Decimal.__str__ has no way to force exact decimal representation

[sim1234]

BPO	43315
Nosy	@rhettinger, @facundobatista, @mdickinson, @stevendaprano, @sim1234

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-03-02.18:43:45.809>
created_at = <Date 2021-02-24.10:59:48.508>
labels = ['type-feature', '3.10']
title = 'Decimal.__str__ has no way to force exact decimal representation'
updated_at = <Date 2021-03-02.18:43:45.809>
user = 'https://github.com/sim1234'

bugs.python.org fields:

activity = <Date 2021-03-02.18:43:45.809>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2021-03-02.18:43:45.809>
closer = 'mark.dickinson'
components = []
creation = <Date 2021-02-24.10:59:48.508>
creator = 'sim1234'
dependencies = []
files = []
hgrepos = []
issue_num = 43315
keywords = []
message_count = 7.0
messages = ['387618', '387621', '387625', '387626', '387724', '387727', '387951']
nosy_count = 5.0
nosy_names = ['rhettinger', 'facundobatista', 'mark.dickinson', 'steven.daprano', 'sim1234']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43315'
versions = ['Python 3.10']

[sim1234]

str(Decimal("0.00000001")) always returns "1E-8" and there is no way to directly get the "0.00000001" string back without writting custom method for converting DecimalTuple to string. This is caused by arbitrary choice of and leftdigits > -6 condition in https://github.com/python/cpython/blob/master/Lib/_pydecimal.py#L1052 .
The hardcoded value of -6 should be parametrizable. This can be done by adding it as argument to the str method or maybe by adding it to the decimal context.

[mdickinson]

This is by design: the choice of -6 is not arbitrary, but follows the standard that the Decimal class is based on. Quoting from http://speleotrove.com/decimal/daconvs.html#reftostr:

If the exponent is less than or equal to zero and the adjusted exponent is greater than or equal to -6, [...]

"str" isn't parameterizable (at least, not without a major change to the way that Python works), so if you're using it you have to accept the compromises it makes. If you want finer control over the string representation of Decimal objects, use Python's formatting capabilities:

    >>> from decimal import Decimal
    >>> x = Decimal("1e-8")
    >>> format(x, 'f')
    '0.00000001'

[stevendaprano]

Python 3.5 to 3.9 are all in feature freeze and can accept no new features; new features can only be added to 3.10.

But having said that, I agree with Mark that the correct solution here is to use format, not str. Szymon, unless you have a very argument against format, I think this should be closed.