What is the perfect counterpart in Python for "while not EOF"

Solution 1:

Loop over the file to read lines:

with open('somefile') as openfileobject:
    for line in openfileobject:
        do_something()

File objects are iterable and yield lines until EOF. Using the file object as an iterable uses a buffer to ensure performant reads.

You can do the same with the stdin (no need to use raw_input():

import sys

for line in sys.stdin:
    do_something()

To complete the picture, binary reads can be done with:

from functools import partial

with open('somefile', 'rb') as openfileobject:
    for chunk in iter(partial(openfileobject.read, 1024), b''):
        do_something()

where chunk will contain up to 1024 bytes at a time from the file, and iteration stops when openfileobject.read(1024) starts returning empty byte strings.

Solution 2:

You can imitate the C idiom in Python.

To read a buffer up to max_size number of bytes, you can do this:

with open(filename, 'rb') as f:
    while True:
        buf = f.read(max_size)
        if not buf:
            break
        process(buf)

Or, a text file line by line:

# warning -- not idiomatic Python! See below...
with open(filename, 'rb') as f:
    while True:
        line = f.readline()
        if not line:
            break
        process(line)

You need to use while True / break construct since there is no eof test in Python other than the lack of bytes returned from a read.

In C, you might have:

while ((ch != '\n') && (ch != EOF)) {
   // read the next ch and add to a buffer
   // ..
}

However, you cannot have this in Python:

 while (line = f.readline()):
     # syntax error

because assignments are not allowed in expressions in Python (although recent versions of Python can mimic this using assignment expressions, see below).

It is certainly more idiomatic in Python to do this:

# THIS IS IDIOMATIC Python. Do this:
with open('somefile') as f:
    for line in f:
        process(line)

Update: Since Python 3.8 you may also use assignment expressions:

 while line := f.readline():
     process(line)

That works even if the line read is blank and continues until EOF.