Python Redirect Input from a File to Standard Input Stream

Redirecting Input from a File

By default, Python reads input from what is called the “standard input stream” – this is normally input typed by the user in the console. The key idea of redirecting standard input is to change where that input comes from to read from a file rather than directly from keyboard input.

Some ways this can be done in Python include:

• Use the sys.stdin variable to open a file for reading to feed its contents into input() calls
• Utilize the fileinput module to simplify redirection using its helpers

Either approach allows input to be smoothly redirected to a script from a text file as needed.

Why Redirect Standard Input?

There are a few reasons why file input redirection can be very helpful:

• Automation – Scripts can run without human interaction by sourcing all input from files
• Testing – Developer can pass different input cases via files to rigorously test their code
• Batch processing – Redirect files into a script to repeatably transform input in bulk

By taking input from files rather than keyboard entry, scripts and programs can run more flexibly and powerfully. Output can also be redirected in similar ways as needed.

Using sys.stdin

Python’s sys module contains system specific parameters and functions, including stdin which is the standard input stream.

By opening a file and assigning it to sys.stdin, anything that reads from input() will now pull data from that file rather than asking the user to type input.

Here is a simple code demonstration:

import sys

test_file = open("test_input.txt")  
sys.stdin = test_file  

input_data = input("Enter input: ") 
print(input_data)

test_file.close()

What happens above:

1. Open our input text file and store reference
2. Redirect sys.stdin to open file handle
3. When input() is called, text input will be read from file
4. Close file when done

Now input serves data from the external file rather than asking the user directly!

Using fileinput Module

Python’s fileinput module offers a more elegant way to handle standard input redirection and supports additional handy features like iterating through multiple input files automatically.

Using fileinput is simple:

import fileinput

for line in fileinput.input(["test1.txt", "test2.txt"]):
    process(line)

This loops through contents of test1.txt and then test2.txt printing each line. Any prompts that rely on input() rather than direct handling of sys.stdin will also now source their data from those files.

Some ways fileinput improves on directly using sys.stdin:

• Automatically open and close files
• Read multiple files in sequence
• Support different modes like stdin, pipes, compressed files
• Helper functions like filename(), lineno()

In general, fileinput offers a more robust way to handle standard input redirection in Python.

Code Examples

Let’s take a look at some examples to demonstrate file input redirection in action.

Basic File Redirect Example

Here is simple script that expects user input using sys.stdin:

#!/usr/bin/env python3
import sys

print("This script expects input")

input1 = input("Enter input> ") 

print("Thanks!", input1)

Normally this would wait and read input typed by the user.

We can redirect to a file input.txt instead:

input.txt

Hello from file!

script.py

#!/usr/bin/env python3
import sys

input_file = open("input.txt")
sys.stdin = input_file  

print("This script expects input")

input1 = input("Enter input> ")  

print("Thanks!", input1)  

input_file.close()

Now when run, it will read the redirected file input rather than asking the user to type input!

Fileinput Module Example

For a simple demonstration of the fileinput module:

test1.txt:

First file line 1
First file line 2

test2.txt:

Second file line 1
Second file line 2

readfiles.py:

#!/usr/bin/env python3 

import fileinput

for line in fileinput.input():
    print(f"Read: {line}", end="")

print("\nAll done!")

Running this script prints:

Read: First file line 1
Read: First file line 2
Read: Second file line 1 
Read: Second file line 2

All done!

It loops through each line across both files!

The power here is simply calling fileinput.input() enables easy multi-file handling and input reading automatically.

Handling Both File and Keyboard Input

A common issue with input redirection is it prevents any interactive keyboard input. Sometimes it is helpful to handle both files and keyboard input.

The tee module allows this by merging an input file with additional keyboard input from the user:

from tee import stdin_all

with stdin_all() as input:
    for line in input:
        print(line)

The stdin_all handler will yield lines from any file passed to the script via standard input redirection. It will also wait and allow a user to type additional lines in the console before resuming reading file lines.

This offers the flexibility to redirect a file and still allow interactive user input during execution.

Conclusion

Being able to redirect standard input from a text file in Python enables scripts to automate based on file contents and removes reliance on constant interactive user input. Knowledge of input manipulation unlocks additional capabilities.

Key takeaways include:

• Use sys.stdin file handling to redirect text files to input()
• The fileinput module makes redirection easy with support for multiple files
• Handle both redirected input and keyboard input using tee.stdin_all()

Input redirection vastly expands the flexibility of Python programs and automation scripts. Both sys.stdin and fileinput approaches have their tradeoffs to factor when handling standard input from text files.

This content helped explain the basics of input redirection in Python to run programs more robustly and automate behavior. Please leave any questions or feedback on ways I can improve in the comments!