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How do you read from stdin in Python?

tehryan
1#
tehryan Published in 2009-09-20 05:48:07Z

I'm trying to do some of the code golf challenges, but they all require the input to be taken from stdin. How do I get that in Python?

zahypeti
2#
zahypeti Reply to 2017-12-10 19:22:29Z

Here's from Learning Python:

import sys
data = sys.stdin.readlines()
print "Counted", len(data), "lines."

On Unix, you could test it by doing something like:

% cat countlines.py | python countlines.py 
Counted 3 lines.

On Windows or DOS, you'd do:

C:\> type countlines.py | python countlines.py 
Counted 3 lines.
SilentGhost
3#
SilentGhost Reply to 2015-08-31 17:34:21Z

You could use the fileinput module:

import fileinput

for line in fileinput.input():
    pass

fileinput will loop through all the lines in the input specified as file names given in command-line arguments, or the standard input if no arguments are provided.

Zero Piraeus
4#
Zero Piraeus Reply to 2017-09-11 18:47:00Z

Python also has built-in functions input() and raw_input(). See the Python documentation under Built-in Functions.

For example,

name = raw_input("Enter your name: ")   # Python 2.x

or

name = input("Enter your name: ")   # Python 3
Massimiliano Torromeo
5#
Massimiliano Torromeo Reply to 2013-12-12 10:33:01Z

The answer proposed by others:

for line in sys.stdin:
  print line

is very simple and pythonic, but it must be noted that the script will wait until EOF before starting to iterate on the lines of input.

This means that tail -f error_log | myscript.py will not process lines as expected.

The correct script for such a use case would be:

while 1:
    try:
        line = sys.stdin.readline()
    except KeyboardInterrupt:
        break

    if not line:
        break

    print line

UPDATE
From the comments it has been cleared that on python 2 only there might be buffering involved, so that you end up waiting for the buffer to fill or EOF before the print call is issued.

Simon Liu
6#
Simon Liu Reply to 2012-12-24 06:18:36Z

This will echo standard input to standard output:

import sys
line = sys.stdin.readline()
while line:
    print line,
    line = sys.stdin.readline()
javadba
7#
javadba Reply to 2015-08-24 12:53:25Z

Building on all the anwers using sys.stdin, you can also do something like the following to read from an argument file if at least one argument exists, and fall back to stdin otherwise:

import sys
f = open(sys.argv[1]) if len(sys.argv) > 1 else sys.stdin    
for line in f:
#     Do your stuff

and use it as either

$ python do-my-stuff.py infile.txt

or

$ cat infile.txt | python do-my-stuff.py

or even

$ python do-my-stuff.py < infile.txt

That would make your Python script behave like many GNU/Unix programs such as cat, grep and sed.

Boubakr
8#
Boubakr Reply to 2013-08-27 15:43:43Z

Try this:

import sys

print sys.stdin.read().upper()

and check it with:

$ echo "Hello World" | python myFile.py
Tanveer Alam
9#
Tanveer Alam Reply to 2015-02-26 10:52:10Z

You can read from stdin and then store inputs into "data" as follows:

data = ""
for line in sys.stdin:
    data += line
estani
10#
estani Reply to 2014-10-29 14:57:06Z

I had some issues when getting this to work for reading over sockets piped to it. When the socket got closed it started returning empty string in an active loop. So this is my solution to it (which I only tested in linux, but hope it works in all other systems)

import sys, os
sep=os.linesep

while sep == os.linesep:
    data = sys.stdin.readline()               
    sep = data[-len(os.linesep):]
    print '> "%s"' % data.strip()

So if you start listening on a socket it will work properly (e.g. in bash):

while :; do nc -l 12345 | python test.py ; done

And you can call it with telnet or just point a browser to localhost:12345

rogerdpack
11#
rogerdpack Reply to 2018-02-02 20:01:02Z

The following chip of code will help you (it will read all of stdin blocking unto EOF, into one string):

import sys
input_str = sys.stdin.read()
print input_str.split()
szeitlin
12#
szeitlin Reply to 2016-09-17 09:35:29Z

Regarding this:

for line in sys.stdin:

I just tried it on python 2.7 (following someone else's suggestion) for a very large file, and I don't recommend it, precisely for the reasons mentioned above (nothing happens for a long time).

