Reword comment, tidy blank line, test colon-path branch

Address review: frame the fast path as 'no colon means it cannot be a
URL' (a file path may legitimately contain ':' on POSIX), add the blank
line before the lazy import, and cover a ':'-containing argument that is
not a URL (single-letter drive, colon in the name) reaching the file
path branch.

Author: gaborbernat

Lib/mimetypes.py

@@ -126,10 +126,11 @@ def guess_type(self, url, strict=True):
 
         # TODO: Deprecate accepting file paths (in particular path-like objects).
         url = os.fspath(url)
-        # A URL scheme requires a ':'; a plain file path (the common case) has
-        # none, so skip the relatively expensive urlparse() for it.
+        # Without a ':' the argument cannot carry a URL scheme, so it cannot
+        # be a URL; skip the relatively expensive urlparse() in that case.
         if isinstance(url, str) and ':' not in url:
             return self.guess_file_type(url, strict=strict)
+
         import urllib.parse
         p = urllib.parse.urlparse(url)
         if p.scheme and len(p.scheme) > 1:

Lib/test/test_mimetypes.py

@@ -375,6 +375,15 @@ def test_url(self):
         result = self.db.guess_type('http://example.com/host.html?q=x.tar')
         self.assertSequenceEqual(result, ('text/html', None))
 
+    def test_path_with_colon_but_no_url_scheme(self):
+        # A ':' that does not introduce a real URL scheme -- a single-letter
+        # Windows drive, or a colon elsewhere in the name -- is treated as a
+        # file path rather than a URL.
+        eq = self.assertSequenceEqual
+        eq(self.db.guess_type("c:fake.html"), ("text/html", None))
+        eq(self.db.guess_type(r"c:\dir\fake.html"), ("text/html", None))
+        eq(self.db.guess_type("note 12:30.txt"), ("text/plain", None))
+
     def test_guess_all_types(self):
         # First try strict.  Use a set here for testing the results because if
         # test_urllib2 is run before test_mimetypes, global state is modified