io module provides two types of in-memory file-like objects. Such objects may be helpful for interacting with interfaces that only support files without the need of creating one. The obvious example is unit-testing.
These two types are BytesIO and StringIO that works with bytes and string respectively.
In : f = StringIO()
In : f.write('first\n')
Out: 6
In : f.write('second\n')
Out: 7
In : f.seek(0)
Out: 0
In : f.readline()
Out: 'first\n'
In : f.readline()
Out: 'second\n'