Using multipe operators in Python sequentially

Question

I’m trying to understand how Python handles using multiple sequential operators to add and subtract numbers.

Here is an example of what I mean:

>>> 5+-2
3
>>> 5-+2
3
>>> 5+-+-+2
7
>>> 5+-+-+-2
3
>>> 5+-+---+2
7
>>> 5-+-+---+2
3
>>> 5-+-+---+-2
7
>>> 5++++-++++--+-+++2
7
>>> 5+----2
7
>>> 5++++-2
3
>>> 5++-++-2
7
>>>

I don’t understand what decides whether to add or subtract these two integers.
I’ve used Python 3.11.1 for this example.

Asked By: Dušan Stokić

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Source

Answer 1

To understand how those expressions are evaluated you can use the ast (abstract syntax tree) module.

>>> import ast
>>> def pretty_print_ast(code: str) -> None:
...     print(ast.dump(ast.parse(code), indent=4))

Now let’s ask how Python evaluates 5+-2?

>>> pretty_print_ast("5+-2")
    Module(
        body=[
            Expr(
                value=BinOp(
                    left=Constant(value=5),
                    op=Add(),
                    right=UnaryOp(
                        op=USub(),
                        operand=Constant(value=2))))],
        type_ignores=[])

So it’s getting parsed as 5+(-2). Now let’s take another example 5+-+-+2.

>>> pretty_print_ast("5+-+-+2")
Module(
    body=[
        Expr(
            value=BinOp(
                left=Constant(value=5),
                op=Add(),
                right=UnaryOp(
                    op=USub(),
                    operand=UnaryOp(
                        op=UAdd(),
                        operand=UnaryOp(
                            op=USub(),
                            operand=UnaryOp(
                                op=UAdd(),
                                operand=Constant(value=2)))))))],
    type_ignores=[])

As you can see, it’s getting parsed as 5+(-(+(-(+2)))).

Please note that the UnaryOp node in the ast corresponds to the unary operations.

Answered By: Abdul Niyas P M

Answer 2

I strongly recommend you to look at the concept of interpreter and stack. And you may research for programming languages concepts. This question is more of a question of how a programming language works.

Python read the line left to right. Let’s assume we have a stack. Python read first value and check it’s type if numeric or operator.

For example 2+-3
saved to stack

step we got 2 (read line until operator)
step: we got operator + (we and python already know if the operator is *, we should check coming char is * because ** another operator).
Saved to stack
step: we got – operator. But we can not use + with another operator. So we should check if this operator shows us number is negative or positive.
saved to stack -> -3
step: no coming char. So we have valid command. Otherwise we could throw exception (eg. SyntaxError)
step: calculate stack

This is basically how mathematical operations are done. Of course there are more of them.

Answered By: notNowOnlyCoding

Answer 3

There is no such thing as a ++ operator in Python.
There is no such thing as a — operator in Python.
Everything else is based on order of operations. See Operator Precedence in Python

Answered By: Pravin Damle

Using multipe operators in Python sequentially

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