What is the perfect counterpart in Python for “while not EOF”

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Question :

What is the perfect counterpart in Python for “while not EOF”

To read some text file, in C or Pascal, I always use the following snippets to read the data until EOF:

while not eof do begin

Thus, I wonder how can I do this simple and fast in Python?

Answer #1:

Loop over the file to read lines:

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

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:

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''):

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.

Answered By: Martijn Pieters

Answer #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:

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:

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:

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

 while line := f.readline():
Answered By: dawg

Answer #3:

The Python idiom for opening a file and reading it line-by-line is:

with open('filename') as f:
    for line in f:

The file will be automatically closed at the end of the above code (the with construct takes care of that).

Finally, it is worth noting that line will preserve the trailing newline. This can be easily removed using:

line = line.rstrip()
Answered By: NPE

Answer #4:

You can use below code snippet to read line by line, till end of file

line = obj.readline()
while(line != ''):

    # Do Something

    line = obj.readline()
Answered By: A R

Answer #5:

While there are suggestions above for “doing it the python way”, if one wants to really have a logic based on EOF, then I suppose using exception handling is the way to do it —

    line = raw_input()
    ... whatever needs to be done incase of no EOF ...
except EOFError:
    ... whatever needs to be done incase of EOF ...


$ echo test | python -c "while True: print raw_input()"
Traceback (most recent call last):
  File "<string>", line 1, in <module> 
EOFError: EOF when reading a line

Or press Ctrl-Z at a raw_input() prompt (Windows, Ctrl-Z Linux)

Answered By: user5472996

Answer #6:

In addition to @dawg’s great answer, the equivalent solution using walrus operator (Python >= 3.8):

with open(filename, 'rb') as f:
    while buf := f.read(max_size):
Answered By: Infinity

Answer #7:

You can use the following code snippet. readlines() reads in the whole file at once and splits it by line.

line = obj.readlines()
Answered By: Aditeya Pandey

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