WinXP, 3.0b2
>>>f=open('somefile', 'r')
>>>f.readline(somefloat)
Traceback (most recent call last):
  File "<pyshell#70>", line 1, in <module>
    f.readline(1.0)
  File "C:\Program Files\Python30\lib\io.py", line 1766, in readline
    return line[:endpos]
TypeError: slice indices must be integers or None or have an __index__
method

At this point, any f.read or f.readline calls fail with a similar
TypeError.  The error recovery seems incomplete.  The same does *not*
happen with f.read(float).  Recovery is complete and subsequent
f.read/f.readline calls work.

In 2.5, float size arg worked with a deprecation warning, so issue does
not arise.  I presume same is true in 2.6.

Good catch!

This looks pretty simple to fix; I think a type check is missing
(actually several, there are at least two implementations in io.py).

In fact, the 2.6 version of io.py already has the necessary checks. 
(But merging those forward is not 100% trivial since in 3.0 the 'long'
type doesn't exist.)

Patch attached, suggested test below.

def test_readline():
    for mode in ('r', 'rb', 'r+', 'r+b'):
        f = open(__file__, mode)
        try:
            f.readline(0.1)
        except TypeError:
            tmp = f.readline()
        f.close()
    print('OK')

test_readline()