homepage

When using a format specifier with leading zero, the format spec mini-language (as documented at https://docs.python.org/3/library/string.html#format-specification-mini-language>) says:
> '=' [...] becomes the default when ‘0’ immediately precedes the field width.
When the ‘=’ option is only implied, the error message "ValueError: '=' alignment not allowed in string format specifier" becomes surprising and incomprehensible to someone who does not know that implied behaviour.
In issue 15560, Terry Reedy says:
> If the spec string is still available, it could be searched and the message adjusted if '=' is not present. That proposal should be a new issue if someone wants to push it.
This issue raises that proposal.
The error message should be changed so that:
* It makes sense whether or not the ‘=’ option is explicit in the format specifier.
Or:
* Different messages are produced when the ‘=’ option is explicit versus when it is implicit.
I think the former option is better, but either will satisfy this request.