[Eric]

> Is that the "0" here: [...]

Yes, sorry; that could have been clearer.

> In particular, what if you specify a different alignment type with the pre-width 0?

Right, that's the critical question here. For floats and ints, an explicitly-specified alignment type overrides the implicit "=". But Decimal raises. The Decimal behaviour seems more reasonable, but the float and int behaviours are more historically baked-in, and riskier to change.

And then there's a parallel question with the fill character: should an explicitly-given fill override the "0"-fill character that's implicit in that "[0]"? int and float say "yes". Decimal says "ValueError".