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surprise - byte in set

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  surprise - byte in set patrick vrijlandt <pvrijlandt@gmail.com> - 2015-01-03 19:50 +0100
    Re: surprise - byte in set Jason Friedman <jsf80238@gmail.com> - 2015-01-03 12:12 -0700
    Re: surprise - byte in set Dan Stromberg <drsalists@gmail.com> - 2015-01-03 11:15 -0800
    Re: surprise - byte in set Gary Herron <gherron@digipen.edu> - 2015-01-03 12:05 -0800
    Re: surprise - byte in set patrick vrijlandt <pvrijlandt@gmail.com> - 2015-01-03 23:08 +0100
    Re: surprise - byte in set Amir Arsalan <am1r.ar3alan@gmail.com> - 2015-01-03 22:38 +0330

#83173 — surprise - byte in set

Hello list,

Let me first wish you all the best in 2015!

Today I was trying to test for occurrence of a byte in a set ...

 >>> sys.version
'3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit 
(Intel)]'
 >>> 'b' in 'abc'
True
 >>> b'b' in b'abc'
True
 >>> 'b' in set('abc')
True
 >>> b'b' in set(b'abc')
False

I was surprised by the last result. What happened?
(Examples simplified; I was planning to manipulate the set)

Patrick

---
Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware.
http://www.avast.com

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#83174

>>>> sys.version
> '3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit
> (Intel)]'
>>>> 'b' in 'abc'
> True
>>>> b'b' in b'abc'
> True
>>>> 'b' in set('abc')
> True
>>>> b'b' in set(b'abc')
> False
>
> I was surprised by the last result. What happened?
> (Examples simplified; I was planning to manipulate the set)

I'm no expert, but I see:

>>> for i in set(b'abc'):
...     print(type(i))
...
<class 'int'>
<class 'int'>
<class 'int'>

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#83175

On Sat, Jan 3, 2015 at 10:50 AM, patrick vrijlandt <pvrijlandt@gmail.com> wrote:
> Hello list,
>
> Let me first wish you all the best in 2015!
>
> Today I was trying to test for occurrence of a byte in a set ...


In the last case, the set has integers in it.

Try:
b'b'[0] in set(b'abc')

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#83177

On 01/03/2015 10:50 AM, patrick vrijlandt wrote:
> Hello list,
>
> Let me first wish you all the best in 2015!
>
> Today I was trying to test for occurrence of a byte in a set ...
>
> >>> sys.version
> '3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit 
> (Intel)]'
> >>> 'b' in 'abc'
> True
> >>> b'b' in b'abc'
> True
> >>> 'b' in set('abc')
> True
> >>> b'b' in set(b'abc')
> False
>
> I was surprised by the last result. What happened?
> (Examples simplified; I was planning to manipulate the set)

The surprise is really that the 3rd test is True not that the fourth is 
False.

First, as should be expected, a byte string is a sequence of (small) 
ints.  So b'b' is a (short) byte string and the set set(b'abc') is 
composed of three ints.  You should not expect your inclusion test to 
return True when testing for a bytes-type object in a set of int-type 
objects.  And that explains your False result in the 4th test.

 >>> type(b'abc')
<class 'bytes'>
 >>> type(b'abc'[0])
<class 'int'>


But things are different for strings.  You might think a string is a 
sequence of characters, but Python does not have a character type. In 
fact the elements of a string are just 1 char long strings:

 >>> type('abc')
<class 'str'>
 >>> type('abc'[0])
<class 'str'>

You would not logically expect to find a string 'b' in a set of 
characters in, say C++,  where the two types are different.  But that's 
not the Python way.  In Python a set of characters set('abc') is really 
a set of (short) strings, and the character 'b' is really a (short) 
string, so the inclusion test works.

Python's way of returning a 1-byte string when indexing a string 
(instead of returning an element of type character) allows this 
surprising result.

 >>> 'abc'[0]
'a'
 >>> 'abc'[0][0]
'a'
 >>> 'abc'[0][0][0]
'a'
 >>> 'abc'[0][0][0][0]
'a'
...


I've never considered this a problem, but a infinitely indexable object 
*is* a bit of an oddity.




>
> Patrick
>
> ---
> Dit e-mailbericht is gecontroleerd op virussen met Avast 
> antivirussoftware.
> http://www.avast.com
>


-- 
Dr. Gary Herron
Department of Computer Science
DigiPen Institute of Technology
(425) 895-4418

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#83182

Dear all,

Many thanks for your responses. I never realised this difference between 
'bytes' and 'string'.

Thanks,

Patrick

---
Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware.
http://www.avast.com

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#83233

[Multipart message — attachments visible in raw view] — view raw

Hi my friends.

On last result you have  a , b , c  binary.

>> set('abc')
{'a','b','c')
>> set(b'abc')
{97,98,99}
On Jan 3, 2015 10:25 PM, "patrick vrijlandt" <pvrijlandt@gmail.com> wrote:

> Hello list,
>
> Let me first wish you all the best in 2015!
>
> Today I was trying to test for occurrence of a byte in a set ...
>
> >>> sys.version
> '3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit
> (Intel)]'
> >>> 'b' in 'abc'
> True
> >>> b'b' in b'abc'
> True
> >>> 'b' in set('abc')
> True
> >>> b'b' in set(b'abc')
> False
>
> I was surprised by the last result. What happened?
> (Examples simplified; I was planning to manipulate the set)
>
> Patrick
>
> ---
> Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware.
> http://www.avast.com
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>

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