Python allow multiple + or - after * and / operation without reporting exception "Invalid argument to operation" ?

[ylwsas]

Bug report

Bug description:

The plus and minus symbols can be regarded as numeric sign flag before an operands, but it seems Python can put multiple plus or minus after operators and before operands , and Python do not report any exception something like invalid argument to operation. The operations results depends on the number of signs for minus symbols, that is quite wired.

For Java or other language, they do report syntax errors. Think about the following codes, they all work.
Should we report syntax error for this situation?

# 
a=3; b=4
c=a *+ b
print(c)

a=3; b=4
c=a *++++++ b
print(c)


a=3; b=4
c=a *- b
print(c)

a=3; b=4
c=a *-- b
print(c)

a=3; b=4
c=a *--- b
print(c)

a=3; b=4
c=a /+++--- b
print(c)

'''Output: 
12
12
-12
12
-12
-0.75
'''

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

[JelleZijlstra]

This invokes the unary + and - operators a number of times. Somewhat confusing behavior but it follows logically from the grammar.

You can see this with the ast module:

>>> print(ast.dump(ast.parse('a+++b'), indent=True))
Module(
 body=[
  Expr(
   value=BinOp(
    left=Name(id='a', ctx=Load()),
    op=Add(),
    right=UnaryOp(
     op=UAdd(),
     operand=UnaryOp(
      op=UAdd(),
      operand=Name(id='b', ctx=Load())))))])

And in the bytecode:

>>> dis.dis('a+++b')
  0           RESUME                   0

  1           LOAD_NAME                0 (a)
              LOAD_NAME                1 (b)
              CALL_INTRINSIC_1         5 (INTRINSIC_UNARY_POSITIVE)
              CALL_INTRINSIC_1         5 (INTRINSIC_UNARY_POSITIVE)
              BINARY_OP                0 (+)
              RETURN_VALUE

[ylwsas]

Although syntactically logical, this is a defect in the syntax checking stage of the programming language. Unary operations usually only act on one subsequent operand, can we apply the constraint to 1. You can try other languages, they can report error.

[skirpichev]

Although syntactically logical

Not "logical", just correct.

You can try other languages

If you try C, not all your examples report errors, roughly less than a half.