tests: simplify and document the sorting of pyflake messages

The pyflake messages are simply ordered by message type, path, line no (and
message text).
The message type is taken from the order of the filters.

The previous ordering looks complicated and illogically.
It was the following order (r'\3:\5:\4:\1:\2:' + line):
  message (\3 and \5)
  var name (\4)
  path (\1)
  line no (\2)
  line reference
Ordering by var name before path looks illogically for me.

tests/filterpyflakes.py

@@ -4,35 +4,48 b''
 
 import sys, re, os
 
-def makekey(message):
-    # "path/file:line: message"
-    match = re.search(r"(line \d+)", message)
-    line = ''
-    if match:
-        line = match.group(0)
-        message = re.sub(r"(line \d+)", '', message)
-    return re.sub(r"([^:]*):([^:]+):([^']*)('[^']*')(.*)$",
-                  r'\3:\5:\4:\1:\2:' + line,
-                  message)
+def makekey(typeandline):
+    """
+    for sorting lines by: msgtype, path/to/file, lineno, message
+
+    typeandline is a sequence of a message type and the entire message line
+    the message line format is path/to/file:line: message
+
+    >>> makekey((3, 'example.py:36: any message'))
+    (3, 'example.py', 36, ' any message')
+    >>> makekey((7, 'path/to/file.py:68: dummy message'))
+    (7, 'path/to/file.py', 68, ' dummy message')
+    >>> makekey((2, 'fn:88: m')) > makekey((2, 'fn:9: m'))
+    True
+    """
+
+    msgtype, line = typeandline
+    fname, line, message = line.split(":", 2)
+    # line as int for ordering 9 before 88
+    return msgtype, fname, int(line), message
+
 
 lines = []
 for line in sys.stdin:
-    # We whitelist tests
+    # We whitelist tests (see more messages in pyflakes.messages)
     pats = [
             r"imported but unused",
             r"local variable '.*' is assigned to but never used",
             r"unable to detect undefined names",
            ]
-    if not re.search('|'.join(pats), line):
-        continue
+    for msgtype, pat in enumerate(pats):
+        if re.search(pat, line):
+            break # pattern matches
+    else:
+        continue # no pattern matched, next line
     fn = line.split(':', 1)[0]
     f = open(os.path.join(os.path.dirname(os.path.dirname(__file__)), fn))
     data = f.read()
     f.close()
     if 'no-check-code' in data:
         continue
-    lines.append(line)
+    lines.append((msgtype, line))
 
-for line in sorted(lines, key = makekey):
+for msgtype, line in sorted(lines, key = makekey):
     sys.stdout.write(line)
 print