Re: [Qemu-devel] [PATCH] configure: automatically pick python3 is available

[Eric Blake <eblake@redhat.com>]

On 3/1/19 10:33 AM, Daniel P. Berrangé wrote:
> Unless overridden via an env var or configure arg, QEMU will only look
> for the 'python' binary in $PATH. This is unhelpful on distros which
> are only shipping Python 3.x (eg Fedora) in their default install as,
> if they comply with PEP 394, the bare 'python' binary won't exist.
> 
> This changes configure so that by default it will search for all three
> common python binaries, preferring to find Python 3.x versions.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  configure | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/configure b/configure
> index 540bee19ba..ef2e5bf9f5 100755
> --- a/configure
> +++ b/configure
> @@ -902,7 +902,10 @@ fi
>  
>  : ${make=${MAKE-make}}
>  : ${install=${INSTALL-install}}
> -: ${python=${PYTHON-python}}
> +# We prefer python 3.x. A bare 'python' is traditionally
> +# python 2.x, but some distros have it as python 3.x, so
> +# we check that before python2
> +: ${python=${PYTHON-python3 python python2}}

If I run with PYTHON='/path with spaces/to/mypython',

>  : ${smbd=${SMBD-/usr/sbin/smbd}}
>  
>  # Default objcc to clang if available, otherwise use CC
> @@ -1797,8 +1800,20 @@ EOF
>  exit 0
>  fi
>  
> -if ! has $python; then
> -  error_exit "Python not found. Use --python=/path/to/python"
> +try_python="$python"
> +python=
> +for binary in $try_python

then this ignores my request (trying '/path', 'with', and
'spaces/to/mypython', none of which works).

I think a better approach would be:

python=
for binary in "${PYTHON-python3}" python python2

> +do
> +    if has $binary

if has "$binary"

> +    then
> +       python=$binary
> +       break
> +    fi
> +done
> +
> +if test -z "$python"
> +then
> +    error_exit "Python not found. Use --python=/path/to/python"
>  fi
>  
>  # Note that if the Python conditional here evaluates True we will exit
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org