@@ -55,7 +55,7 @@
 
 __all__ = ('Git',)
 
-if is_win():
+if is_win:
     WindowsError = OSError
 
 if PY3:
@@ -239,7 +239,7 @@ def dict_to_slots_and__excluded_are_none(self, d, excluded=()):
 ## CREATE_NEW_PROCESS_GROUP is needed to allow killing it afterwards,
 # seehttps://docs.python.org/3/library/subprocess.html#subprocess.Popen.send_signal
 PROC_CREATIONFLAGS = (CREATE_NO_WINDOW | subprocess.CREATE_NEW_PROCESS_GROUP
-                      if is_win()
+                      if is_win
                       else 0)
 
 
@@ -630,7 +630,7 @@ def execute(self, command,
         env["LC_ALL"] = "C"
         env.update(self._environment)
 
-        if is_win():
+        if is_win:
             cmd_not_found_exception = WindowsError
             if kill_after_timeout:
                 raise GitCommandError('"kill_after_timeout" feature is not supported on Windows.')
@@ -650,13 +650,13 @@ def execute(self, command,
                          stderr=PIPE,
                          stdout=PIPE if with_stdout else open(os.devnull, 'wb'),
                          shell=self.USE_SHELL,
-                         close_fds=(is_posix()),  # unsupported on windows
+                         close_fds=(is_posix),  # unsupported on windows
                          universal_newlines=universal_newlines,
                          creationflags=PROC_CREATIONFLAGS,
                          **subprocess_kwargs
                          )
         except cmd_not_found_exception as err:
-            raise GitCommandNotFound(str(err))
+            raise GitCommandNotFound('%s: %s' % (command[0], err))
 
         if as_process:
             return self.AutoInterrupt(proc, command)
 
@@ -24,6 +24,9 @@
 )
 
 PY3 = sys.version_info[0] >= 3
+is_win = (os.name == 'nt')
+is_posix = (os.name == 'posix')
+is_darwin = (os.name == 'darwin')
 defenc = sys.getdefaultencoding()
 
 if PY3:
@@ -78,17 +81,6 @@ def __new__(cls, name, nbases, d):
             return meta(name, bases, d)
     return metaclass(meta.__name__ + 'Helper', None, {})
 
-def is_win():
-    return os.name == 'nt'
-
-
-def is_posix():
-    return os.name == 'posix'
-
-
-def is_darwin():
-    return os.name == 'darwin'
-
 
 ## From https://docs.python.org/3.3/howto/pyporting.html
 class UnicodeMixin(object):
 
@@ -137,7 +137,7 @@ def _set_cache_(self, attr):
             # which happens during read-tree.
             # In this case, we will just read the memory in directly.
             # Its insanely bad ... I am disappointed !
-            allow_mmap = (is_win() or sys.version_info[1] > 5)
+            allow_mmap = (is_win or sys.version_info[1] > 5)
             stream = file_contents_ro(fd, stream=True, allow_mmap=allow_mmap)
 
             try:
 
@@ -76,7 +76,7 @@ def run_commit_hook(name, index):
                            stdout=subprocess.PIPE,
                            stderr=subprocess.PIPE,
                            cwd=index.repo.working_dir,
-                           close_fds=(is_posix()),
+                           close_fds=(is_posix),
                            creationflags=PROC_CREATIONFLAGS,)
     stdout, stderr = cmd.communicate()
     cmd.stdout.close()
 
@@ -376,7 +376,7 @@ def __init__(self, repo, name):
         self.repo = repo
         self.name = name
 
-        if is_win():
+        if is_win:
             # some oddity: on windows, python 2.5, it for some reason does not realize
             # that it has the config_writer property, but instead calls __getattr__
             # which will not yield the expected results. 'pinging' the members
 
@@ -370,7 +370,7 @@ def delete_remote(self, remote):
     def _get_config_path(self, config_level):
         # we do not support an absolute path of the gitconfig on windows ,
         # use the global config instead
-        if is_win() and config_level == "system":
+        if is_win and config_level == "system":
             config_level = "global"
 
         if config_level == "system":
@@ -884,7 +884,7 @@ def _clone(cls, git, url, path, odb_default_type, progress, **kwargs):
         prev_cwd = None
         prev_path = None
         odbt = kwargs.pop('odbt', odb_default_type)
-        if is_win():
+        if is_win:
             if '~' in path:
                 raise OSError("Git cannot handle the ~ character in path %r correctly" % path)
 
