Indentation error.  Also no need to store `f` on the instance.  This should probably be rewritten as:

```python
def read_wwinp(self, filename):
    """ This method creates a Wwinp object from the WWINP file <filename>.
    """
    with open(filename, 'r') as f: 
         self._read_block1(f)
         self._read_block2(f)
         self._read_block3(f)
```

You'll need to modify the signatures of the `_read_blockX()` to now accept a file handle.