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How to pack a string variable of length 1 as a char using struct.pack?

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  How to pack a string variable of length 1 as a char using struct.pack? danwgrace@gmail.com - 2014-08-05 05:15 -0700
    Re: How to pack a string variable of length 1 as a char using struct.pack? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-05 22:28 +1000
    Re: How to pack a string variable of length 1 as a char using struct.pack? Thomas Orozco <thomas@orozco.fr> - 2014-08-05 14:30 +0200

#75730 — How to pack a string variable of length 1 as a char using struct.pack?

Hi,
How to pack a string variable of length 1 as a char using struct.pack?
The following works fine:
p = struct.pack('c', b'1')

Whereas this causes an error "char format requires a bytes object of length 1":
s = '1'
p = struct.pack('c', s)

I need to pack a variable rather than a literal.

Thanks.

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#75731

danwgrace@gmail.com wrote:

> Hi,
> How to pack a string variable of length 1 as a char using struct.pack?
> The following works fine:
> p = struct.pack('c', b'1')

Here you use a byte string of length 1, b'1'.


> Whereas this causes an error "char format requires a bytes object of
> length 1": 
> s = '1' 
> p = struct.pack('c', s)

Here you use a Unicode string of length 1, '1'.

Do this instead:

s = b'1'
p = struct.pack('c', s)


-- 
Steven

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#75733

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On Tue, Aug 5, 2014 at 2:15 PM, <danwgrace@gmail.com> wrote:

> Hi,
> How to pack a string variable of length 1 as a char using struct.pack?
> The following works fine:
> p = struct.pack('c', b'1')
>
> Whereas this causes an error "char format requires a bytes object of
> length 1":
> s = '1'
> p = struct.pack('c', s)
>
> I need to pack a variable rather than a literal.
>

I assume you are using Python 3. In Python 3, s = '1' is a *unicode
string*, not a *bytes object*.

You need to convert your string to a bytes object by encoding it.

However, be mindful that some characters may actually require multiple
bytes to be encoded:

    struct.pack('c', s.encode('ascii'))

(You can of course use e.g. 'utf-8' as the encoding here)

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