[Python-ideas] Retain string form of AST Numbers

[Chris Angelico]

Currently, parsing a Python script into an AST discards a huge amount
of information. Some of that (line and column information) is retained
as metadata, but a lot isn't. One of the potentially-useful things
that's currently dropped is the string form of numbers:

>>> print(ast.dump(ast.parse("x = 1 + 2.000 + 3.0j")))
Module(body=[Assign(targets=[Name(id='x', ctx=Store())],
value=BinOp(left=BinOp(left=Num(n=1), op=Add(), right=Num(n=2.0)),
op=Add(), right=Num(n=3j)))])

The three Num nodes have an n with the already-constructed
int/float/complex. At this point, it's impossible to return to the
original string that was entered; any rounding will have already taken
place.

If the Num nodes also had a .str member with the unparsed string ("1",
"2.000", and "3.0j"), it would be possible to influence their
interpretation with an AST transform - for instance, turning them into
decimal.Decimal construction calls, or wrapping them in something that
automatically keeps track of significant figures (which would mean
that "2.000" is considered more precise than a mere "2.0" or "2.").

It shouldn't be too difficult to do, and it could simply be ignored in
most cases; any sort of change to the actual number could leave the
original string out of sync with it, which isn't a problem.

Thoughts?

ChrisA