How to download source package using only console?

["Alexander V. Makartsev" <avbetev@gmail.com>]

    Hello, fellow Debian users.
    
    When I need to build a backport of a package, I sometimes find it
    difficult to obtain actual source package(-s) from Debian repos
    using console.
    Following advice from a wiki page [1], after "apt update", doesn't
    do it:
> $ apt source
> lego/testing                                                                                                                    
>          
>       Reading package lists...
> Done                                                                                                                                       
>          
>       Picking 'golang-github-xenolf-lego' as
>         source package instead of
> 'lego'                                                                                             
>          
>       E: Can not find version '3.2.0-3.1' of
>         package
> 'lego'                                                                                                               
>          
>       E: Unable to find a source package for
>         golang-github-xenolf-lego
>     
    Any other seemingly intended ways also fail in a similar fashion:
> $ dget
> lego=4.9.1-1                                                                                                                          
>          
>         dget: no hostnames in apt-cache policy lego for version 4.9.1-1
>         found
>       
> $ apt-get source lego -t
> testing                                                                                                             
>          
>         Reading package lists...
> Done                                                                                                                                       
>          
>         E: The value 'testing' is invalid for APT::Default-Release as
>         such a release is not available in the
>         sources                                                        
>          
>         E: Unable to find a source package for
>     
    Trying to do the same for another package seems to work:
> $ apt source ipcalc/testing
>       Reading package lists... Done
>       Selected version '0.42-2' (testing) for
>         ipcalc
>       Need to get 33,7 kB of source archives.
>       Get:1 https://mirror.yandex.ru/debian
>         testing/main ipcalc 0.42-2 (dsc) [1 692 B]
>       Get:2 https://mirror.yandex.ru/debian
>         testing/main ipcalc 0.42-2 (tar) [25,9 kB]
>       Get:3 https://mirror.yandex.ru/debian
>         testing/main ipcalc 0.42-2 (diff) [6 144 B]
>       Fetched 33,7 kB in 1s (52,5 kB/s)     
>       dpkg-source: info: extracting ipcalc in
>         ipcalc-0.42
>       dpkg-source: info: unpacking
>         ipcalc_0.42.orig.tar.gz
>       dpkg-source: info: unpacking
>         ipcalc_0.42-2.debian.tar.xz
>       dpkg-source: info: using patch list from
>         debian/patches/series
>       dpkg-source: info: applying 01-paths.patch
>     
    
    So why those fail for a "lego" package and is there a way to solve
    this once and for all?
    I know I can go to a packages website [2] and manually download
    ".dsc" file and feed it to "dget" utility, or download source files
    directly from said website, but there has to be a better way.
    
    Some useful info:
> $ cat /etc/apt/sources.list |
>         grep -iE "testing"
>       #deb https://mirror.yandex.ru/debian/
>         testing main contrib non-free 
>       deb-src https://mirror.yandex.ru/debian/
>         testing main contrib non-free
>     
> $ rmadison lego
>       lego       | 0.3.1-5+b13   | oldstable  |
>         amd64, arm64, armel, armhf, i386, mips, mips64el, mipsel,
>         ppc64el, s390x
>       lego       | 3.2.0-3.1+b5  | stable     |
>         amd64, arm64, armel, armhf, i386, mips64el, mipsel, ppc64el,
>         s390x
>       lego       | 4.9.1-1       | testing    |
>         amd64, arm64, armel, armhf, i386, mips64el, mipsel, ppc64el,
>         s390x
>       lego       | 4.9.1-1       | unstable   |
>         amd64, arm64, armel, armhf, i386, mips64el, mipsel, ppc64el,
>         s390x
>     
> $ uname -a
>       Linux host0 5.10.0-22-amd64 #1 SMP Debian
>         5.10.178-3 (2023-04-22) x86_64 GNU/Linux
>     
    
    
    [1] https://wiki.debian.org/SimpleBackportCreation
    [2] https://packages.debian.org/bookworm/lego

--
With kindest regards, Alexander.

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