cli/lib/test/test_util.py

#
# Copyright (C) 2016 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

"""Utility functions and classes to help with unit tests."""


import StringIO
import sys


class OutputCapture(object):
    """Captures stdout/stderr for easy access to what was printed.

    Examples:
        # Easiest is to use this as a context manager.
        with stubs.OutputCapture() as output:
            print('foo')
            print('bar', file=sys.stderr)
        self.assertEqual('foo\n', output.stdout)
        self.assertEqual('bar\n', output.stderr)

        # Can also be used manually if needed.
        output = stubs.OutputCapture()
        output.replace_sys_streams()
        ...
        output.restore_sys_streams()
    """

    def __init__(self):
        self._replacement_streams = StringIO.StringIO(), StringIO.StringIO()
        self._saved_streams = None, None

    @property
    def stdout(self):
        """Returns the stdout string."""
        return self._replacement_streams[0].getvalue()

    @property
    def stderr(self):
        """Returns the stderr string."""
        return self._replacement_streams[1].getvalue()

    def replace_sys_streams(self):
        """Replaces sys.stdout and sys.stderr."""
        self._saved_streams = sys.stdout, sys.stderr
        sys.stdout, sys.stderr = self._replacement_streams

    def restore_sys_streams(self):
        """Restores saved sys.stdout and sys.stderr."""
        sys.stdout, sys.stderr = self._saved_streams

    def __enter__(self):
        self.replace_sys_streams()
        return self

    def __exit__(self, exc_type, value, traceback):
        self.restore_sys_streams()