 
@@ -74,7 +74,7 @@ def _mktemp(*args):
     prefixing /private/ will lead to incorrect paths on OSX."""
     tdir = tempfile.mktemp(*args)
     # See :note: above to learn why this is comented out.
-    # if is_darwin():
+    # if is_darwin:
     #     tdir = '/private' + tdir
     return tdir
 
@@ -84,7 +84,7 @@ def _rmtree_onerror(osremove, fullpath, exec_info):
     Handle the case on windows that read-only files cannot be deleted by
     os.remove by setting it to mode 777, then retry deletion.
     """
-    if is_win() or osremove is not os.remove:
+    if is_win or osremove is not os.remove:
         raise
 
     os.chmod(fullpath, 0o777)
@@ -141,7 +141,7 @@ def repo_creator(self):
 
 
 def launch_git_daemon(temp_dir, ip, port):
-    if is_win():
+    if is_win:
         ## On MINGW-git, daemon exists in .\Git\mingw64\libexec\git-core\,
         #  but if invoked as 'git daemon', it detaches from parent `git` cmd,
         #  and then CANNOT DIE!
@@ -242,7 +242,7 @@ def remote_repo_creator(self):
                         gd.proc.terminate()
                     log.warning('git(%s) ls-remote failed due to:%s',
                                 rw_repo.git_dir, e)
-                    if is_win():
+                    if is_win:
                         msg = textwrap.dedent("""
                         MINGW yet has problems with paths, and `git-daemon.exe` must be in PATH
                         (look into .\Git\mingw64\libexec\git-core\);
 
@@ -118,7 +118,7 @@ def test_with_rw_remote_and_rw_repo(self, rw_repo, rw_remote_repo):
         assert rw_remote_repo.config_reader("repository").getboolean("core", "bare")
         assert os.path.isdir(os.path.join(rw_repo.working_tree_dir, 'lib'))
 
-    @skipIf(sys.version_info < (3,) and is_win(),
+    @skipIf(sys.version_info < (3,) and is_win,
             "Unicode woes, see https://github.com/gitpython-developers/GitPython/pull/519")
     @with_rw_repo('0.1.6')
     def test_add_unicode(self, rw_repo):
@@ -135,7 +135,7 @@ def test_add_unicode(self, rw_repo):
 
         open(file_path, "wb").write(b'something')
 
-        if is_win():
+        if is_win:
             # on windows, there is no way this works, see images on
             # https://github.com/gitpython-developers/GitPython/issues/147#issuecomment-68881897
             # Therefore, it must be added using the python implementation
 
@@ -85,7 +85,7 @@ def test_it_transforms_kwargs_into_git_command_arguments(self):
 
         # order is undefined
         res = self.git.transform_kwargs(**{'s': True, 't': True})
-        assert ['-s', '-t'] == res or ['-t', '-s'] == res
+        self.assertEqual(set(['-s', '-t']), set(res))
 
     def test_it_executes_git_to_shell_and_returns_result(self):
         assert_match('^git version [\d\.]{2}.*$', self.git.execute(["git", "version"]))
@@ -117,7 +117,7 @@ def test_persistent_cat_file_command(self):
         g.stdin.write(b"b2339455342180c7cc1e9bba3e9f181f7baa5167\n")
         g.stdin.flush()
         obj_info_two = g.stdout.readline()
-        assert obj_info == obj_info_two
+        self.assertEqual(obj_info, obj_info_two)
 
         # read data - have to read it in one large chunk
         size = int(obj_info.split()[2])
@@ -127,18 +127,19 @@ def test_persistent_cat_file_command(self):
         # now we should be able to read a new object
         g.stdin.write(b"b2339455342180c7cc1e9bba3e9f181f7baa5167\n")
         g.stdin.flush()
-        assert g.stdout.readline() == obj_info
+        self.assertEqual(g.stdout.readline(), obj_info)
 
