Python read stdin line by line

How to read line by line from stdin in python

The file-like object sys.stdin is automatically iterated over line by line; if you call .readline() on it, you only read the first line (and iterate over that character-by-character); if you call read() , then you’ll read the entire input into a single string and iterate over that character-by.character.

Solution 2

The answer from Tim Pietzcker is IMHO the correct one. There are 2 similar ways of doing this. Using:

for line in sys.stdin.readlines(): 

The second option is closer to your original code. The difference between these two options is made clear by using e.g. the following modification of the for-loop body and using keyboard for input:

for line in sys.stdin.readlines(): line_len = len(line) print('Last line was', line_len, 'chars long.') chrCounter += len(line) 

If you use the first option ( for line in sys.stdin: ), then the lines are processed right after you hit enter.

If you use the second option ( for line in sys.stdin.readlines(): ), then the whole file is first read, split into lines and only then they are processed.

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Comments

Everyone knows how to count the characters from STDIN in C. However, when I tried to do that in python3, I find it is a puzzle. (counter.py)

import sys chrCounter = 0 for line in sys.stdin.readline(): chrCounter += len(line) print(chrCounter) 

The answer is only the length of the first line «import sys». In fact, the program ONLY read the first line from the standard input, and discard the rest of them. It will be work if I take the place of sys.stdin.readline by sys.stdin.read()

import sys print(len(sys.stdin.read())) 

However, it is obviously, that the program is NOT suitable for a large input. Please give me a elegant solution. Thank you!

Can you explain what do you mean by right after you hit enter ? I have a shell script which prints output line by line, and if I consume that using a pipe ( $ build.sh | my-python-file.py ) using this method for line in sys.stdin: , I do not get anything but the second option works fine.

It is already some time ago, since I answered this question. By «right after you hit enter» I meant «as soon as a newline character is read from standard input, i.e. once the iterator of the file is ready to provide next line to the calling code. I am not able to say why the first approach does not work for your case, but the second one probably waits until all lines are produced and then pipes them to my-python-file.py where they are processed all at once.

Ok, thank you very much. I will investigate why option 1 does not work for me and post here if any clue I get.

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How to Read from stdin in Python

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

1. Using sys.stdin to read from standard input

Python sys module stdin is used by the interpreter for standard input. Internally, it calls the input() function. The input string is appended with a newline character (\n) in the end. So, you can use the rstrip() function to remove it. Here is a simple program to read user messages from the standard input and process it. The program will terminate when the user enters “Exit” message.

import sys for line in sys.stdin: if 'Exit' == line.rstrip(): break print(f'Processing Message from sys.stdin **********') print("Done") 

Hi Processing Message from sys.stdin *****Hi ***** Hello Processing Message from sys.stdin *****Hello ***** Exit Done 

Notice the use of rstrip() to remove the trailing newline character so that we can check if the user has entered “Exit” message or not.

2. Using input() function to read stdin data

We can also use Python input() function to read the standard input data. We can also prompt a message to the user. Here is a simple example to read and process the standard input message in the infinite loop, unless the user enters the Exit message.

while True: data = input("Please enter the message:\n") if 'Exit' == data: break print(f'Processing Message from input() **********') print("Done") 

The input() function doesn’t append newline character to the user message.

3. Reading Standard Input using fileinput module

We can also use fileinput.input() function to read from the standard input. The fileinput module provides utility functions to loop over standard input or a list of files. When we don’t provide any argument to the input() function, it reads arguments from the standard input. This function works in the same way as sys.stdin and adds a newline character to the end of the user-entered data.

import fileinput for fileinput_line in fileinput.input(): if 'Exit' == fileinput_line.rstrip(): break print(f'Processing Message from fileinput.input() **********') print("Done") 

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How to Read From stdin in Python

Reading from stdin (standard input) in Python is a common programming task that helps create interactive I/O programs. Stdin reads keyboard inputs from a user, files, or data streams.

Python offers several different methods to read from stdin depending on the type of problem.

This guide provides three methods to show how to read standard input from stdin in Python.

Method 1: Read From stdin Using sys.stdin

The sys module contains a stdin method for reading from standard input. Use this method to fetch user input from the command line.

Import the sys library and use the stdin method in a for loop. See the example below for a demonstration:

import sys print('Write a message and press Enter: ') for line in sys.stdin: line = line.rstrip() print(f'Message from stdin: ') break

The code does the following:

• Reads user input from the keyboard ( sys.stdin ).
• Removes new lines from the user entry ( rstrtip() ).
• Prints back the entered message by appending an f-string.
• Exits the for loop ( break ).

The program expects user input, prints the entered text stream, and returns to the command line.

Method 2: Read From stdin Using the input() function

The second way to read input from stdin in Python is with the built-in input() function. Enter a descriptive message as a value, and save the output into a variable.

message = input('Enter a message:') print(f'Your name is: ')

The program saves the input from stdin into a variable ( name ) and returns the value.

Method 3: Read from stdin by Using fileinput.input()

The fileinput Python library contains the input() function, which allows reading file names from standard input.

The method allows passing file names as a function or command-line arguments. In both cases, the file’s contents are read as input from stdin. Below is a demonstration of both use cases.

Files as Function Arguments

To pass file names as the fileinput.input() arguments, the files must be located in the same directory as the script.

For example, if the current directory contains two files (file1.txt and file2.txt), use the following code to print the file contents line by line:

import fileinput with fileinput.input(files=('file1.txt', 'file2.txt')) as f: for line in f: print(line)

The with statement helps open a file stream, uses the file names as the function’s input, then goes through each line in the file using a for loop. As a result, the contents of the files print to the console.

Files as Command Line Arguments

An alternative way to read files is to provide the file names as command-line arguments. Use this method when reading different files each time.

The following code shows how to read files provided as command-line arguments:

import fileinput for f in fileinput.input(): print(f)

Save the code and run with the following:

python3 test.py file1.txt file2.txt

The code goes through the list of files and prints the file contents to the console.

You now know three different methods to read input from stdin in Python. Standard input is an essential programming skill to get a user’s input into a program.

Next, learn about file handling in Python or see how to take user input and create a calculator with Python.

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