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Read user input (STDIN) in Python

David Y.

The Problem

In a Python script, how can I read from standard input (stdin)? For example, I want to prompt the user for their name and use it later on in my code.

The Solution

Python provides a few methods for reading from stdin in different contexts. The simplest and most commonly used is the default input function, which prompts the user to enter a line into stdin and returns it as a string once they press Enter. For example:

name = input("Enter your name: ")
print(f"Hi {name}!")

Running this script will look like this:

$ python main.py
Enter your name: Jane Doe
Hi Jane Doe!

If we want to read multiple lines from stdin and don’t need to provide a prompt, we can use sys.stdin from Python’s built-in sys module. This allows us to treat stdin like a file. For example:

import sys

for line in sys.stdin:
    print(f"Received: {line.strip()}")

We could then pipe a file (lines.txt) into our script:

python stdin_reader.py < lines.txt

For a lines.txt with the following content:

Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
The sky above the port was the color of television, tuned to a dead channel.
It was a bright cold day in April, and the clocks were striking thirteen.

We would get this output:

Received: Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
Received: The sky above the port was the color of television, tuned to a dead channel.
Received: It was a bright cold day in April, and the clocks were striking thirteen.

As with a file, we could also read all the data at once instead of going line by line, using sys.stdin.read:

import sys

lines = sys.stdin.read()
print(f"Received:\n{lines}")

Using lines.txt again, this would produce the output below:

Received:
Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
The sky above the port was the color of television, tuned to a dead channel.
It was a bright cold day in April, and the clocks were striking thirteen.

Note that in both cases, we could enter lines into stdin interactively as we would with input instead of piping a file into the script. If we run our script without piped input, it will wait for us to enter something into stdin. We can then type or paste in multiple lines, finishing with Ctrl-D:

$ python stdin_reader.py
Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
The sky above the port was the color of television, tuned to a dead channel.
It was a bright cold day in April, and the clocks were striking thirteen.
^D
Received:
Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
The sky above the port was the color of television, tuned to a dead channel.
It was a bright cold day in April, and the clocks were striking thirteen.

Finally, sys.argv provides a list of command line arguments passed to our script upon execution, starting with the script name. For example, the script below will print a list of its arguments:

import sys

print(sys.argv)

We could use it as follows:

$ python args.py 1 2 3 4
['args.py', '1', '2', '3', '4']

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