How to Redirect Input from a File to Standard Input Stream in Python

Python’s Standard Input Stream

The standard input stream in Python, stdin, is what receives input data by default, like from input() or console input. This allows a user to dynamically provide input to your script.

Under the hood, stdin is a file-like object that reads from the keyboard input buffer. But you can swap out where it reads from to any file by redirecting stdin.

The sys Module

Python’s sys module contains attributes for accessing standard streams like stdin, stdout and stderr.

To redirect stdin input to a file, you need to import sys and access the stdin attribute:

import sys

input_stream = sys.stdin

Now input_stream represents the stdin file object you can redirect.

Opening the File to Redirect

To redirect a file’s contents to stdin, first open the file using Python’s built-in open() function:

file = open('data.txt')

This opens data.txt for reading input.

Redirecting the File to sys.stdin

Pass the opened file object to sys.stdin using:

sys.stdin = file

Now any input read from stdin will come from data.txt instead of manual input!

Reading from the Redirected Stdin

With the redirection set up, you can read lines using .readline() as if from a user:

print(sys.stdin.readline())

This will print the first line of data.txt.

For automated batch processing, you can iterate over all lines:

for line in sys.stdin:
  print(line)

# Process line

This lets you transparently process file input as if it were manual input to your Python program.

Resetting Standard Input

To revert stdin to keyboard input, you can close the file and reset sys.stdin:

file.close()
sys.stdin = sys.__stdin__

Now stdin is restored for interactive user input.

Full Example Code

Here is complete code to demonstrate file input redirection in Python:

import sys

print('Stdin before:', sys.stdin)

file = open('data.txt')
print('Redirecting file to stdin') 

sys.stdin = file
print('Stdin after:', sys.stdin) 

print(sys.stdin.readline())

for line in sys.stdin:
  print(line)

print('Closing file and resetting stdin')

file.close()  
sys.stdin = sys.__stdin__

print('Stdin after reset:', sys.stdin)

print('Type some manual input:')
manual_input = input() # Now from keyboard again

This shows the full process of redirecting a file to stdin, reading the redirected input, then resetting it back to normal.

Key Takeaways

The key points to remember about redirecting input from a file to stdin in Python are:

• Import sys to access stdin for redirection
• Open the file with open() in read mode
• Redirect using sys.stdin = file_object
• Read redirected input with sys.stdin
• Reset stdin using sys.stdin = sys.__stdin__ after

Redirecting file input to stdin is extremely useful for automating Python scripts and decoupling input sources from code. Master input redirection in Python to write flexible, batchable programs ready for real-world data pipelines.