TypeError: 'NoneType' object is not callable

想写一个五子棋，但是在写一个类的时候，当不注释第八行的代码（self.is_end = None）时，运行这段代码就会出现 TypeError: ‘NoneType’ object is not callable的错误，这是为什么呢

目前发现错误的原因：由于State类中，由于变量is_end和is_end函数使用了相同的名称，所以会出现调用出错，这是目前我发现的错误

import numpy as np
import time

BOzRD_ROWS = 15
BOARD_COLS = 15
class State:
    def __init__(self):
        self.data = np.zeros((BOARD_ROWS,BOARD_COLS))
        # self.is_end = None  为什么在这里加这一句就会出现调用错误？？
        
    def next_state(self, row, col, symbol):
        new_state = State()
        new_state.data = self.data.copy()
        new_state.data[row,col] = symbol
        return new_state

    # 最后落子在（row,col）处时是否结束比赛
    def is_end(self, row, col, symbol):
        directions = [[(-1,0),(1,0)],[(0,-1),(0,1)],[(-1,1),(1,-1)],[(-1,-1),(1,1)]]
        for one_direction in directions:
            count = 1
            for direction in one_direction:
                row_new = row
                col_new = col
                while True:
                    row_new += direction[0]
                    col_new += direction[1]
                    print(row_new,col_new)
                    if row_new < 0 or row_new > 14 or col_new < 0 or col_new > 14:
                        break
                    if self.data[row_new,col_new] == symbol:
                        count += 1
                        if count >= 5:
                            self.is_end = True
                            return self.is_end
                    else:
                        break
        self.is_end = False
        return self.is_end

    def print_state(self):
        for i in range(BOARD_ROWS):
            out = str()
            for j in range(BOARD_COLS):
                if self.data[i,j] == 0:
                    token = '+ '
                elif self.data[i,j] == 1:
                    token = '* '
                else:
                    token = 'x '
                out += token
            print(out)

class Human_player():
    def __init__(self):
        self.state = None
        self.symbol = None

    def set_state(self, state):
        self.state = state

    def set_symbol(self, symbol):
        self.symbol = symbol

    def act(self):
        location = input('please input you location: \n') # type: str
        location_row = int(location.split(' ')[0]) - 1
        location_col = int(location.split(' ')[1]) - 1
        return [location_row, location_col, self.symbol]

def PVP():
    state = State()
    state.print_state()
    player1 = Human_player()
    player2 = Human_player()
    player1.symbol = 1
    player2.symbol = -1
    while True:
        [loc_row, loc_col, symbol] = player1.act()
        state = state.next_state(loc_row,loc_col, symbol)
        state.print_state()
        is_end = state.is_end(loc_row, loc_col, symbol)
        print(is_end)
        if is_end:
            return 0

        [loc_row, loc_col, symbol] = player2.act()
        state = state.next_state(loc_row, loc_col, symbol)
        is_end = state.is_end(loc_row, loc_col, symbol)
        state.print_state()
        if is_end:
            return 0

PVP()