Index: tests/test_diff_script.py
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+# This file is part of Adblock Plus <https://adblockplus.org/>,
+# Copyright (C) 2006-present eyeo GmbH
+#
+# Adblock Plus is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# Adblock Plus is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Functional tests for the diff script."""
+
+from __future__ import unicode_literals
+
+import pytest
+import subprocess
+import io
+
+from test_differ import BASE, LATEST
+
+
+@pytest.fixture
+def rootdir(tmpdir):
+    """Directory with example filter lists."""
+    rootdir = tmpdir.join('root')
+    rootdir.mkdir()
+    rootdir.join('base.txt').write_text(BASE, encoding='utf8')
+    rootdir.join('latest.txt').write_text(LATEST, encoding='utf8')
+
+    return rootdir
+
+
+@pytest.fixture
+def dstfile(tmpdir):
+    """Destination file for saving the diff output."""
+    return tmpdir.join('dst')
+
+
+def run_script(*args, **kw):
+    """Run diff rendering script with given arguments and return its output."""
+    cmd = ['fldiff'] + list(args)
+
+    proc = subprocess.Popen(cmd, stderr=subprocess.PIPE,
+                            stdout=subprocess.PIPE, stdin=subprocess.PIPE,
+                            **kw)
+    stdout, stderr = proc.communicate()
+    return proc.returncode, stderr.decode('utf-8'), stdout.decode('utf-8')
+
+
+def test_diff_with_outfile(rootdir, dstfile):
+    run_script(str(rootdir.join('base.txt')),
+               str(rootdir.join('latest.txt')),
+               str(dstfile))
+    with io.open(str(dstfile), encoding='utf-8') as dst:
+        result = dst.read()
+    assert '+ &ad_channel=\xa3' in result
+
+
+def test_no_outfile(rootdir):
+    _, _, out = run_script(str(rootdir.join('base.txt')),
+                           str(rootdir.join('latest.txt')))
+    assert '[Adblock Plus Diff]' in out
+
+
+def test_no_base_file(rootdir):
+    code, err, _ = run_script('wrong.txt', str(rootdir.join('latest.txt')))
+    assert code == 1
+    assert 'No such file or directory' in err
+
+
+def test_no_latest_file(rootdir):
+    code, err, _ = run_script(str(rootdir.join('base.txt')), 'wrong.txt')
+    assert code == 1
+    assert 'No such file or directory' in err
+
+
+def test_diff_to_self(rootdir):
+    _, _, out = run_script(str(rootdir.join('latest.txt')),
+                           str(rootdir.join('latest.txt')))
+    assert out == '[Adblock Plus Diff]\n'