Python stdin – Taking input

There are a number of ways of taking input in Python, most commonly through the input() function. However, for some use cases you may prefer other methods which work slightly differently. One of these other methods is the Python stdin, which is a part of the sys module.

The unique feature about Python stdin is that it allows you to take input directly from the command prompt. You’ll find these options useful on Linux systems in particular.

Using Python Stdin

The first thing to do is import sys from the standard library.

import sys

No need to download this library separately, as its part of the standard library (which comes bundled with Python).

There are several ways we can use sys.stdin in Python, so let’s take a look at them one by one.

import sys

for line in sys.stdin:
    print(line)

The above technique allows you to continuously input data into the Python Program from the command prompt or IDE terminal. All you have to do is press enter for every line/word you wrote to be sent back to the program, where it will be printed out.

This technique is a bit tricky, but it’s a pretty useful way of taking input. stdin can accept multiple lines, without having to type them all out first. If you press enter, Python stdin will return the currently entered data then simply move to the next line, waiting for more input, instead of exiting.

You can only break out of this infinite input loop by stdin when the CTRL + C keys are pressed. (Some of you may have to use CTRL + Z, depending on your OS).

Python stdin automatically adds a “\n” newline character after taking input from the user. Right now our input looks a bit off, because if we print it out we end up with a blank line in between. This is because print() also adds its own newline character by default.

We can change this behavior by modifying the print function as shown below.

import sys

for line in sys.stdin:
    print(line, end="")

Alternatively, you can do the following:

import sys

for line in sys.stdin:
    if 'exit' == line.rstrip():
        break
    print(line)

If the word “exit” is entered, then the loop will be broken. This is useful if you want more control over how you can exit from stdin.

This is similar to the first technique, but instead it reads a whole block of data at once, instead of line by line.

import sys

inp = sys.stdin.read()
print(inp)

Similarly to the previous technique, you can only break out of this when CTRL + C keys are pressed.

Here’s another way of taking input which is useful for counting the number of lines entered. It takes each line and stores it into a list, which it returns back to the program.

import sys

inp = sys.stdin.readlines()
print(len(inp))
print(inp)

Here is the input and the corresponding output for the above code. “Hello World” was the input that we split across two lines, and the last two lines show the output.

Hello 
World
2
['Hello \n', 'World\n']

This marks the end of the Python Stdin Tutorial. Any suggestions or contributions for CodersLegacy are more than welcome. Questions regarding the tutorial content can be asked in the comments section below.