When a non-literal is given to literal_eval, attempt to be more
helpful with the message, rather than calling it 'malformed'.

I afraid this makes an error message more confusing and misleading.

>>> ast.literal_eval('~2')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/ast.py", line 93, in literal_eval
    return _convert(node_or_string)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 92, in _convert
    return _convert_signed_num(node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 65, in _convert_signed_num
    return _convert_num(node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 56, in _convert_num
    raise ValueError('%s not allowed in literal' % type(node).__name__)
ValueError: UnaryOp not allowed in literal

This is not true since + and - are allowed:

>>> ast.literal_eval('-2')
-2

>>> ast.literal_eval('2+3')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/ast.py", line 93, in literal_eval
    return _convert(node_or_string)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 92, in _convert
    return _convert_signed_num(node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 65, in _convert_signed_num
    return _convert_num(node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 56, in _convert_num
    raise ValueError('%s not allowed in literal' % type(node).__name__)
ValueError: BinOp not allowed in literal

But:

>>> ast.literal_eval('2+3j')
(2+3j)

>>> ast.literal_eval('"a"+"b"')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/ast.py", line 93, in literal_eval
    return _convert(node_or_string)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 85, in _convert
    left = _convert_signed_num(node.left)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 65, in _convert_signed_num
    return _convert_num(node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 56, in _convert_num
    raise ValueError('%s not allowed in literal' % type(node).__name__)
ValueError: Str not allowed in literal

But Str is allowed:

>>> ast.literal_eval('"ab"')
'ab'

Hmm, I think I see what I broke there. It was part of the merge conflict resolution - I moved the check into the function, which is actually incorrect. It wasn't misleading like that in the original patch. Will fix that.

Actually, it's a bit more complicated than I thought. Current proposed solution: Track the context of each conversion, thus allowing different errors to be distinguished.

(BTW, by "proposed" I mean that the change that I describe is in the PR.)

Please wait until issue32892 and issue32893 be committed or rejected before merging this PR.