diff --git a/mediaproxy/tls.py b/mediaproxy/tls.py
index 3f92ece..b07b4be 100644
--- a/mediaproxy/tls.py
+++ b/mediaproxy/tls.py
@@ -1,84 +1,89 @@
 
 """TLS support"""
 
 __all__ = ['X509Credentials']
 
 import os
 import stat
 
 from application.process import process
 from gnutls import crypto
 from gnutls.interfaces import twisted
 
 from mediaproxy.configuration import TLSConfig
 
 
 class FileDescriptor(object):
     def __init__(self, name, type):
         certs_path = os.path.normpath(TLSConfig.certs_path)
         self.path = os.path.join(certs_path, name)
         self.klass = type
         self.timestamp = 0
         self.object = None
     def get(self):
         path = process.config_file(self.path)
         if path is None:
-            raise RuntimeError("missing or unreadable file: %s" % self.path)
+            raise RuntimeError('missing or unreadable file: %s' % self.path)
         mtime = os.stat(path)[stat.ST_MTIME]
         if self.timestamp < mtime:
             f = open(path)
             try:
                 self.object = self.klass(f.read())
                 self.timestamp = mtime
             finally:
                 f.close()
         return self.object
 
 
 class X509Entity(object):
     type = None
+
     def __init__(self, name_attr):
         self.name_attr = name_attr
         self.descriptors = {}
+
     def __get__(self, obj, type_=None):
         name = getattr(obj or type_, self.name_attr, None)
         if name is None:
             return None
         descriptor = self.descriptors.setdefault(name, FileDescriptor(name, self.type))
         return descriptor.get()
+
     def __set__(self, obj, value):
-        raise AttributeError("cannot set attribute")
+        raise AttributeError('cannot set attribute')
+
     def __delete__(self, obj):
-        raise AttributeError("cannot delete attribute")
+        raise AttributeError('cannot delete attribute')
 
 
 class X509Certificate(X509Entity):
     type = crypto.X509Certificate
 
+
 class X509PrivateKey(X509Entity):
     type = crypto.X509PrivateKey
 
+
 class X509CRL(X509Entity):
     type = crypto.X509CRL
 
 
 class X509Credentials(twisted.X509Credentials):
     """SIPThor X509 credentials"""
 
-    X509cert_name = None ## will be defined by each instance
-    X509key_name  = None ## will be defined by each instance
+    X509cert_name = None  # will be defined by each instance
+    X509key_name  = None  # will be defined by each instance
     X509ca_name   = 'ca.pem'
     X509crl_name  = 'crl.pem'
 
     X509cert = X509Certificate(name_attr='X509cert_name')
     X509key  = X509PrivateKey(name_attr='X509key_name')
     X509ca   = X509Certificate(name_attr='X509ca_name')
     X509crl  = X509CRL(name_attr='X509crl_name')
 
     def __init__(self, cert_name):
         self.X509cert_name = '%s.crt' % cert_name
         self.X509key_name = '%s.key' % cert_name
         twisted.X509Credentials.__init__(self, self.X509cert, self.X509key, [self.X509ca], [self.X509crl])
         self.verify_peer = True
         self.verify_period = TLSConfig.verify_interval
-