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decimal docs: specification link and examples (#128698)

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Co-authored-by: RUANG (James Roy) <longjinyii@outlook.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
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Doc/library/decimal.rst
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@ -34,10 +34,12 @@ The :mod:`decimal` module provides support for fast correctly rounded
decimal floating-point arithmetic. It offers several advantages over the
:class:`float` datatype:
* Decimal "is based on a floating-point model which was designed with people
in mind, and necessarily has a paramount guiding principle -- computers must
provide an arithmetic that works in the same way as the arithmetic that
people learn at school." -- excerpt from the decimal arithmetic specification.
* Decimal "is based on a `floating-point model
<https://speleotrove.com/decimal/damodel.html#refnumber>`__ which was designed
with people in mind, and necessarily has a paramount guiding principle --
computers must provide an arithmetic that works in the same way as the
arithmetic that people learn at school." -- excerpt from the decimal
arithmetic specification.
* Decimal numbers can be represented exactly. In contrast, numbers like
``1.1`` and ``2.2`` do not have exact representations in binary
@ -238,6 +240,26 @@ floating-point flying circus:
>>> c % a
Decimal('0.77')
Decimals can be formatted (with :func:`format` built-in or :ref:`f-strings`) in
fixed-point or scientific notation, using the same formatting syntax (see
:ref:`formatspec`) as builtin :class:`float` type:
.. doctest::
>>> format(Decimal('2.675'), "f")
'2.675'
>>> format(Decimal('2.675'), ".2f")
'2.68'
>>> f"{Decimal('2.675'):.2f}"
'2.68'
>>> format(Decimal('2.675'), ".2e")
'2.68e+0'
>>> with localcontext() as ctx:
... ctx.rounding = ROUND_DOWN
... print(format(Decimal('2.675'), ".2f"))
...
2.67
And some mathematical functions are also available to Decimal:
>>> getcontext().prec = 28