         # same can be achived using the respective command functions
         hexsha, typename, size = self.git.get_object_header(hexsha)
         hexsha, typename_two, size_two, data = self.git.get_object_data(hexsha)
-        assert typename == typename_two and size == size_two
+        self.assertEqual(typename, typename_two)
+        self.assertEqual(size, size_two)
 
     def test_version(self):
         v = self.git.version_info
-        assert isinstance(v, tuple)
+        self.assertIsInstance(v, tuple)
         for n in v:
-            assert isinstance(n, int)
+            self.assertIsInstance(n, int)
         # END verify number types
 
     def test_cmd_override(self):
@@ -174,28 +175,28 @@ def test_insert_after_kwarg_raises(self):
     def test_env_vars_passed_to_git(self):
         editor = 'non_existant_editor'
         with mock.patch.dict('os.environ', {'GIT_EDITOR': editor}):
-            assert self.git.var("GIT_EDITOR") == editor
+            self.assertEqual(self.git.var("GIT_EDITOR"), editor)
 
     @with_rw_directory
     def test_environment(self, rw_dir):
         # sanity check
-        assert self.git.environment() == {}
+        self.assertEqual(self.git.environment(), {})
 
         # make sure the context manager works and cleans up after itself
         with self.git.custom_environment(PWD='/tmp'):
-            assert self.git.environment() == {'PWD': '/tmp'}
+            self.assertEqual(self.git.environment(), {'PWD': '/tmp'})
 
-        assert self.git.environment() == {}
+        self.assertEqual(self.git.environment(), {})
 
         old_env = self.git.update_environment(VARKEY='VARVALUE')
         # The returned dict can be used to revert the change, hence why it has
         # an entry with value 'None'.
-        assert old_env == {'VARKEY': None}
-        assert self.git.environment() == {'VARKEY': 'VARVALUE'}
+        self.assertEqual(old_env, {'VARKEY': None})
+        self.assertEqual(self.git.environment(), {'VARKEY': 'VARVALUE'})
 
         new_env = self.git.update_environment(**old_env)
-        assert new_env == {'VARKEY': 'VARVALUE'}
-        assert self.git.environment() == {}
+        self.assertEqual(new_env, {'VARKEY': 'VARVALUE'})
+        self.assertEqual(self.git.environment(), {})
 
         path = os.path.join(rw_dir, 'failing-script.sh')
         stream = open(path, 'wt')
@@ -214,11 +215,11 @@ def test_environment(self, rw_dir):
                 try:
                     remote.fetch()
                 except GitCommandError as err:
-                    if sys.version_info[0] < 3 and is_darwin():
-                        assert 'ssh-origin' in str(err)
-                        assert err.status == 128
+                    if sys.version_info[0] < 3 and is_darwin:
+                        self.assertIn('ssh-orig, ' in str(err))
+                        self.assertEqual(err.status, 128)
                     else:
-                        assert 'FOO' in str(err)
+                        self.assertIn('FOO', str(err))
                 # end
             # end
         # end if select.poll exists
 
@@ -577,7 +577,7 @@ def mixed_iterator():
         assert len(entries) == 1 and entries[0].hexsha != null_hex_sha
 
         # add symlink
-        if not is_win():
+        if not is_win:
             for target in ('/etc/nonexisting', '/etc/passwd', '/etc'):
                 basename = "my_real_symlink"
 
@@ -630,7 +630,7 @@ def mixed_iterator():
         index.checkout(fake_symlink_path)
 
         # on windows we will never get symlinks
-        if is_win():
+        if is_win:
             # simlinks should contain the link as text ( which is what a
             # symlink actually is )
             open(fake_symlink_path, 'rb').read() == link_target
@@ -711,7 +711,7 @@ def make_paths():
             assert fkey not in index.entries
 
         index.add(files, write=True)
-        if is_win():
+        if is_win:
             hp = hook_path('pre-commit', index.repo.git_dir)
             hpd = os.path.dirname(hp)
             if not os.path.isdir(hpd):