Test: Added testcases for Python tell/truncate

Signed-off-by: Andreas Maier <andreas.r.maier@gmx.de>

tests/unit/test_python_tell.py

@@ -0,0 +1,90 @@
+"""
+Tests for Python tell() behavior.
+
+This test exists because certain uses of tell() behave differently between
+Windows and Unix-like platforms on Python 2.7.
+
+They document these differences, and verify that the approach used in
+test_build_mockenv() works on all platforms.
+"""
+
+from __future__ import absolute_import, print_function
+
+import sys
+import os
+import pytest
+
+
+@pytest.fixture()
+def file_path():
+    """
+    Create a text file with one line, and remove it after the test.
+
+    Returns the file path of the text file.
+    """
+
+    _file_path = 'tmp_file.txt'
+    with open(_file_path, 'w') as fp:
+        fp.write('first line\n')
+
+    # The yield causes the remainder of this fixture to be executed at the
+    # end of the test.
+    yield _file_path
+
+    os.remove(_file_path)
+
+
+def test_tell_append_diff(file_path):
+    # pylint: disable=redefined-outer-name
+    """
+    Test the different behavior of tell() after opening for appending on
+    Python 2 on Windows.
+    """
+
+    size = os.stat(file_path).st_size
+
+    # Get the stream position via tell() and append a line
+    with open(file_path, 'a') as fp:
+        pos = fp.tell()
+        fp.write('second line\n')
+
+    # Remove the line again by truncating the file to the previous stream
+    # position.
+    with open(file_path, 'a') as fp:
+        fp.seek(pos)
+        fp.truncate()
+
+    new_size = os.stat(file_path).st_size
+    if sys.platform == 'win32' and sys.version_info[0] == 2:
+        exp_size = 0
+    else:
+        exp_size = size
+    assert new_size == exp_size, \
+        "original size: {}, tell() result: {}, actual new size: {}, " \
+        "expected new size: {}". \
+        format(size, pos, new_size, exp_size)
+
+
+def test_truncate_append_same(file_path):
+    # pylint: disable=redefined-outer-name
+    """
+    Test the behavior of truncate(size) after opening for appending.
+
+    This is the approach used in test_build_mockenv().
+    """
+
+    size = os.stat(file_path).st_size
+
+    # Append a line
+    with open(file_path, 'a') as fp:
+        fp.write('second line\n')
+
+    # Remove the line again by truncating the file using truncate() with size.
+    with open(file_path, 'a') as fp:
+        fp.truncate(size)
+
+    new_size = os.stat(file_path).st_size
+    exp_size = size
+    assert new_size == exp_size, \
+        "original size: {}, actual new size: {}, expected new size: {}". \
+        format(size, new_size, exp_size)