Python uses {variablename:formatting} to format strings:
https://docs.python.org/3/library/string.html#formatspec

The formatting is inspired by printf, but not 100% printf syntax.

Use python's : to separate the formatting:

  {1:%010.2f} => $_=$ARG[1]; printf("%010.2f",$_)
  {:% 10s}    => printf("% 10s",$_)
  {=2 $_/=2; :% 10s=}  => $_=$ARG[2]; $_/=2; printf("% 10s",$_)
  {namedvar:%8.2f} => {4:%8.2f} => $_=$ARG[4]; printf("%8.2f",$_)

Potential problem:

  {=s:a:b:g=} :g is not a formatting instruction.

Solved by forcing :% to be the start for printf formatting.

So this will work:

   {:%d pct} => printf("%d pct",$_)

But this will not:

   {:pct=%d} => printf("pct=%d",$_)

because :pct does not start with :% and thus cannot reliably be detected from perl's s::: function.

--plus defines {:num} inspired from bash. num can be an negative and positive integer. But it can never start with :%, so it will be unabiguous whether you use printf syntax or bash.

We cannot use % by itself, because --plus defines {%regexp}.

Benefit: If you need values to be formatted as %04d, it is now easy:

    parallel echo {:%04d} ::: 12.34 123.4 1234

It can be used on all replacement strings.