMAIL PROTECTED]>
wrote:
> On 9/27/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> 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 .

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


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Re: True of False

[Shriphani]

[EMAIL PROTECTED] 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?
> Thanks.
>
> Kou

Hello,
Just try :

a = ["a","b","c","d","e","f"]
if "c" in a:
print "yes"

That is going to work as the statement '"c" in a' itself is true. You
could try that by typing "c" in a at the interpreter.

regards,
Shriphani Palakodety

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Re: True of False

[Marc 'BlackJack' Rintsch]

On Thu, 27 Sep 2007 09:33:34 -0700, koutoo 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?

Wow that's odd:

In [265]: a = list('abcdef')

In [266]: a
Out[266]: ['a', 'b', 'c', 'd', 'e', 'f']

In [267]: 'c' in a
Out[267]: True

In [268]: 'c' in a == True
Out[268]: False

In [269]: ('c' in a) == True
Out[269]: True

In [270]: 'c' in (a == True)
---
 Traceback (most recent call last)

/home/bj/ in ()

: argument of type 'bool' is not iterable


What's going on there?

Ciao,
Marc 'BlackJack' Rintsch
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Re: True of False

[skulka3]

On Sep 27, 11:33 am, [EMAIL PROTECTED] 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?
> Thanks.
>
> Kou

,
You may want to include paren around ("c" in a) and a lower case p for
Print, i.e. print, and  it should work

so eg:
a = ["a", "b", "c", "d", "e", "f"]

 if ("c" in a) == True:
  print "Yes"

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Re: True of False

[Simon Brunning]

On 9/27/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> 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 .

-- 
Cheers,
Simon B.
[EMAIL PROTECTED]
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True of False

[koutoo]

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?
Thanks.

Kou

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