{"id":6607,"date":"2023-06-05T02:22:47","date_gmt":"2023-06-05T02:22:47","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=6607"},"modified":"2023-06-05T02:28:09","modified_gmt":"2023-06-05T02:28:09","slug":"python-thread-return-value","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-concurrency\/python-thread-return-value\/","title":{"rendered":"How to Return Values from a Thread"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to return values from a child thread to the main thread by extending the threading.Thread<\/code> class.<\/p>\n\n\n\n

When a Python program starts, it has a single thread called the main thread<\/strong>. Sometimes, you want to offload I\/O tasks to new threads<\/a> and execute them concurrently. Additionally, you may also want to get the return values from these threads from the main thread.<\/p>\n\n\n\n

To return a value from a thread, you can extend the Thread class<\/a> and store that value in the instance of the class.<\/p>\n\n\n\n

The following example illustrates how to check a specified URL and return its HTTP status code in a separate thread:<\/p>\n\n\n

from<\/span> threading import<\/span> Thread\nimport<\/span> urllib.request\n\n\nclass<\/span> HttpRequestThread<\/span>(Thread)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, url: str)<\/span> -> None<\/span>:<\/span>\n        super().__init__()\n        self.url = url\n        self.http_status_code = None<\/span>\n        self.reason = None<\/span>\n\n    def<\/span> run<\/span>(self)<\/span> -> None<\/span>:<\/span>\n        try<\/span>:\n            response = urllib.request.urlopen(self.url)\n            self.http_status_code = response.code\n        except<\/span> urllib.error.HTTPError as<\/span> e:\n            self.http_status_code = e.code\n        except<\/span> urllib.error.URLError as<\/span> e:\n            self.reason = e.reason\n\n\ndef<\/span> main<\/span>()<\/span> -> None<\/span>:<\/span>\n    urls = [\n        'https:\/\/httpstat.us\/200'<\/span>,\n        'https:\/\/httpstat.us\/400'<\/span>\n    ]\n\n    # create new threads<\/span>\n    threads = [HttpRequestThread(url) for<\/span> url in<\/span> urls]\n\n    # start the threads<\/span>\n    [t.start() for<\/span> t in<\/span> threads]\n\n    # wait for the threads to complete<\/span>\n    [t.join() for<\/span> t in<\/span> threads]\n\n    # display the URLs with HTTP status codes<\/span>\n    [print(f'{t.url}<\/span>: {t.http_status_code}<\/span>'<\/span>) for<\/span> t in<\/span> threads]\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    main()\n\n\ndef<\/span> main<\/span>()<\/span> -> None<\/span>:<\/span>\n    urls = [\n        'https:\/\/httpstat.us\/200'<\/span>,\n        'https:\/\/httpstat.us\/400'<\/span>\n    ]\n\n    # create new threads<\/span>\n    threads = [HttpRequestThread(url) for<\/span> url in<\/span> urls]\n\n    # start the threads<\/span>\n    [t.start() for<\/span> t in<\/span> threads]\n\n    # wait for the threads to complete<\/span>\n    [t.join() for<\/span> t in<\/span> threads]\n\n    # display the URLs with HTTP status codes<\/span>\n    [print(f'{t.url}<\/span>: {t.http_status_code}<\/span>'<\/span>) for<\/span> t in<\/span> threads]\n\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
ttps:\/\/httpstat.us\/200<\/span>: 200<\/span>\nhttps:\/\/httpstat.us\/400<\/span>: 400<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define the HttpRequestThread<\/code> class that extends the Thread<\/code> class:<\/p>\n\n\n
class<\/span> HttpRequestThread<\/span>(Thread)<\/span>:<\/span>\n# ...<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, define the __init__()<\/code><\/code> method that accepts a URL. Inside the __init__()<\/code><\/code> method adds url<\/code>, http_status_code<\/code><\/code>, and reason<\/code> instance variables. The http_status_code<\/code><\/code> will store the status code of the url<\/code> at the time of checking and the reason<\/code> will store the error message in case an error occurs:<\/p>\n\n\n
def<\/span> __init__<\/span>(self, url: str)<\/span> -> None<\/span>:<\/span>\n    super().__init__()\n    self.url = url\n    self.http_status_code = None<\/span>\n    self.reason = None<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, override the run()<\/code> method that uses the urllib<\/code> library to get the HTTP status code of the specified URL and assigns it to http_status_code<\/code> field. If an error occurs, it assigns the error message to the reason<\/code> field: <\/p>\n\n\n
def<\/span> run<\/span>(self)<\/span> -> None<\/span>:<\/span>\n    try<\/span>:\n        response = urllib.request.urlopen(self.url)\n        self.http_status_code = response.code\n    except<\/span> urllib.error.HTTPError as<\/span> e:\n        self.http_status_code = e.code\n    except<\/span> urllib.error.URLError as<\/span> e:\n        self.reason = e.reason<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Later, we can access the url<\/code>, http_status_code<\/code>, and reason<\/code> from the instance of the HttpRequestThread<\/code> class.<\/p>\n\n\n\n
Fourth, define the main()<\/code> function that creates instances of the HttpRequestThread<\/code> class, start the threads, wait for them to complete, and get the result from the instances:<\/p>\n\n\n
def<\/span> main<\/span>()<\/span> -> None<\/span>:<\/span>\n    urls = [\n        'https:\/\/httpstat.us\/200'<\/span>,\n        'https:\/\/httpstat.us\/400'<\/span>\n    ]\n\n    # create new threads<\/span>\n    threads = [HttpRequestThread(url) for<\/span> url in<\/span> urls]\n\n    # start the threads<\/span>\n    [t.start() for<\/span> t in<\/span> threads]\n\n    # wait for the threads to complete<\/span>\n    [t.join() for<\/span> t in<\/span> threads]\n\n    # display the URLs with HTTP status codes<\/span>\n    [print(f'{t.url}<\/span>: {t.http_status_code}<\/span>'<\/span>) for<\/span> t in<\/span> threads]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • Extend the Thread<\/code> class and set the instance variables inside the subclass to return the values from a child thread to the main thread.<\/li>\n<\/ul>\n
    \n\t
    \n\t\t
    Was this tutorial helpful ?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
    \n\t\t
    \n\t\t\t
    <\/div>\n\t\t\t