avoid importing a module twice

due to the way utils.import_module is implemented,
"ryu-manager ryu/app/switches.py" ends up with loading
switches.py module twice.
this commit fixes that by checking if module pathnames specified
on the command line is aliases of already loaded modules.
while the check is incomplete, it should cover the most of useful cases.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/utils.py

@@ -39,15 +39,46 @@ import re
 LOG = logging.getLogger('ryu.utils')
 
 
+def chop_py_suffix(p):
+    for suf in ['.py', '.pyc', '.pyo']:
+        if p.endswith(suf):
+            return p[:-len(suf)]
+    return p
+
+
+def _likely_same(a, b):
+    if os.path.samefile(a, b):
+        return True
+    if chop_py_suffix(a) == chop_py_suffix(b):
+        return True
+    return False
+
+
+def _find_loaded_module(modpath):
+    # copy() to avoid RuntimeError: dictionary changed size during iteration
+    for k, m in sys.modules.copy().iteritems():
+        if not hasattr(m, '__file__'):
+            continue
+        if _likely_same(m.__file__, modpath):
+            return m
+    return None
+
+
 def import_module(modname):
     try:
         __import__(modname)
     except:
-        sys.path.append(os.path.dirname(os.path.abspath(modname)))
+        abspath = os.path.abspath(modname)
+        mod = _find_loaded_module(abspath)
+        if mod:
+            return mod
+        opath = sys.path
+        sys.path.append(os.path.dirname(abspath))
         name = os.path.basename(modname)
         if name.endswith('.py'):
             name = name[:-3]
         __import__(name)
+        sys.path = opath
         return sys.modules[name]
     return sys.modules[modname]