Redirect stdout to a file in Python?

Navigating stdout redirection

When managing stdout redirection, several aspects should be considered to ensure seamless operation.

The contextlib way for redirection

For Python 3.4 and later versions, the contextlib module provides a neat method:

import contextlib

with open('output.txt', 'w') as f, contextlib.redirect_stdout(f):
    print('Voila! Output.txt now gets all the attention.')
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This context manager automates redirection and restoration, making your code easier to comprehend and less prone to errors.

Handling in older Python versions

If you're indulging in Python versions prior to 3.4, define a custom context manager using contextlib.contextmanager:

from contextlib import contextmanager
import sys

@contextmanager
def magic_redirect(file_name):
    original_stdout = sys.stdout
    with open(file_name, 'w') as file:
        sys.stdout = file
        yield  # Abracadabra
        sys.stdout = original_stdout

with magic_redirect('output.txt'):  # Now you see it
    print('Find me in "output.txt".')  # Now you don't
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Catching stderr and other streams

To sweep streams other than stdout off their feet, replace sys.stdout with sys.stderr or any other dashing stream:

sys.stderr = open('error_log.txt', 'w')  # stderr's new love interest
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Avoiding the tricky parts

Fair warning - some libraries might write straight to the system's core stdout, bypassing Python's swoon-worthy output capturing. Test the behavior if you're using external modules.

Flush buffers with sys.stdout.flush() to ensure all content is written before redirection. Why keep the data waiting, especially in Python 3 where C stdio buffers must be handled correctly?

Diving deeper: Advanced redirection techniques

Beyond basic redirections, there are some expert-level techniques and considerations.

File descriptors: We're going deeper

To redirect stdout at the file descriptor level, you know, to make sure subprocesses also fall in line, use os.dup2():

import os
import sys

original_fd = sys.stdout.fileno()
with open('output.txt', 'w') as f:
    os.dup2(f.fileno(), original_fd)  # Hide and seek level 1000
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Custom stdout class: The master of redirection

For more flexible control, subclass sys.stdout and override the write() method:

import sys

class SmartyStdout:
    # For those who like to customize
    def __init__(self, file):
        self.file = file

    def write(self, message):
        # Your very own control room
        self.file.write(message)

    def flush(self):
        pass  # If needed, otherwise chill

with open('output.txt', 'w') as f:
    original_stdout = sys.stdout
    sys.stdout = SmartyStdout(f)
    try:
        # Redirect party starts now
    finally:
        sys.stdout = original_stdout  # Party's over folks!
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On recovery duty

Don't forget to restore the original stdout after your tasks are done. Leave it just like you found it.

# Now you see the magic
sys.stdout = sys.__stdout__
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