Add full test coverage for fs_str func (#784)

piptools/utils.py

@@ -235,11 +235,6 @@ def fs_str(string):
     On Python 3 returns the string as is, since Python 3 uses unicode
     paths and the input string shouldn't be bytes.
 
-    >>> fs_str(u'some path component/Something')
-    'some path component/Something'
-    >>> assert isinstance(fs_str('whatever'), str)
-    >>> assert isinstance(fs_str(u'whatever'), str)
-
     :type string: str|unicode
     :rtype: str
     """

tests/test_utils.py

@@ -1,3 +1,4 @@
+import six
 from pytest import mark, raises
 
 from piptools.utils import (
@@ -6,6 +7,7 @@
     flat_map,
     format_requirement,
     format_specifier,
+    fs_str,
     get_hashes_from_ireq,
     is_pinned_requirement,
     name_from_req,
@@ -147,3 +149,15 @@ def test_name_from_req_with_project_name(from_line):
     ireq = from_line("foo==1.8")
     ireq.req.project_name = "bar"
     assert name_from_req(ireq.req) == "bar"
+
+
+def test_fs_str():
+    assert fs_str(u"some path component/Something") == "some path component/Something"
+    assert isinstance(fs_str("whatever"), str)
+    assert isinstance(fs_str(u"whatever"), str)
+
+
+@mark.skipif(six.PY2, reason="Not supported in py2")
+def test_fs_str_with_bytes():
+    with raises(AssertionError):
+        fs_str(b"whatever")