Mocking "the filesystem" by stubbing built-in open() in Python

listshellaliases.py

"""
Example function that reads a file and does something with it.
"""
import re
import sys


def list_shell_aliases(script):
    """Find all command aliases defined in a shell script.
    Aliases are created though the ``alias`` command::

        alias foo='bar -x'

    :param script: Path to shell script file
    :return: Dictionary mapping alias name to its definition
    """
    with open(script, 'r') as f:
        script_code = f.read()

    alias_re = re.compile(r'''
        ^ \s* alias \s+ (\w+) = ['"](.*)['"] $
    ''', re.VERBOSE | re.MULTILINE)

    return dict((m.group(1), m.group(2))
                for m in alias_re.finditer(script_code))

test_listshellaliases.py

#!/usr/bin/env python

try:
    import __builtin__ as builtins  # Python 2.x
except ImportError:
    import builtins
import os
import sys

from mocktest import mock, TestCase, when



def mock_file_read(filename, content):
    fp = mock('open_result')
    old__open = open
    when(builtins).open.then_return(old__open)
    when(builtins).open(filename, 'r').then_return(fp)

    when(fp).__enter__.then_return(fp)
    when(fp).read().then_return(content)
    when(fp).__exit__.then_return(False)  # don't squelch exceptions



import listshellaliases as __unit__


class ListShellAliases(TestCase):
    SCRIPT = '~/.fooshrc'

    def test_empty_script(self):
        mock_file_read(self.SCRIPT, '')
        self.assertEquals({}, __unit__.list_shell_aliases(self.SCRIPT))

    def test_no_aliases(self):
        mock_file_read(self.SCRIPT, os.linesep.join([
            'if [ -f ~/.bash_aliases ]; then',
            '    source ~/.bash_aliases',
            'fi',
        ]))
        self.assertEquals({}, __unit__.list_shell_aliases(self.SCRIPT))

    def test_one(self):
        mock_file_read(self.SCRIPT, os.linesep.join([
            'alias foo="bar -x"',
        ]))
        self.assertEquals({'foo': 'bar -x'},
                          __unit__.list_shell_aliases(self.SCRIPT))


if __name__ == '__main__':
    import unittest
    unittest.main()