How to duplicate sys.stdout to a log file?

Question

Edit: Since it appears that there\'s either no solution, or I\'m doing something so non-standard that nobody knows - I\'ll revise my question to also ask: What is the best w

Answer 1

The print statement will call the write() method of any object you assign to sys.stdout.

I would spin up a small class to write to two places at once...

import sys

class Logger(object):
    def __init__(self):
        self.terminal = sys.stdout
        self.log = open("log.dat", "a")

    def write(self, message):
        self.terminal.write(message)
        self.log.write(message)  

sys.stdout = Logger()

Now the print statement will both echo to the screen and append to your log file:

# prints "1 2" to  AND log.dat
print "%d %d" % (1,2)

This is obviously quick-and-dirty. Some notes:

• You probably ought to parametize the log filename.
• You should probably revert sys.stdout to if you won't be logging for the duration of the program.
• You may want the ability to write to multiple log files at once, or handle different log levels, etc.

These are all straightforward enough that I'm comfortable leaving them as exercises for the reader. The key insight here is that print just calls a "file-like object" that's assigned to sys.stdout.