from mock import mock_open, patch

# works: consume entire "file"
with patch('__main__.open', mock_open(read_data='bibble')) as m:
    with open('foo') as h:
        result = h.read()

assert result == 'bibble'  # ok

# works: consume one line
with patch('__main__.open', mock_open(read_data='bibblenbobble')) as m:
    with open('foo') as h:
        result = h.readline()

assert result == 'bibblen'  # ok

# consume first 3 bytes of the "file"
with patch('__main__.open', mock_open(read_data='bibble')) as m:
    with open('foo') as h:
        result = h.read(3)

assert result == 'bib', 'result of read: {}'.format(result)  # fails

Traceback (most recent call last):
  File "/tmp/t.py", line 25, in <module>
    assert result == 'bib', 'result of read: {}'.format(result)
AssertionError: result of read: bibble