Unsupported format string passed to mpf.__format__

$ python
Python 3.12.3 (main, Feb  4 2025, 14:48:35) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mpmath
>>> mpmath.__version__
'1.3.0'
>>> val = mpmath.mpf('0.1')
>>> f"{val:.016f}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to mpf.__format__
>>>

Can you fix this bug?

Could you reproduce this bug on development version of the mpmath?

Here's what I get with the development version and Python 3.12.8:

In [1]: import mpmath

In [2]: mpmath.__version__
Out[2]: '1.4.0a5.dev2+g5e57cb8'

In [3]: val = mpmath.mpf('0.1')

In [4]: f"{val:.016f}"
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[4], line 1
----> 1 f"{val:.016f}"

File ~/repos/git/forks/mpmath/mpmath/ctx_mp_python.py:428, in _mpf.__format__(s, format_spec)
    360 """
    361 ``mpf`` objects allow for formatting similar to Python floats:
    362 
   (...)
    424 
    425 """
    427 _, _, (prec, _) = s._ctxdata
--> 428 return format_mpf(s._mpf_, format_spec, prec)

File ~/repos/git/forks/mpmath/mpmath/libmp/libmpf.py:1804, in format_mpf(num, format_spec, prec)
   1803 def format_mpf(num, format_spec, prec):
-> 1804     format_dict = read_format_spec(format_spec)
   1806     sign, digits = format_digits(num, format_dict, prec)
   1808     nchars = len(digits) + len(sign)

File ~/repos/git/forks/mpmath/mpmath/libmp/libmpf.py:1485, in read_format_spec(format_spec)
   1483         format_dict['precision'] = 6
   1484 else:
-> 1485     raise ValueError("Invalid format specifier '{}'".format(format_spec))
   1487 return format_dict

ValueError: Invalid format specifier '.016f'

The format works with a Python float:

In [5]: val = 0.1

In [6]: f"{val:.016f}"
Out[6]: '0.1000000000000000'

Are you satisfied with Warren's answer?

Relax requirements on precision format, allow 0-prefixes

@WarrenWeckesser

@WarrenWeckesser, thanks (I asked from phone). PR is ready: #916

But see python/cpython#130662

Maybe we shouldn't replicate this Python feature.

Relax requirements on precision format, allow 0-prefixes

In the realm of arbitrary precision numbers, I think there is no such thing as meaningless leading or trailing zero.
A leading or trailing zero is supposed to be a correctly computed digit.
For example, 1.000000000000000 is different from 1.000000000000000123463563456.
Presentation is important for readability and clarity.
Leading or trailing zeros can be used to justify a table of numbers left or right respectively.
For example, to me the following table could be improved.
+---------+--------------------+
| float | mpf |
+---------+--------------------+
| -1.00 | 0.3678794411714423 |
| -0.75 | 0.4723665527410147 |
| -0.50 | 0.6065306597126334 |
| -0.25 | 0.7788007830714049 |
| 0.00 | 1.000000000000000 |
| 0.25 | 1.284025416687741 |
| 0.50 | 1.648721270700128 |
| 0.75 | 2.117000016612675 |
+---------+--------------------+
Should be
+---------+--------------------+
| float | mpf |
+---------+--------------------+
| -1.00 | 0.3678794411714423 |
| -0.75 | 0.4723665527410147 |
| -0.50 | 0.6065306597126334 |
| -0.25 | 0.7788007830714049 |
| 0.00 | 1.0000000000000000 |
| 0.25 | 1.284025416687741X |
| 0.50 | 1.648721270700128Y |
| 0.75 | 2.117000016612675Z |
+---------+--------------------+
where X, Y, and Y are the relevant digits in each row.

Reopened, until python/cpython#130662 will be fixed. Meanwhile 849e185 was reverted in #936.

Revert "Revert 849e185 and (partially) f4cef9c (same for with field)"

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skirpichev commented on Feb 27, 2025

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skirpichev commented on Feb 28, 2025

ant1sir commented on Feb 28, 2025

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skirpichev commented on Mar 4, 2025

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Unsupported format string passed to mpf.format #915