keyword 'in' not returning a bool?

[c james]

Try this

>>sample = {'t':True, 'f':False}
>>'t' in sample

True

>>type('t' in sample)

<type 'bool'>

>>'t' in sample == True

False

Why is this? Now try

>>bool('t' in sample) == True

True

Can someone explain what is going on?

[Larry Bates]

Re: keyword 'in' not returning a bool?

c james wrote:

Try this
>

>>>sample = {'t':True, 'f':False}
>>>'t' in sample

True

>>>type('t' in sample)

<type 'bool'>

>>>'t' in sample == True

False
>
Why is this? Now try

>>>bool('t' in sample) == True

True
>
Can someone explain what is going on?
>

Precedence. In:

't' in sample == True

sample == True is evaluated first
then 't' in that which is false

you should write

('t' in sample) == True

but that is unnecessary because

t' in sample

is easier to read and to code and less prone to this problem.

-Larry

[Richard Brodie]

Re: keyword 'in' not returning a bool?


"c james" <cjames@callone .netwrote in message
news:mailman.52 1.1202490602.92 67.python-list@python.org ...

>>>'t' in sample == True

False

It's comparison operator chaining:

't' in sample == True is like 't' == sample == True

and is equivalent to 't' in sample and sample == True

[Arnaud Delobelle]

Re: keyword 'in' not returning a bool?

On Feb 8, 5:09 pm, c james <cja...@callone .netwrote:

Try this
>

>sample = {'t':True, 'f':False}
>'t' in sample

True

>type('t' in sample)

<type 'bool'>

>'t' in sample == True

>
False
>
Why is this?  Now try>>bool('t' in sample) == True
>
True
>
Can someone explain what is going on?

This is because in Python '==' and 'in' are comparison operators and
in Python you can chain comparisons.

Typical usage is:

if 3 <= x < 9: # meaning (3 <= x) and (x < 9)
# do something...

So 't' in sample == True means the same as

('t' in sample) and (sample == True)

Which will never be true as sample is a dictionary.

To solve this, use brackets:

>>('t' in sample) == True

True

HTH

--
Arnaud

[Sion Arrowsmith]

Re: keyword 'in' not returning a bool?

In article <mailman.521.12 02490602.9267.p ython-list@python.org >,
c james <cjames@callone .netwrote:

>>>sample = {'t':True, 'f':False}
>>>'t' in sample == True

>False
>
>Why is this?

http://docs.python.org/lib/comparisons.html

"Comparison s can be chained arbitrarily; for example, x < y <= z is
equivalent to x < y and y <= z, except that y is evaluated only once
[ ... ]
Two more operations with the same syntactic priority, "in" and "not
in", are supported only by sequence types"

So:
't' in sample == True
is equivalent to:
't' in sample and sample == True
and obviously:

>>sample == True

False

--
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