Types `f` and `e` in format spec does not remove trailing zeros for floats if precision is not specified

[Prometheus3375]

Bug report

Bug description:

I will report this as a bug, however, this can be an issue with documentation.

By the docs of format specification mini-language:

'f' -- Fixed-point notation. For a given precision p, formats the number as a decimal number with exactly p digits following the decimal point. With no precision given, uses a precision of 6 digits after the decimal point for float, and uses a precision large enough to show all coefficient digits for Decimal. If no digits follow the decimal point, the decimal point is also removed unless the # option is used.

I am focusing on the bold part. Let's test it.

Integers, floats and complex numbers:

>>> f'{1:f}'
'1.000000'
>>> f'{1.:f}'
'1.000000'
>>> f'{1+1j:f}'
'1.000000+1.000000j'
>>> f'{.1:f}'
'0.100000'

Decimals:

>>> from decimal import Decimal
>>> f'{Decimal(1):f}'
'1'
>>> f'{Decimal(1.):f}'
'1'
>>> f'{Decimal(.1):f}'
'0.1000000000000000055511151231257827021181583404541015625'

If no digits follow the decimal point, the decimal point is also removed unless the # option is used.

The sentence goes after mentioning Decimal. So maybe the sentence speaks only about them? If so, then note about # must work.

>>> from decimal import Decimal
>>> d = Decimal(1)
>>> f'{d:#f}'
ValueError: invalid format string

As intended, because per # documentation:

The '#' option causes the “alternate form” to be used for the conversion. The alternate form is defined differently for different types. This option is only valid for integer, float and complex types

The further docs of # also suggest that f must remove trailing zeros:

For float and complex the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result.

The same issue is true for format types e, F and E.

CPython versions tested on:

3.11

Operating systems tested on:

Windows

[serhiy-storchaka]

The behavior corresponds to the documentation. Where do you see the bug?

[Prometheus3375]

The documentation says:

If no digits follow the decimal point, the decimal point is also removed unless the # option is used.

An integer does not have digits after the decimal point, but f{1:f} returns 1.000000.

[serhiy-storchaka]

Other examples:

>>> f'{1:.0f}'
'1'
>>> f'{1.0:.0f}'
'1'
>>> f'{1+1j:.0f}'
'1+1j'

☝️ "If no digits follow the decimal point, the decimal point is also removed..."

>>> f'{1:#.0f}'
'1.'
>>> f'{1.0:#.0f}'
'1.'
>>> f'{1+1j:#.0f}'
'1.+1.j'

☝️ "... unless the # option is used."

[serhiy-storchaka]

f{1:f} returns 1.000000 which has digits after the decimal point.

[Prometheus3375]

Oh, I see. Thank you for the answer! I got a bit confused with docs which places comment about no digits after mentioning the case with no precision given.