surprise - byte in set

Sat Jan 3 15:05:49 EST 2015

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
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-- 
Dr. Gary Herron
Department of Computer Science
DigiPen Institute of Technology
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