Python, how to send output to both file and terminal

Question

I want to use Python to send output to both a file log.txt and STDOUT on the terminal. Here is what I have:

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

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

sys.stdout = Logger("log.txt")
print "Hello world !"            #This line is saved in log.txt and STDOUT

This program sends output to the file and stdout. My question is: How did the write function to the file get called?

Answer 1

From the documentation for sys.stdout:

stdout and stderr needn’t be built-in file objects: any object is acceptable as long as it has a write() method that takes a string argument.

Answer 2

More specifically, the print function (in Python 2.X it is still a keyword, but it doesn't matter here) does something like this

import sys
def print(message):
    sys.stdout.write(message)

so that, when you call it it will print your message on sys.stdout. However, if you overwrite sys.stdout with an object containing a .write method, well, it will call that method. That's the magic of duck-typing.