哈哈，看了以下两个帖子，综合下捣鼓出了这个对比的功能
@ianxiaohanxu
http://testerhome.com/topics/1389
@monkey
http://testerhome.com/topics/202

其实在https://github.com/gb112211/Adb-For-Test 里面有一个截取 element 进行对比的方法，但是在使用 appium 时是无法使用的，因为其用到了 uiautomator 命令。。。

在 appium 中截取界面某个 element，也就是截取屏幕的部分区域进行对比，在以图片对比结果作为判断依据的时候还是有用的，直接上代码：
extend.py

#!/usr/bin/env python
#coding=utf-8

import os
import platform
import tempfile
import shutil

from PIL import Image

PATH = lambda p: os.path.abspath(p)
TEMP_FILE = PATH(tempfile.gettempdir() + "/temp_screen.png")

class Appium_Extend(object):
    def __init__(self, driver):
        self.driver = driver

    def get_screenshot_by_element(self, element):
        #先截取整个屏幕，存储至系统临时目录下
        self.driver.get_screenshot_as_file(TEMP_FILE)

        #获取元素bounds
        location = element.location
        size = element.size
        box = (location["x"], location["y"], location["x"] + size["width"], location["y"] + size["height"])

        #截取图片
        image = Image.open(TEMP_FILE)
        newImage = image.crop(box)
        newImage.save(TEMP_FILE)

        return self

    def get_screenshot_by_custom_size(self, start_x, start_y, end_x, end_y):
        #自定义截取范围
        self.driver.get_screenshot_as_file(TEMP_FILE)
        box = (start_x, start_y, end_x, end_y)

        image = Image.open(TEMP_FILE)
        newImage = image.crop(box)
        newImage.save(TEMP_FILE)

        return self

    def write_to_file( self, dirPath, imageName, form = "png"):
        #将截屏文件复制到指定目录下
        if not os.path.isdir(dirPath):
            os.makedirs(dirPath)
        shutil.copyfile(TEMP_FILE, PATH(dirPath + "/" + imageName + "." + form))

    def load_image(self, image_path):
        #加载目标图片供对比用
        if os.path.isfile(image_path):
            load = Image.open(image_path)
            return load
        else:
            raise Exception("%s is not exist" %image_path)

    #http://testerhome.com/topics/202
    def same_as(self, load_image, percent):
        #对比图片，percent值设为0，则100%相似时返回True，设置的值越大，相差越大
        import math
        import operator

        image1 = Image.open(TEMP_FILE)
        image2 = load_image

        histogram1 = image1.histogram()
        histogram2 = image2.histogram()

        differ = math.sqrt(reduce(operator.add, list(map(lambda a,b: (a-b)**2, \
                                                         histogram1, histogram2)))/len(histogram1))
        if differ <= percent:
            return True
        else:
            return False

接着跑了个 appium 脚本简单测试了下：
extend_test.py

#coding=utf-8

import unittest
import os

from extend import Appium_Extend
from appium import webdriver

class Test(unittest.TestCase):
    #初始化环境
    def setUp(self):
        desired_caps = {}
        desired_caps["platformName"] = "Android"
        desired_caps["platformVersion"] = "4.3"
        desired_caps["deviceName"] = "788a6ab5"
        desired_caps["appPackage"] = "com.android.settings"
        desired_caps["appActivity"] = ".Settings"

        self.driver = webdriver.Remote("http://127.0.0.1:4723/wd/hub", desired_caps)

        self.extend = Appium_Extend(self.driver)

        #回到主屏幕
        self.driver.press_keycode(3)

    #退出测试
    def tearDown(self):
        self.driver.quit()

    def test_get_screen_by_element(self):
        element = self.driver.find_element_by_id("com.android.deskclock:id/imageview")

        self.extend.get_screenshot_by_element(element).write_to_file("f:\\screen", "image")
        self.assertTrue(os.path.isfile("f:\\screen\\image.png"))

    def test_same_as(self):
        element = self.driver.find_element_by_id("com.android.deskclock:id/imageview")

        load = self.extend.load_image("f:\\screen\\image.png")
        #要求百分百相似
        result = self.extend.get_screenshot_by_element(element).same_as(load, 0)
        self.assertTrue(result)

if __name__ == "__main__":
    suite = unittest.TestSuite()
    suite.addTest(Test("test_get_screen_by_element"))
    suite.addTest(Test("test_same_as"))
    #执行测试
    unittest.TextTestRunner().run(suite)

这里截取的图片是下面这张图中的时间插件：

截取后的图片：

另外批量截图、批量对比就针对需求再做扩展了！