How to stop iterating using itertools.islice when EOF is reached

Question

I would like to use itertools.islice(self._f, 0, 100, None) to read in a file piece by piece (in blocks of 100 lines) as follows:

f = open('test.dat', 'r')

while (some condition I look for):
    f = open(fileName, 'r') 

    x = itertools.islice(f, 0, 100, None)

    doSomethingWithX(x)

My problem is, I do not know how long the file is and I am looking for a condition to stop the while loop when the end of the file is reached. But I cannot figure out how it is done.

EDIT: Ok, I see the difficulty. Maybe I should reformulate the question when the itertools.islice is capsuled in a class like here:

class reader:
    def __init__()
        self._f = open('test.dat', 'r')

    def getNext():
        return itertools.islice(self._f, 0, 100, None)




R = reader()
while (some condition I look for):
   x = R.getNext()
   doSomethingWithX(x)

Answer 1

If you don't mind getting list slices, you can use iter:

with  open(filename, 'r')  as f:
    for x in iter(lambda: list(itertools.islice(f, 100)), []):
        doSomethingWithX(x)

Not sure which file you are using as you have f = .. twice and have self_.f in there too.

Using your edited code:

class reader:
    def __init__(self):
        self._f = open('out.csv', 'r')

    def getNext(self):
        return itertools.islice(self._f, 100)

R = reader()
import itertools
for x in iter(lambda: list(R.getNext()),[]):
    print(x)

using a test file with the following and your class code using itertools.islice(self._f, 2):

1
2
3
4
5
6
7
8
9
10

outputs:

In [15]: R = reader()

In [16]: import itertools

In [17]: for x in iter(lambda: list(R.getNext()),[]):
   ....:         print(x)
   ....:     
['1\r\n', '2\r\n']
['3\r\n', '4\r\n']
['5\r\n', '6\r\n']
['7\r\n', '8\r\n']
['9\r\n', '10']