text_to_be_present_in_element(locator,text)是指定页面元素的文本位置，
一般用于验证一个文本信息或者错误的信息，我们任然以百度登录为案例，
用户名和密码为空，点击登录按钮，验证在指定的时间范围内会不会出现
"请您填写手机/邮箱/用户名"的错误信息，见实现的代码：
# -*-coding:utf-8-*-
from selenium import webdriver
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions
import time as t
driver = webdriver.Chrome()
driver.maximize_window()
driver.get('https://www.baidu.com')
driver.find_element_by_id('u1').find_element_by_class_name('lb').click()
t.sleep(3)
driver.find_element_by_id('TANGRAM__PSP_10__footerULoginBtn').click()
t.sleep(3)
driver.find_element_by_id('TANGRAM__PSP_10__submit').click()
t.sleep(2)
element = driver.find_element_by_css_selector('#TANGRAM__PSP_10__error')
WebDriverWait(driver, 10).until(expected_conditions.text_to_be_present_in_element((By.CSS_SELECTOR,"#TANGRAM__PSP_10_error"), u'请您填写手机/邮箱/用户名'))
driver.quit()
driver.quit()