Python - 在某个点之后打印语句不会进入 sys.stdout 文件答案

【问题标题】：Python - print statements not going into sys.stdout file after a certain pointPython - 在某个点之后打印语句不会进入 sys.stdout 文件
【发布时间】：2020-10-28 15:52:07
【问题描述】：

我有一个使用 Youtube_dlc 的小型 PyQt5 GUI。我想将一些打印语句放入外部日志文件中进行测试。但是，在某个点之后（在我将视频元数据存储到变量中之后），我的程序不再将打印语句保存到文件中。只有当我在我的应用程序上按 X 并关闭它时，它才会一次保存所有行。但是，当我不将标准输出重定向到文件时，它会像平常一样打印。

这是我的代码：

import youtube_dlc
from audioclipextractor import AudioClipExtractor
from PyQt5 import QtWidgets
import Output
import sys
import os
import datetime

ffmpegpath = "ffmpeg.exe"
sys.stdout = open("test.txt", "w")
open("test.txt", "w").truncate(0)



class youtubeDownloader(QtWidgets.QMainWindow, Output.Ui_MainWindow):
    def __init__(self, parent=None):
        super(youtubeDownloader, self).__init__(parent)
        self.setupUi(self)
        self.button_download.clicked.connect(lambda: self.add_youtubeaudio(url=self.textbox_url.toPlainText(), start=self.textbox_start.toPlainText(), end=self.textbox_end.toPlainText(), file=self.textbox_filename.toPlainText()))

    def add_youtubeaudio(self, url, start=None, end=None, file=None):
        ydl_opts = {

            'format': 'bestaudio/best',
            'postprocessors': [{
                'key': 'FFmpegExtractAudio',
                'preferredcodec': 'mp3',
                'preferredquality': '192',
            }],
        }
        with youtube_dlc.YoutubeDL(ydl_opts) as ydl:
            print("{} # Downloading Youtube video {}".format(datetime.datetime.now(),url))
            ydl.download([url])
            print("{} # Downloading Youtube video {} complete".format(datetime.datetime.now(),url))
            self.textBrowser_Output.setPlainText("{} wurde heruntergeladen".format(file))
            print("{} # Downloading Metadata {}".format(datetime.datetime.now(), url))
            video_info = (ydl.extract_info(url))['id']
            print("{} # Downloading Metadata {} complete".format(datetime.datetime.now(), url))
            for x in os.listdir(os.curdir):
                if str(video_info)+".mp3" in x:
                    videokeep = x
            print("{} # Checking altarnative arguments".format(datetime.datetime.now()))
            if start and end:
                print("{} # Cutting video {} from {} to {}".format(datetime.datetime.now(),url,start,end))
                ext = AudioClipExtractor(str(videokeep), ffmpegpath)
                specs = str(start) + " " + str(end)
                ext.extract_clips(specs)
                print("{} # Cutting video {} from {} to {} complete".format(datetime.datetime.now(),url, start, end))
                try:
                    print("{} # Renaming file to {}".format(datetime.datetime.now(),file))
                    os.rename('clip1.mp3',str(file)+".mp3")
                    print("{} # Renaming file to {} complete".format(datetime.datetime.now(),file))
                    self.textBrowser_Output.setPlainText("{} wurde in clip geschnitten".format(file))
                except:
                    print("{} # Output file {} already exists".format(datetime.datetime.now(),file))
                    self.textBrowser_Output.setPlainText("{} existiert bereits".format(file))
            else:
                try:
                    print("{} # Renaming file to {}".format(datetime.datetime.now(),file))
                    os.rename(videokeep, str(file)+".mp3")
                    print("{} # Renaming file to {} complete".format(datetime.datetime.now(),file))
                except:
                    print("{} # Output file {} already exists".format(datetime.datetime.now(),file))

        if self.checkBox_deletevid.isChecked():
            try:
                os.remove(videokeep)
            except:
                pass




def main():
    app = QtWidgets.QApplication(sys.argv)
    form = youtubeDownloader()
    form.show()
    app.exec_()


if __name__ == '__main__':
    main()

（https://github.com/FlyingThunder/YoutubeDownloader 的完整代表）

像这样运行它并使用默认输入将其保存到文件中：

[youtube] C0DPdy98e4c: Downloading webpage
2020-10-28 16:33:05.376083#Downloading Youtube video https://www.youtube.com/watch?v=C0DPdy98e4c
[download] Destination: TEST VIDEO-C0DPdy98e4c.m4a

