b = a*+2 is valid syntax. Why?

Question

Why does python (3.7.9) allow this syntax?

a = 3
b = a*+2

It find this annoying because if you want to type b = a**2 it can happen very quickly on the german keyboard layout that you write instead b = a*+2. With no sytnax error, finding such a bug can be very time consuming. Furthermore, it violates the usual mathematical rule that adjacent operators should be separated with brackets.

That's allowed in many other languages (e.g. C, Go, JavaScript). — bereal, Nov 25 '21 at 08:04

Abdul Niyas P M · Accepted Answer · 2021-11-28T12:02:01.900

This is because python supports unary arithmetic operations. You can also inspect how python compiles down the source expression to abstract syntax tree using ast module.

>>> import ast
>>>
>>> print(ast.dump(ast.parse("b = a*+2"), indent=4))
Module(
    body=[
        Assign(
            targets=[
                Name(id='b', ctx=Store())],
            value=BinOp(
                left=Name(id='a', ctx=Load()),
                op=Mult(),
                right=UnaryOp(
                    op=UAdd(),
                    operand=Constant(value=2))))],
    type_ignores=[])

b = a*+2 is valid syntax. Why?

1 Answers1