[New-bugs-announce] [issue27772] Refer to actual format string when creating “zero padding” error message

[Ben Finney]

New submission from Ben Finney:

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.

----------
messages: 272827
nosy: bignose
priority: normal
severity: normal
status: open
title: Refer to actual format string when creating “zero padding” error message
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27772>
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