Using readlines() instead of readline() was added in 4dbbf322a9df for performance. But it looks that now this is not needed. Naive implementation with readline() is about 2 times slower, but with careful optimization we can achieve the same performance (or better).

Here are results of benchmarks.

Unpatched:

$ mkdir testdir
$ for i in `seq 10`; do for j in `seq 1000`; do echo "$j"; done >"testdir/file$i"; done
$ ./python -m timeit -s "import fileinput, glob; files = glob.glob('testdir/*')" -- "f = fileinput.input(files)" "while f.readline(): pass"
10 loops, best of 3: 56.4 msec per loop
$ ./python -m timeit -s "import fileinput, glob; files = glob.glob('testdir/*')" -- "list(fileinput.input(files))"10 loops, best of 3: 68.4 msec per loop

Patched:

$ ./python -m timeit -s "import fileinput, glob; files = glob.glob('testdir/*')" -- "f = fileinput.input(files)" "while f.readline(): pass"
10 loops, best of 3: 47.4 msec per loop
$ ./python -m timeit -s "import fileinput, glob; files = glob.glob('testdir/*')" -- "list(fileinput.input(files))"
10 loops, best of 3: 63.1 msec per loop

The patch also fixes original issue.

It also fixes yet one issue. Currently lines are buffered and you need to enter many lines first then get first line:

>>> import fileinput
>>> fi = fileinput.input()
>>> line = fi.readline()
qwerty
asdfgh
zxcvbn
^D
>>> line
'qwerty\n'

With the patch you get the line just as it entered.