How to use the testtools.matchers.FileExists function in testtools

self.copy_project_to_cwd('hg-branch')

        self.init_source_control()
        subprocess.check_call(
            ['hg', 'branch', 'second'], stdout=subprocess.DEVNULL)
        open('second', 'w').close()
        self.commit('second', 'second')
        subprocess.check_call(
            ['hg', 'branch', 'default'], stdout=subprocess.DEVNULL)
        open('default', 'w').close()
        self.commit('default', 'default')

        self.run_snapcraft('pull')
        self.assertThat(
            os.path.join(self.parts_dir, 'mercurial', 'src', 'second'),
            FileExists())

        self.run_snapcraft('pull')
        self.assertThat(
            os.path.join(self.parts_dir, 'mercurial', 'src', 'second'),
            FileExists())

def test_pull_with_pip_requirements_file(self):
        self.run_snapcraft("build", "pip-requirements-file")
        self.assertThat(
            glob(
                os.path.join(
                    self.parts_dir,
                    "python2",
                    "install",
                    "lib",
                    "python2*",
                    "site-packages",
                    "argparse.py",
                )
            )[0],
            FileExists(),
        )
        self.assertThat(
            glob(
                os.path.join(
                    self.parts_dir,
                    "python3",
                    "install",
                    "lib",
                    "python3*",
                    "site-packages",
                    "argparse.py",
                )
            )[0],
            FileExists(),
        )

def test_use_of_source_subdir_picks_up_setup_py(self):
        # Regression test for LP: #1752481
        self.run_snapcraft("stage", "python-with-source-subdir")

        self.assertThat(os.path.join(self.stage_dir, "bin", "hello"), FileExists())

def test_snap_filter_is_deprecated(self):
        output = self.run_snapcraft(
            ['prime', 'snap-keyword'], 'prime-filter')

        # Verify that the `snap` keyword is deprecated.
        self.assertThat(
            output, Contains(
                "DEPRECATED: The 'snap' keyword has been replaced by 'prime'."
                "\nSee http://snapcraft.io/docs/deprecation-notices/dn1 "
                "for more information."))

        # Verify that only the `snap1` file made it into prime (i.e. `snap2`
        # was filtered out).
        self.assertThat(os.path.join(self.prime_dir, 'snap1'), FileExists())
        self.assertThat(
            os.path.join(self.prime_dir, 'snap2'), Not(FileExists()))

def test_builtin_function_from_stage(self):
        os.mkdir("stagedir")

        runner = _runner.Runner(
            part_properties={"override-stage": "snapcraftctl stage"},
            sourcedir="sourcedir",
            builddir="builddir",
            stagedir="stagedir",
            primedir="primedir",
            builtin_functions={"stage": _fake_stage},
        )

        runner.stage()

        self.assertThat(os.path.join("stagedir", "fake-stage"), FileExists())

def test_builtin_function_from_prime(self):
        os.mkdir("primedir")

        runner = _runner.Runner(
            part_properties={"override-prime": "snapcraftctl prime"},
            sourcedir="sourcedir",
            builddir="builddir",
            stagedir="stagedir",
            primedir="primedir",
            builtin_functions={"prime": _fake_prime},
        )

        runner.prime()

        self.assertThat(os.path.join("primedir", "fake-prime"), FileExists())

def test_build(self):
        plugin = nodejs.NodePlugin("test-part", self.options, self.project)

        self.create_assets(plugin)

        plugin.build()

        self.assertThat(os.path.join(plugin.installdir, "bin", "run"), FileExists())

        expected_env = dict(PATH=os.path.join(plugin._npm_dir, "bin"))
        if self.http_proxy is not None:
            expected_env["http_proxy"] = self.http_proxy
        if self.https_proxy is not None:
            expected_env["https_proxy"] = self.https_proxy
        if self.package_manager == "npm":
            expected_run_calls = [
                mock.call(
                    [self.get_npm_cmd(plugin), "install", "--unsafe-perm"],
                    cwd=plugin.builddir,
                    env=expected_env,
                ),
                mock.call(
                    [self.get_npm_cmd(plugin), "pack"],
                    cwd=plugin.builddir,

def test_clean_build_step_single_part(self):
        self.assert_files_exist()

        self.run_snapcraft(['clean', 'part1', '--step=build'])
        self.assertThat(os.path.join(self.stage_bindir, 'file1'),
                        Not(FileExists()))
        self.assertThat(os.path.join(self.stage_bindir, 'file2'), FileExists())
        self.assertThat(os.path.join(self.snap_bindir, 'file1'),
                        Not(FileExists()))
        self.assertThat(os.path.join(self.snap_bindir, 'file2'), FileExists())

        self.assertThat(self.parts['part1']['builddir'], Not(DirExists()))
        self.assertThat(self.parts['part1']['installdir'], Not(DirExists()))
        self.assertThat(self.parts['part1']['sourcedir'], DirExists())

        self.assertThat(
            os.path.join(self.parts['part2']['builddir'], 'file2'),
            FileExists())
        self.assertThat(
            os.path.join(self.parts['part2']['bindir'], 'file2'),
            FileExists())

        # Now try to prime again
        self.run_snapcraft('prime')
        self.assert_files_exist()

def test_migrate(self):
        """
        The legacy bootstrap flag file gets migrated to a local state flag.
        """
        with open(paths.LEGACY_BOOTSTRAP_FLAG, "w") as fd:
            fd.write("")
        self.configuration.migrate()
        self.assertThat(paths.LEGACY_BOOTSTRAP_FLAG, Not(FileExists()))