[download]   0.4% of 274.02KiB at 167.00KiB/s ETA 00:01
[download]   1.1% of 274.02KiB at 501.01KiB/s ETA 00:00
[download]   2.6% of 274.02KiB at  1.14MiB/s ETA 00:00 
[download]   5.5% of 274.02KiB at  2.10MiB/s ETA 00:00 
[download]  11.3% of 274.02KiB at  1.38MiB/s ETA 00:00 
[download]  23.0% of 274.02KiB at  1.67MiB/s ETA 00:00 
[download]  46.3% of 274.02KiB at  2.22MiB/s ETA 00:00 
[download]  93.1% of 274.02KiB at  3.08MiB/s ETA 00:00 
[download] 100.0% of 274.02KiB at  3.27MiB/s ETA 00:00 
[download] 100% of 274.02KiB in 00:00                  
[ffmpeg] Correcting container in "TEST VIDEO-C0DPdy98e4c.m4a"
[ffmpeg] Destination: TEST VIDEO-C0DPdy98e4c.mp3
Deleting original file TEST VIDEO-C0DPdy98e4c.m4a (pass -k to keep)
[youtube] C0DPdy98e4c: Downloading webpage
2020-10-28 16:33:07.722789#Downloading Youtube video https://www.youtube.com/watch?v=C0DPdy98e4c complete
2020-10-28 16:33:07.722789#Downloading Metadata https://www.youtube.com/watch?v=C0DPdy98e4c
[download] Destination: TEST VIDEO-C0DPdy98e4c.m4a

[download]   0.4% of 274.02KiB at 143.29KiB/s ETA 00:01
[download]   1.1% of 274.02KiB at 429.88KiB/s ETA 00:00
[download]   2.6% of 274.02KiB at 1003.04KiB/s ETA 00:00
[download]   5.5% of 274.02KiB at  1.84MiB/s ETA 00:00  
[download]  11.3% of 274.02KiB at 878.15KiB/s ETA 00:00 
[download]  23.0% of 274.02KiB at  1.24MiB/s ETA 00:00  
[download]  46.3% of 274.02KiB at  1.76MiB/s ETA 00:00  
[download]  93.1% of 274.02KiB at  2.64MiB/s ETA 00:00  
[download] 100.0% of 274.02KiB at  2.75MiB/s ETA 00:00  
[download] 100% of 274.02KiB in 00:00                   
[ffmpeg] Correcting container in "TEST VIDEO-C0DPdy98e4c.m4a"
[ffmpeg] Destination: TEST VIDEO-C0DPdy98e4c.mp3
Deleting original file TEST VIDEO-C0DPdy98e4c.m4a (pass -k to keep)

当注释掉sys.stdout 行时，我在控制台中得到了这个：

2020-10-28 16:46:57.879111 # Downloading Youtube video https://www.youtube.com/watch?v=C0DPdy98e4c
[youtube] C0DPdy98e4c: Downloading webpage
[download] Destination: TEST VIDEO-C0DPdy98e4c.m4a
[download] 100% of 274.02KiB in 00:00                   
[ffmpeg] Correcting container in "TEST VIDEO-C0DPdy98e4c.m4a"
[ffmpeg] Destination: TEST VIDEO-C0DPdy98e4c.mp3
Deleting original file TEST VIDEO-C0DPdy98e4c.m4a (pass -k to keep)
2020-10-28 16:47:00.811446 # Downloading Youtube video https://www.youtube.com/watch?v=C0DPdy98e4c complete
2020-10-28 16:47:00.811446 # Downloading Metadata https://www.youtube.com/watch?v=C0DPdy98e4c
[youtube] C0DPdy98e4c: Downloading webpage
[download] Destination: TEST VIDEO-C0DPdy98e4c.m4a
[download] 100% of 274.02KiB in 00:00                  
[ffmpeg] Correcting container in "TEST VIDEO-C0DPdy98e4c.m4a"
[ffmpeg] Destination: TEST VIDEO-C0DPdy98e4c.mp3
Deleting original file TEST VIDEO-C0DPdy98e4c.m4a (pass -k to keep)
2020-10-28 16:47:02.566841 # Downloading Metadata https://www.youtube.com/watch?v=C0DPdy98e4c complete
2020-10-28 16:47:02.566841 # Checking altarnative arguments
2020-10-28 16:47:02.566841 # Cutting video https://www.youtube.com/watch?v=C0DPdy98e4c from 5 to 7
2020-10-28 16:47:02.715639 # Cutting video https://www.youtube.com/watch?v=C0DPdy98e4c from 5 to 7 complete
2020-10-28 16:47:02.715639 # Renaming file to test
2020-10-28 16:47:02.717630 # Renaming file to test complete

为什么我不使用sys.stdout时，最后几条打印语句没有立即保存到文件中，而是正常打印出来？

【问题讨论】：

标签： python stdout youtube-dl

【解决方案1】：

我没有分析你的代码，但它听起来就像你被文件的缓冲行为所困扰。进入终端的东西通常是行缓冲的，但是当你打开一个磁盘文件时，它会被缓冲成更大的块。

参见open 的buffering 参数

【讨论】：