How to Duplicate Python\'s sys.stdout to a Log File, Including System Call Output?

Duplicating sys.stdout to a Log File

In this programming conundrum, the goal is to duplicate all output from a Python application, including system call output, to a separate log file. However, this task has proven challenging for the user.

Initially, the user attempted a solution by reversing the order of dup2 calls, but to no avail. The code below demonstrates their unsuccessful attempt:

import os, sys

so = se = open("a.log", 'w', 0)
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

os.dup2(sys.stdout.fileno(), so.fileno())
os.dup2(sys.stderr.fileno(), se.fileno())

print("foo bar")

os.spawnve("P_WAIT", "/bin/ls", ["/bin/ls"], {})
os.execve("/bin/ls", ["/bin/ls"], os.environ)

With this approach, the file "a.log" should have contained the same output that was displayed on the screen, but it did not.

Solution

Fortunately, there is a solution to this problem. The code below provides a class called Tee that successfully duplicates sys.stdout to a specified log file:

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 the user to create a Tee instance and specify the name and mode of the log file. Subsequently, sys.stdout is redirected to the Tee instance, effectively duplicating all output to the log file while still preserving output on the screen.

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