python tesserocr xpath

如下代码中xpath查询，contains(.,"登录成功")这个方法，我把“.”去掉，网页会不停的登录进去再退出去再登录，而我加上“.”就正常，感觉好奇怪，不知道为什么会影响?

WebDriverWait(browser,10).until(EC.presence_of_element_located((By.XPATH,'//h2[contains(.,"登录成功")]')))

import re,time,tesserocr
from selenium import webdriver
from io import BytesIO
from PIL import Image
from retrying import retry
from selenium.webdriver.support.wait import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
from selenium.common.exceptions import TimeoutException
from selenium.webdriver.common.by import By
import numpy as np

# def preprocess(image):
#     image = image.convert('L')
#     array = np.array(image)
#     array = np.where(array > 150,255,0)
#     image = Image.fromarray(array.astype('uint8'))
#     return image

@retry(stop_max_attempt_number=10, retry_on_result=lambda x: x is False)
def login():
    browser.get('https://captcha7.scrape.center/')
    browser.find_element(by=By.CSS_SELECTOR,value='.username input[type="text"]').send_keys('admin')
    browser.find_element(by=By.CSS_SELECTOR,value='.password input[type="password"]').send_keys('admin')
    captcha = browser.find_element(by=By.CSS_SELECTOR,value='#captcha')
    image = Image.open(BytesIO(captcha.screenshot_as_png))
    # image = preprocess(image)
    captcha = tesserocr.image_to_text(image)
    captcha = re.sub('[^A-Za-z0-9]','',captcha)
    browser.find_element(by=By.CSS_SELECTOR,value='.captcha input[type="text"]').send_keys(captcha)
    browser.find_element(by=By.CSS_SELECTOR,value='.login').click()
    try:
        WebDriverWait(browser,10).until(EC.presence_of_element_located((By.XPATH,'//h2[contains(.,"登录成功")]')))
        time.sleep(10) 
        browser.close()
        return True
    except TimeoutException:
        return False

if __name__ == '__main__':
    browser = webdriver.Chrome()
    login()