You don’t need any pull up or pull down resistors, use 

pull_up_down=GPIO.PUD_DOWN

instead.

The setup:

The wiring:

The test program: test.py

# Door open/closed detector
#
# (c) Alan Lupsha 2020
# wiki.lupsha.com


import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM) # GPIO Numbers instead of board numbers

MAGNET_GPIO = 18
GPIO.setup(MAGNET_GPIO, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # GPIO Assign mode

print("monitoring in progress...")

try:
    while (True):

        print('MAGNET_GPIO is: %s' % GPIO.input(MAGNET_GPIO))

        time.sleep(0.1)

# If user presses ^C cleanup the GPIO
except KeyboardInterrupt:
    GPIO.cleanup()
print("Exiting program, bye.")

Run it.

python3 test.py

Open:

Closed:

A more complex example: (2 files)

door.py

# Door open/closed detector
#
# This programs monitors a reed switch connected to GPIO 18 and 3VDC
# When the reed switch is open, the sendalert method is called, which
# contacts other alarm systems. When the reed switch is closed, the 
# alarm mode is reset and sendstopalert is called.
#
# (c) Alan Lupsha 2020
# wiki.lupsha.com


import RPi.GPIO as GPIO
import time
from datetime import datetime
import urllib3
import requests, json
#import http.client as httplib
#import sys
import ledstuff

GPIO.setmode(GPIO.BCM) # GPIO Numbers instead of board numbers

MAGNET_GPIO = 18
GPIO.setup(MAGNET_GPIO, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # GPIO Assign mode

#LED_GPIO = 23
#GPIO.setup(LED_GPIO, GPIO.OUT) # the red LED

# initially there is no alert
alert = False


def loginfo(message):
    rightnow = datetime.now().strftime("[%Y/%m/%d %H:%M:%S]")
    print(rightnow, ' [INFO] ', message)
    return
def logerror(message):
    rightnow = datetime.now().strftime("[%Y/%m/%d %H:%M:%S]")
    print(rightnow, ' [ERROR] ', message)
    return
def logexception(message, exception):
    rightnow = datetime.now().strftime("[%Y/%m/%d %H:%M:%S]")
    print(rightnow, ' [ERROR] ', message, ' ==> ', exception)
    return
def urlhit(httpurl, userMessage):
    sample = {
        "message": userMessage
    }
    jsonData = json.dumps( sample )

    loginfo('urlhit %s with message %s' % ( httpurl, jsonData))
    try:
        response = requests.post(httpurl, jsonData)
    except urllib3.exceptions.NewConnectionError as e:
        logexception('new connection error 1 yo', e)
    except urllib3.exceptions.MaxRetryError as e:
        logexception('max retry error 2 yo', e)
    except requests.exceptions.ConnectionError as e:
        logexception('connection error 3 yo', e)
    return

def startalert( str ):
    # GPIO.output(LED_GPIO, True)
    # Send an alert into the cloud
    now = datetime.now()
    rightnow = now.strftime("[%Y/%m/%d %H:%M:%S]")
    message = "Door open at " + rightnow
    loginfo(message)
    urlhit('http://localhost:8080/door/open', message)
    return

def stopalert( str ):
    # GPIO.output(LED_GPIO, False)
    # Send END of alert into the cloud
    now = datetime.now()
    rightnow = now.strftime("[%Y/%m/%d %H:%M:%S]")
    message = "Door closed at " + rightnow
    loginfo(message)
    urlhit('http://localhost:8080/door/close', message)
    return

loginfo("monitoring in progress...")
ledstuff.arm()

totalcounts = 0
try:
    while (True):

        totalcounts = totalcounts + 1

        # MAGNET_GPIO = 1 means the circuit is closed, there is NO alert, = 0 means ALERT
        # print('******************** MAGNET_GPIO is: %s' % GPIO.input(MAGNET_GPIO))

        # a break took place just now
        if alert == False and GPIO.input(MAGNET_GPIO) == 0:
            alert = True
            startalert( time )
            loginfo('=> alert now is: DOOR OPEN')
            ledstuff.ledon()

        # an ongoing breach is STILL taking place
        if alert == True and GPIO.input(MAGNET_GPIO) == 0:
        #    loginfo('=> breach still taking place')
            ledstuff.ledon()

        # the breach has ended, we are resetting the alert mode as well
        if alert == True and GPIO.input(MAGNET_GPIO) == 1:
            alert = False # reset it
            stopalert( time )
            loginfo('=> alert is reset, door is now CLOSED')

        # there is no breach, sensor is closed
        #if alert == False and GPIO.input(MAGNET_GPIO) == 1:
        #    loginfo('=> all ok, GPIO.INPUT(MAGNET_GPIO) is %s' % GPIO.input(MAGNET_GPIO))

        # every x times, blink the LED
        if totalcounts == 10:
            ledstuff.blink()
            totalcounts = 0

        # wait a bit
        time.sleep(1)

# If user presses ^C cleanup the GPIO
except KeyboardInterrupt:
    GPIO.cleanup()

loginfo("Exiting program, bye.")

ledstuff.py

# LED on/off tester
# (c) Alan Lupsha 2020
# wiki.lupsha.com


import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM) # GPIO Numbers instead of board numbers

LED_GPIO = 21
GPIO.setup(LED_GPIO, GPIO.OUT) # GPIO Assign mode

def ledon():
    #print('led ON')
    GPIO.output(LED_GPIO, True)
    return
def ledoff():
    #print('led OFF')
    GPIO.output(LED_GPIO, False)
    return

# short blinks every X seconds
def blink():
    ledon()
    time.sleep(0.1)
    ledoff()
    return

def angry():
    ledon()
    time.sleep(0.5)
    ledoff()
    return

# blink a number of times per second, to signify alertness
def blinkpersecond(timestoblink):
    delay = 1 / timestoblink
    for i in range( 0, timestoblink ):
        #print('delay = ' + str(delay) + ' blink = ' + str(i))
        ledon()
        time.sleep(delay)
        ledoff()
        time.sleep(delay)
    return

# arming
def arm():
        blinkpersecond(2)
        blinkpersecond(5)
        blinkpersecond(10)
        ledoff()
        return

def disarm():
    ledoff()
    return