gh-121798: Add class method Decimal.from_number()

[serhiy-storchaka]

It is an alternate constructor which only accepts a single numeric argument. Unlike to Decimal.from_float() it accepts also Decimal. Unlike to the standard constructor, it does not accept strings and tuples.

• Issue: Add class method Decimal.from_number() #121798

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[eendebakpt]

Why this particular selection? We could add Fraction or Rational as well, which is perhaps more in line with #121797 and #121800

Update: the Decimal constructor does not accept Fraction, so the method from_nunber here only accepts types that the constructor can handle

[serhiy-storchaka]

Yes, these are the only numeric types accepted by the constructor. Exact conversion from Fraction to Decimal is not possible in general case (e.g. 1/3).

[picnixz]

Instead of using number, maybe you can directly say "only accepts instances of float, int or Decimal" since otherwise people might wonder why objects implementing the Number protocol are not allowed.

By the way, this function is essentially a shortcut to avoid an if isinstance(...) right?

[serhiy-storchaka]

Done.

Yes, you can look at it from such point. If your function needs a Decimal or a number that can be converted to Decimal, it can use Decimal.from_number() without additional type check. It will reject strings and tuples that are accepted by the constructor.

[mdickinson]

+1 for the general idea. I haven't reviewed line-by-line.