Python输出重定向

  In Python, file objects sys.stdin, sys.stdout and sys.stderr are repectively the standard input stream, the standard output stream and standard error stream of interpreter. At program startup, the initial values of these objects are saved by sys.__stdin__, __sys.stdout__ and sys.__stderr__, for restoring standard stream objects during finalization.

 

1. Console Output Redirection

 In console, output redirection using > and >>, and > represents overwriting content, >> represents adding content.

echo print('hello world') > test.py
test.py > test.txt

2. print Redirection

import sys
print('hello', file=sys.stderr)

3. sys.stdout Redirection

 Assigning a writable object to sys. stdout can cause subsequent print statements to output to that object. After the redirection is completed, sys.stdout should be restored to its original default value, which is standard output.

import sys
savedStdout = sys.stdout  #保存标准输出流
with open('out.txt', 'w+') as file:
    sys.stdout = file  #标准输出重定向至文件
    print('This message is for file!')

sys.stdout = savedStdout  #恢复标准输出流
print(This message is for screen!')

from PyQt5.QtWidgets import QApplication, QMainWindow, QPlainTextEdit
import sys
class Edit(QPlainTextEdit):
    def __init__(self, parent=None):
        super().__init__()
        self.setParent(parent)
        self.setGeometry(0, 0, 200, 200)
        self.appendPlainText('123')
    def write(self, s=None):	#  必须有write函数的对象才可以赋值给sys.stdout
        self.appendPlainText(s)
if __name__ == '__main__':
    app = QApplication(sys.argv)
    std = sys.stdout
    window = QMainWindow()
    window.setGeometry(0, 0, 500, 500)
    edit = Edit(parent=window)
    sys.stdout = edit
    window.show()
    print('yes')
    sys.stdout = std
    print('yes')
    app.exec_()

4. Logger Redirection

from PyQt5.QtWidgets import QApplication, QMainWindow, QPlainTextEdit
import sys
import logging

class Edit(QPlainTextEdit, logging.Handler):
    def __init__(self, parent=None):
        super().__init__()
        self.setParent(parent)
        self.setGeometry(0, 0, 200, 200)
        self.appendPlainText('123')
        self.setReadOnly(True)

        self.setFormatter(logging.Formatter('%(asctime)s - %(levelname)s - %(message)s'))
    def emit(self, s=None):
        s = self.format(s)
        self.appendPlainText(s)
if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = QMainWindow()
    window.setGeometry(0, 0, 500, 500)
    edit = Edit(parent=window)
    logging.getLogger().addHandler(edit)
    logging.getLogger().setLevel(logging.DEBUG)
    window.show()
    logging.info('\nhello')
    app.exec_()

 Because print can be used in every Python code and logging is limitted, then the print output redirection is recommanded.