# coding=utf-8
# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
# Licensed under the Apache License, Version 2.0 (see LICENSE).

from __future__ import absolute_import, division, print_function, unicode_literals

import ast

from six import PY3

from pants.contrib.python.checks.checker.common import CheckstylePlugin


class ConstantLogic(CheckstylePlugin):
    """Check for constants provided to boolean operators which result in a constant expression."""

    @classmethod
    def name(cls):
        return "constant-logic"

    @classmethod
    def iter_bool_ops(cls, tree):
        for ast_node in ast.walk(tree):
            if isinstance(ast_node, ast.BoolOp) and isinstance(ast_node.op, (ast.And, ast.Or)):
                yield ast_node

    @classmethod
    def is_name_constant(cls, expr):
        if PY3:
            if isinstance(expr, ast.NameConstant):
                return True
        else:
            if isinstance(expr, ast.Name) and expr.id in ["True", "False", "None"]:
                return True
        return False

    @classmethod
    def is_probably_constant(cls, expr):
        if isinstance(expr, (ast.Num, ast.Str)):
            return True
        if cls.is_name_constant(expr):
            return True
        return False

    def nits(self):
        for bool_op in self.iter_bool_ops(self.python_file.tree):
            # We don't try to check anything in the middle of logical expressions with more than two
            # values.
            leftmost = bool_op.values[0]
            rightmost = bool_op.values[-1]
            if self.is_probably_constant(leftmost):
                yield self.error(
                    "T804",
                    "You are using a constant on the left-hand side of a logical operator. "
                    "This is probably an error.",
                    bool_op,
                )
            if isinstance(bool_op.op, ast.And) and self.is_probably_constant(rightmost):
                yield self.error(
                    "T805",
                    "You are using a constant on the right-hand side of an `and` operator. "
                    "This is probably an error.",
                    bool_op,
                )