I ended up with a slightly more pythonic solution (and it works on bigger files):

with open(sys.argv[1], 'r') as f:
    for line in f:

Then I can run the script locally as:

python myscript.py "0 1 2 3 4..." # can be a multi-line string or filename - any std.in input will work
Aaron Hall
13#
Aaron Hall Reply to 2017-10-26 17:43:30Z

How do you read from stdin in Python?

I'm trying to do some of the code golf challenges, but they all require the input to be taken from stdin. How do I get that in Python?

You just need to read from sys.stdin.

TLDR: compatible with Python 2 and 3, Windows, Unix

Say you have a file, inputs.txt, we can accept that file and write it back out:

python -c "import sys; sys.stdout.write(sys.stdin.read())" < inputs.txt

Longer answer

Here's a complete, easily replicable demo, using two methods, the builtin function, input (use raw_input in Python 2), and sys.stdin. The data is unmodified, so the processing is a non-operation.

To begin with, let's create a file for inputs:

$ python -c "print('foo\nbar\nbaz')" > inputs.txt

And using the code we've already seen, we can check that we've created the file:

$ python -c "import sys; sys.stdout.write(sys.stdin.read())" < inputs.txt 
foo
bar
baz

Here's the help on sys.stdin.read from Python 3:

read(size=-1, /) method of _io.TextIOWrapper instance
    Read at most n characters from stream.

    Read from underlying buffer until we have n characters or we hit EOF.
    If n is negative or omitted, read until EOF.

Builtin function, input (raw_input in Python 2)

The builtin function input reads from standard input up to a newline, which is stripped (complementing print, which adds a newline by default.) This occurs until it gets EOF (End Of File), at which point it raises EOFError.

Thus, here's how you can use input in Python 3 (or raw_input in Python 2) to read from stdin - so we create a Python module we call stdindemo.py:

$ python -c "print('try:\n    while True:\n        print(input())\nexcept EOFError:\n    pass')" > stdindemo.py 

And let's print it back out to ensure it's as we expect:

$ python -c "import sys; sys.stdout.write(sys.stdin.read())" < stdindemo.py 
try:
    while True:
        print(input())
except EOFError:
    pass

Again, input reads up until the newline and essentially strips it from the line. print adds a newline. So while they both modify the input, their modifications cancel. (So they are essentially each other's complement.)

And when input gets the end-of-file character, it raises EOFError, which we ignore and then exit from the program.

And on Linux/Unix, we can pipe from cat:

$ cat inputs.txt | python -m stdindemo
foo
bar
baz

Or we can just redirect the file from stdin:

$ python -m stdindemo < inputs.txt 
foo
bar
baz

We can also execute the module as a script:

$ python stdindemo.py < inputs.txt 
foo
bar
baz

Here's the help on the builtin input from Python 3:

input(prompt=None, /)
    Read a string from standard input.  The trailing newline is stripped.

    The prompt string, if given, is printed to standard output without a
    trailing newline before reading input.

    If the user hits EOF (*nix: Ctrl-D, Windows: Ctrl-Z+Return), raise EOFError.
    On *nix systems, readline is used if available.

sys.stdin

Here we make a demo script using sys.stdin. The efficient way to iterate over a file-like object is to use the file-like object as an iterator. The complementary method to write to stdout from this input is to simply use sys.stdout.write:

$ python -c "print('import sys\nfor line in sys.stdin:\n    sys.stdout.write(line)')" > stdindemo2.py

Print it back out to make sure it looks right:

$ python -c "import sys; sys.stdout.write(sys.stdin.read())" < stdindemo2.py 
import sys
for line in sys.stdin:
    sys.stdout.write(line)

And redirecting the inputs into the file:

$ python -m stdindemo2 < inputs.txt
foo
bar
baz

Golfed into a command:

$ python -c "import sys; sys.stdout.write(sys.stdin.read())" < inputs.txt
foo
bar
baz

File Descriptors for Golfing

Since the file descriptors for stdin and stdout are 0 and 1 respectively, we can also pass those to open in Python 3 (not 2, and note that we still need the 'w' for writing to stdout).

