How to read line by line from stdin in python

python stdin
Storm-Eyes picture Storm-Eyes · Dec 28, 2013 · Viewed 47.8k times · Source

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)

Then I try to test the program by

python3 counter.py < counter.py

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!

Answer

Tim Pietzcker picture Tim Pietzcker · Dec 28, 2013

It's simpler:

for line in sys.stdin:
    chrCounter += len(line)

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.

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