How Can I Duplicate Python\'s sys.stdout to a Log File in Both Interactive and Daemon Modes?

Duplicating sys.stdout to a Log File

Duplicating sys.stdout to a log file allows you to capture all output, including system calls, from a Python application. Despite its simplicity, this task poses a unique challenge when the application operates in both interactive and daemon modes.

Interactive vs. Daemon Mode

In interactive mode, you want both the screen and a log file to receive output. However, in daemon mode, all output should be directed to the log file. The os.dup2() function works well for redirecting output in daemon mode, but it doesn't provide a straightforward solution for duplicating output in interactive mode without modifying system calls.

Duplication Using Tee

To overcome this issue, you can use the Tee class, which provides a way to duplicate stdout to a log file. Here's an example:

class Tee(object):
    def __init__(self, name, mode):
        self.file = open(name, mode)
        self.stdout = sys.stdout
        sys.stdout = self
    def __del__(self):
        sys.stdout = self.stdout
        self.file.close()
    def write(self, data):
        self.file.write(data)
        self.stdout.write(data)
    def flush(self):
        self.file.flush()

This class allows you to create a Tee instance that takes a log file name and mode as parameters. The write() method duplicates the output to both the log file and the stdout.

Implementation Example

To use the Tee class:

with Tee('logfile.txt', 'w') as tee:
    print('Hello world!')
    os.system('ls -la')

In this example, all output, including the system call to 'ls -la', will be written to both the screen and the log file 'logfile.txt'.

This solution effectively logs all output, including system call output, without the need to modify individual system calls.

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