If this works on your system, it will shave off more characters.

$ python -c "open(1,'w').write(open(0).read())" < inputs.txt
baz
bar
foo

Python 2's io.open does this as well, but the import takes a lot more space:

$ python -c "from io import open; open(1,'w').write(open(0).read())" < inputs.txt 
foo
bar
baz

Addressing other comments and answers

One comment suggests ''.join(sys.stdin) but that's actually longer than sys.stdin.read() - plus Python must create an extra list in memory (that's how str.join works when not given a list) - for contrast:

''.join(sys.stdin)
sys.stdin.read()

The top answer suggests:

import fileinput

for line in fileinput.input():
    pass

But, since sys.stdin implements the file API, including the iterator protocol, that's just the same as this:

import sys

for line in sys.stdin:
    pass

Another answer does suggest this. Just remember that if you do it in an interpreter, you'll need to do Ctrl-d if you're on Linux or Mac, or Ctrl-z on Windows (after Enter) to send the end-of-file character to the process. Also, that answer suggests print(line) - which adds a '\n' to the end - use print(line, end='') instead (if in Python 2, you'll need from __future__ import print_function).

The real use-case for fileinput is for reading in a series of files.


PS I spent a lot of effort trying to make these demos work cross-platform. The Python is consistent, but the shell entries are not very flexible. However, making this accessible to people on Windows as well as Linux was important to me. Let me know if you have issues with Windows.

anatoly techtonik
14#
anatoly techtonik Reply to 2016-08-14 05:12:41Z

Read from sys.stdin, but to read binary data on Windows, you need to be extra careful, because sys.stdin there is opened in text mode and it will corrupt \r\n replacing them with \n.

The solution is to set mode to binary if Windows + Python 2 is detected, and on Python 3 use sys.stdin.buffer.

import sys

PY3K = sys.version_info >= (3, 0)

if PY3K:
    source = sys.stdin.buffer
else:
    # Python 2 on Windows opens sys.stdin in text mode, and
    # binary data that read from it becomes corrupted on \r\n
    if sys.platform == "win32":
        # set sys.stdin to binary mode
        import os, msvcrt
        msvcrt.setmode(sys.stdin.fileno(), os.O_BINARY)
    source = sys.stdin

b = source.read()
Uri Goren
15#
Uri Goren Reply to 2016-11-07 10:15:59Z

I am pretty amazed no one had mentioned this hack so far:

python -c "import sys;print (''.join([l for l in sys.stdin.readlines()]))"

compatible with both python2 and python3

Py_minion
16#
Py_minion Reply to 2016-11-21 22:12:15Z

One option is to use input function as shown below. There is also raw_input method if you are not sure about the data type.

#!/usr/bin/env python

number = input("Please enter a number : ")
print("Entered number is %d" % number)

Running the program:

python test1.py

Please enter a number : 55
Entered number is 55

Reference: http://www.python-course.eu/input.php

SummerBreeze
17#
SummerBreeze Reply to 2017-11-02 08:49:46Z

The problem I have with solution

import sys

for line in sys.stdin:
    print(line)

is that if you don't pass any data to stdin, it will block forever. That's why I love this answer: check if there is some data on stdin first, and then read it. This is what I ended up doing:

import sys
import select

# select(files to read from, files to write to, magic, timeout)
# timeout=0.0 is essential b/c we want to know the asnwer right away
if select.select([sys.stdin], [], [], 0.0)[0]:
    help_file_fragment = sys.stdin.read()
else:
    print("No data passed to stdin", file=sys.stderr)
    sys.exit(2)
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