Error in str.format for non number formatting

[TG-Techie]

CircuitPython version

Adafruit CircuitPython 7.2.0-alpha.1-58-g211dc53d4-dirty on 2022-01-10; TG-Watch with nRF52840

Code/REPL

'{:09}'.format('Jan')

Behavior

The behavior circuitpython is:

>>> '{:09}'.format('Jan')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier

Description

the behavior on python 3.10 is:

>>> '{:09}'.format('Jan')
'Jan000000'

It took a good bit to figure out what the error was, I’m not sure if the behavior with this error is intentional.
The error message was decently misleading while debugging my code. (because I didn’t have any =s in the text I was using)

Additional information

Taking a quick look at the source, it looks like that error message is commented as for not numbers but is also the default case... which doesn't necessarily have an = in it

circuitpython/py/objstr.c

Lines 1378 to 1389 in 476b17b

	} else {
	// arg doesn't look like a number
	if (align == '=') {
	#if MICROPY_ERROR_REPORTING <= MICROPY_ERROR_REPORTING_TERSE
	terse_str_format_value_error();
	#else
	mp_raise_ValueError(
	MP_ERROR_TEXT("'=' alignment not allowed in string format specifier"));
	#endif
	}

I'm more than glad to PR a different error message! I’m not sure if this is a large enough edge case to merit a PR?

Or PR a check for an equal and display this message and display a more generic "we don’t know what to do with the given formatter" otherwise.

[Neradoc]

That looks like a new thing in 3.10:

Python 3.9.6 (default, Aug 11 2021, 01:29:00) 
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> '{:09}'.format('Jan')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier
>>> 

[jepler]

Since this is about the core, I've prepared a fix and offered it to micropython. We can make a decision whether to take it ourselves, independently of whether they accept the change: micropython#8156

[jepler]

Python folks considered it a bug fix, as far as I can tell:

• bpo-27772: Make preciding width with 0 valid in string format. python/cpython#11270

[dhalbert]

This is now merged in and is fixed in 9.0.0, at least:

>>> '{:09}'.format('Jan')
'Jan000000'