Why does readline() not work after readlines()?

Question

In Python, say I have:

f = open("file.txt", "r")
    a = f.readlines()
    b = f.readline()
    print a
    print b

print a will show all the lines of the file and print b will show nothing.

Similarly vice versa:

f = open("file.txt", "r")
    a = f.readline()
    b = f.readlines()
    print a
    print b

print a shows the first line but print b will show all lines except the first one.

If both a and b are readlines(), a will show all the lines and b will show nothing.

Why does this happen? Why can't both commands work independently of each other? Is there a workaround for this?

Answer 1

Because doing .readlines() first will consume all of the read buffer leaving nothing behind for .readline() to fetch from. If you want to go back to the start, use .seek(0) as @abccd already mentioned in his answer.

>>> from StringIO import StringIO
>>> buffer = StringIO('''hi there
... next line
... another line
... 4th line''')
>>> buffer.readline()
'hi there\n'
>>> buffer.readlines()
['next line\n', 'another line\n', '4th line']
>>> buffer.seek(0)
>>> buffer.readlines()
['hi there\n', 'next line\n', 'another line\n', '4th line']