pytest with monkeypatch __buildin__.open

conftest.py

import __builtin__
from StringIO import StringIO
import os
import ConfigParser
    
import pytest
 
class MockFileManager():
    def __init__(self):
        self.files = {}
        self._open = __builtin__.open
 
    def open(self, name, mode='r', buffering=-1, **options):
        name = os.path.abspath(name)
        if mode.startswith('r') and name not in self.files:
            # We have to let some files through
            return self._open(name, mode, buffering, **options)  
            # This causes stracktraces not to display          
            #raise IOError(2, "No such file or directory: '%s'" % name)
 
        if mode.startswith('w') or \
           (mode.startswith('a') and name not in self.files):
            buf = StringIO()
            buf.close = lambda: None
            self.files[name] = buf
 
        buf = self.files[name]
 
        if mode.startswith('r'):
            buf.seek(0)
        elif mode.startswith('a'):
            buf.seek(0)
 
        return buf
 
    def write(self, name, text):
        name = os.path.abspath(name)
        buf = StringIO(text)
        buf.close = lambda: None
        self.files[name] = buf
 
    def read(self, name):
        name = os.path.abspath(name)
        if name not in self.files:
            raise IOError(2, "No such file or directory: '%s'" % name)
 
        return self.files[name].getvalue()
 
@pytest.fixture
def mockopen(monkeypatch):
    manager = MockFileManager()
    monkeypatch.setattr(__builtin__, 'open', manager.open)
    return manager

test_monkeypatch_open.py

import pytest

simpleconfig = """[section]\nkey = value\n\n"""

def test_monkeypatch_open_read(mockopen):
    filename = 'somefile.txt'
    mockopen.write(filename, simpleconfig)
 
    parser = ConfigParser.ConfigParser()
    parser.read(filename)
    assert parser.sections() == ['section']
 
def test_monkeypatch_open_write(mockopen):
    parser = ConfigParser.ConfigParser()
    parser.add_section('section')
    parser.set('section', 'key', 'value')
 
    filename = 'somefile.txt'
    parser.write(open(filename, 'wb'))
    assert mockopen.read(filename) == simpleconfig