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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: How to get an integer from a sequence of bytes
Date: Sun, 02 Jun 2013 21:48:36 -0400
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Xref: csiph.com comp.lang.python:46746

On Sun, 02 Jun 2013 21:25:45 +0200, Mok-Kong Shen
<mok-kong.shen@t-online.de> declaimed the following in
gmane.comp.python.general:


> b'7' is the byte with the character 7 in a certain code, so that's
> ok. In other PLs one assigns an int to a byte, with that int in either

	In other languages "byte" is an 8-bit signed/unsigned numeric.

	But what you have is a Python 3.x "bytes" structure -- similar to a
character string in Python 2.x...

> decimal notation or hexadecimal notation, or else one assigns a
> character to it, in which case it gets the value of the character
> in a certain code. What I don't yet understand is why Python is
> apprently different from other PLs in that point in not allowing direct
> coersion of a byte to an int.
>

	As you've been shown, the first step is that you may have to
subscript it; even with just one byte, the structure is still a
"string/array". NOTE: that example doesn't work in 2.7, since
subscripting what is a "string" still returns a substring (of one
character).

	Python doesn't have a "numeric" byte type -- the b"..." is an
"array" of 8-bit values in Python 3.x, and is just a character string in
2.x

	A language like C didn't have a "string" type... "char" was a
pseudonym for "numeric byte" (and some even support "unsigned char" vs
"signed char").

	Maybe you'd like to program in Ada... Where "7" is a "string of
length 1" and '7' is a character -- and you have to do type conversions
to assign the latter to the former.

	Heck:

with Text_IO; use Text_IO;

procedure Bytes is
    
begin
    if "7" = '7' then
	Put_Line ("string 7 is equal to character 7");
    else
	Put_Line ("string 7 is NOT equal to character 7");
    end if;
end Bytes;

WON'T compile... string can not be compared to character!

with Text_IO; use Text_IO;

procedure Bytes is
    
    A_String : String (1 .. 1);
    A_Char : Character := '7';
    
    
begin
    A_String := A_Char;
    
end Bytes;

	The above fails to compile, whereas the following is valid Ada

with Text_IO; use Text_IO;

procedure Bytes is
    
    A_String : String (1 .. 1);
    A_Char : Character := '7';
    
    
begin
    A_String(1) := A_Char;
    
end Bytes;

	Don't even ask about /numeric/ bytes and strings (or characters). Or
lets...

with Text_IO; use Text_IO;

procedure Bytes is
    
    type Byte is mod 256;
    
    A_String : String (1 .. 1);
    Char : Byte := 7;

    
begin
    A_String (1) := Char;
    
end Bytes;

Fails... But...

with Text_IO; use Text_IO;

procedure Bytes is
    
    type Byte is mod 256;
    
    A_String : String (1 .. 1);
    Char : Byte := 7;
    
begin
    A_String (1) := Character'Val (Char);
    
end Bytes;

	That takes a byte data type (unsigned 8-bit value)... Asks for the
CHARACTER data type having the value equivalent to the "position" of the
byte... And then stuff that into the only element of a STRING data type.

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/