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.

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'

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.

Thanks for the replies. The use of format works great. Maybe it's worth mentioning in the documentation because using "%f" % Decimal("0.00000001") leds to loosing precision (cast to float64 I guess) while using f"{"Decimal('0.00000001'):f}" does the thing correctly.

One more thing: It's worth mentioning that by default, you'll get only 28 significant digits of precision, so doing f"{Decimal('1234567890123456789012.12345678').normalize():f}" will produce '1234567890123456789012.123457' (two last digits got rounded).

Sure, but that has nothing to do with string representations or formatting, so I don't see how it's relevant for this issue: the rounding happens when you call `normalize`.

Closing here; the built-in formatting should be sufficient, as discussed.