Read standard input in Python

Use input() for a single prompted line, iterate over sys.stdin for a stream of lines, and use fileinput when a command should accept either filenames or standard input.

Read one line

name = input("Name: ")
print(f"Hello, {name}")

input() returns a string without the trailing newline and raises EOFError when it reaches end of input without reading data. It also works with redirected input, although a prompt is usually undesirable in a pipeline.

Convert numeric input explicitly with int() or float(), catching ValueError when the input is invalid. For passwords, use getpass.getpass() rather than a normal prompt; it attempts to disable echo and warns if the environment cannot support that mode.

Stream lines from a pipe

Save this as filter.py:

import sys

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

Run it with:

cat access.log | python3 filter.py | sort | uniq -c

This processes complete lines as they arrive. It can still wait for a newline or EOF, and upstream buffering can delay delivery.

If the input is small and you need it all at once, use this alternative:

import sys

data = sys.stdin.read()

Do not call read() and then expect a following loop to see the same data: read() has already consumed it. An unbounded read waits for EOF, such as when the producer closes its pipe. In a POSIX terminal, Ctrl-D can signal EOF; behavior varies by terminal and platform.

Accept filenames as well as stdin

import fileinput

with fileinput.input(encoding="utf-8") as lines:
    for line in lines:
        print(line, end="")

With no file arguments, fileinput.input() reads stdin. With arguments, it reads those files in order; a filename of - denotes stdin. The encoding argument is available in Python 3.10 and later.

Detect a terminal

import sys

if sys.stdin.isatty():
    print("Input is connected to a terminal", file=sys.stderr)

isatty() reports whether the stream is connected to a terminal. A false result does not prove that a pipe contains data or that reading will not block; redirected files and other stream types also return false.

References

• Python built-in functions
• Python fileinput reference
• Python password input