为什么这个守护线程阻塞？

Question

为什么以下代码会阻塞 cc.start()？ crawler.py 包含类似于 http://doc.scrapy.org/en/latest/topics/practices.html#run-from-script

的代码

import scrapy
import threading
from subprocess import Popen, PIPE

def worker():
    crawler = Popen('python crawler.py', stdout=PIPE, stderr=PIPE, shell=True)
    while True:
        line = crawler.stderr.readline()
        print(line.strip())

cc = threading.Thread(target=worker())
cc.setDaemon(True)
cc.start()
print "Here" # This is not printed
# Do more stuff

crawler.py 包含以下代码：

from scrapy.crawler import CrawlerProcess
import scrapy

class MySpider(scrapy.Spider):
    name = 'Whosebug'
    start_urls = ['http://whosebug.com/questions?sort=votes']

def parse(self, response):
    for href in response.css('.question-summary h3 a::attr(href)'):
        full_url = response.urljoin(href.extract())
        yield scrapy.Request(full_url, callback=self.parse_question)

    process = CrawlerProcess({
    'USER_AGENT': 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)'
    })

process.crawl(MySpider)
process.start() # the script will block here until the crawling is finished

Answer 1

threading.Thread 将可调用对象作为参数（前函数名），你实际上是在创建线程实例时调用该函数

cc = threading.Thread(target=worker())

你需要做的只是将要调用的函数传递给thread

cc = threading.Thread(target=worker)

为什么这个守护线程阻塞？

Why is this Daemon thread blocking?

subprocess

python-multithreading

python-2.7