format(Fraction(1, 3), '.016f') raises ValueError

[skirpichev]

Bug report

Bug description:

c.f.

>>> format(float(Fraction(1, 3)), '.016f')
'0.3333333333333333'

Looking on docs, I think that float formatting better conforms to the specification.

Similar issue is valid for the width:

>>> format(float(Fraction(1, 3)), '0030.016f')
'0000000000000.3333333333333333'
>>> format(Fraction(1, 3), '0030.016f')
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    format(Fraction(1, 3), '0030.016f')
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/fractions.py", line 577, in __format__
    raise ValueError(
    ...<2 lines>...
    )
ValueError: Invalid format specifier '0030.016f' for object of type 'Fraction'

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-130662: accept leading zeros in precision/width for Fraction's formatting #130663
• gh-130662: refine width/precision specification in format-spec #130717
• gh-130662: accept leading zeros in precision/width for Decimal's formatting #132549

[skirpichev]

CC @mdickinson

BTW, I like more strict formatting rules for Fraction's. Maybe we should keep them, then this should be documented as now docs says: "If the format_spec format specification string ends with one of the presentation types 'e', 'E', 'f', 'F', 'g', 'G' or '%' then formatting follows the rules outlined for the float type in the Format Specification Mini-Language section." Adding more strict processing of width and precision for floats - probably will be too severe compatibility break.

PR is ready for review: #130663

[gvanrossum]

I'm not crazy about the proposed fix. It's harmless, but it perpetuates the questionable support for meaningless extra leading zeros.

I agree we can't tighten float's formatting language, it's not worth breaking working code over.

As far as the docs, maybe we can change the float docs to disallow the redundant leading zeros and explain in a note that CPython does support those for backwards compatibility reasons, but only for float, not for Fraction? Or otherwise mention the difference in the Fraction docs.

[skirpichev]

I agree we can't tighten float's formatting language, it's not worth breaking working code over.

Maybe we can. I doubt someone rely on this "feature". Thus, deprecating this behavior will not affect too much code.

On another hand, it seems that more strict processing - slightly complicates parsing in C (get_integer() helper) and the grammar rules. So, maybe those zeros aren't a big problem: it's easy to support them in the fractions module, see pr. Support this - also not a big problem for external modules, see the mpmath issue.

maybe we can change the float docs to disallow the redundant leading zeros and explain in a note that CPython does support those for backwards compatibility reasons, but only for float, not for Fraction? Or otherwise mention the difference in the Fraction docs.

Formatting docs already (IMO) big and complex. I would prefer to avoid adding more special cases (there is already float vs Decimal differences, etc).

Maybe we could just document more strict requirements for width/precision and soft-deprecate old behavior in WhatsNew?

[serhiy-storchaka]

I think it is better to allow leading zeroes in "width" and "precision" in the Fraction format. The current specification allows leading zeroes, forbidding them would make it even more complex. AFAIK, leading zeroes are allowed in all other programming languages. So forbidding them would complicate the documentation, the implementation, will create unnecessary difference from other programming languages.

[gvanrossum]

I note that Python numbers cannot have leading zeros (to avoid confusion with octal notation in C, C++ and in ancient Python).

But as the simplest (and fully backwards compatible) solution is to allow redundant leading zeros for Fraction, let's just do that.

For consistency I would also make the same change in _GENERAL_FORMAT_SPECIFICATION_MATCHER.

[skirpichev]

I note that Python numbers cannot have leading zeros (to avoid confusion with octal notation in C, C++ and in ancient Python).

Note, that in the given context we have only decimal notation.

BTW, Decimal's seems to be partially affected by this issue (width processing):

>>> f"{Decimal(1.25):0010f}"
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
    f"{Decimal(1.25):0010f}"
      ^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid format string
>>> f"{Decimal(1.25):.010f}"
'1.2500000000'

[gvanrossum]

Yeah, so I am now against “fixing” this. It is easy for users to avoid the issue: don’t use redundant leading zeros.

If there is concern about the docs, let’s change the float docs to no longer promise support for redundant leading zeros. Then users who discover that they work can file issues about getting float fixed, to which we can respond that it is an accidental historic bug that they are supported, and we recommend not using that, it we are reluctant to fix it for fear of unnecessary breaking working code.