How do you read from stdin?

Question

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?

Answer 1

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 can use:

• sys.stdin - A file-like object - call sys.stdin.read() to read everything.
• input(prompt) - pass it an optional prompt to output, it reads from stdin up to the first newline, which it strips. You'd have to do this repeatedly to get more lines, at the end of the input it raises EOFError. (Probably not great for golfing.) In Python 2, this is rawinput(prompt).
• open(0).read() - In Python 3, the builtin function open accepts file descriptors (integers representing operating system IO resources), and 0 is the descriptor of stdin. It returns a file-like object like sys.stdin - probably your best bet for golfing. In Python 2, this is io.open.
• open('/dev/stdin').read() - similar to open(0), works on Python 2 and 3, but not on Windows (or even Cygwin).
• fileinput.input() - returns an iterator over lines in all files listed in sys.argv[1:], or stdin if not given. Use like ''.join(fileinput.input()).

Both sys and fileinput must be imported, respectively, of course.

Quick sys.stdin examples compatible with Python 2 and 3, Windows, Unix

You just need to read from sys.stdin, for example, if you pipe data to stdin:

$ echo foo | python -c "import sys; print(sys.stdin.read())"
foo

We can see that sys.stdin is in default text mode:

>>> import sys
>>> sys.stdin
<_io.TextIOWrapper name='' mode='r' encoding='UTF-8'>

file example

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) for golfing 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.