diff --git a/plugins/dbus_plugin/plugin.py b/plugins/dbus_plugin/plugin.py
index 8bf8d55a3..b8de568e9 100644
--- a/plugins/dbus_plugin/plugin.py
+++ b/plugins/dbus_plugin/plugin.py
@@ -428,7 +428,7 @@ if dbus_support.supported:
             def account_info(self, account):
                 '''show info on account: resource, jid, nick, prio, message'''
                 result = DBUS_DICT_SS()
-                if gajim.connections.has_key(account):
+                if account in gajim.connections:
                     # account is valid
                     con = gajim.connections[account]
                     index = con.connected
diff --git a/src/common/caps_cache.py b/src/common/caps_cache.py
index bbf25c477..6bf9b62ed 100644
--- a/src/common/caps_cache.py
+++ b/src/common/caps_cache.py
@@ -163,7 +163,7 @@ def compute_caps_hash(identities, features, dataforms=[], hash_method='sha-1'):
         hash_ = hashlib.md5(S.encode('utf-8'))
     else:
         return ''
-    return base64.b64encode(hash_.digest())
+    return base64.b64encode(hash_.digest()).decode('utf-8')
 
 
 ################################################################################
diff --git a/src/common/connection_handlers.py b/src/common/connection_handlers.py
index 11bc078fe..df4710caf 100644
--- a/src/common/connection_handlers.py
+++ b/src/common/connection_handlers.py
@@ -431,7 +431,8 @@ class ConnectionVcard:
         if 'PHOTO' in vcard and isinstance(vcard['PHOTO'], dict) and \
         'BINVAL' in vcard['PHOTO']:
             photo = vcard['PHOTO']['BINVAL']
-            photo_decoded = base64.decodestring(photo)
+            photo_decoded = base64.b64decode(photo.encode('utf-8')).decode(
+                'utf-8')
             gajim.interface.save_avatar_files(our_jid, photo_decoded)
             avatar_sha = hashlib.sha1(photo_decoded).hexdigest()
             iq2.getTag('PHOTO').setTagData('SHA', avatar_sha)
@@ -684,7 +685,7 @@ class ConnectionVcard:
         'BINVAL' in vcard['PHOTO']:
             photo = vcard['PHOTO']['BINVAL']
             try:
-                photo_decoded = base64.decodestring(photo)
+                photo_decoded = base64.b64decode(photo.encode('utf-8')).decode('utf-8')
                 avatar_sha = hashlib.sha1(photo_decoded).hexdigest()
             except Exception:
                 avatar_sha = ''
@@ -970,7 +971,7 @@ class ConnectionHandlersBase:
         decmsg = self.gpg.decrypt(encmsg, keyID)
         decmsg = self.connection.Dispatcher.replace_non_character(decmsg)
         # \x00 chars are not allowed in C (so in GTK)
-        obj.msgtxt = helpers.decode_string(decmsg.replace('\x00', ''))
+        obj.msgtxt = decmsg.replace('\x00', '')
         obj.encrypted = 'xep27'
         self.gpg_messages_to_decrypt.remove([encmsg, keyID, obj])
 
@@ -1095,7 +1096,7 @@ class ConnectionHandlersBase:
             jid = gajim.get_jid_without_resource(jid)
 
         try:
-            return self.sessions[jid].values()
+            return list(self.sessions[jid].values())
         except KeyError:
             return []
 
@@ -1157,7 +1158,7 @@ class ConnectionHandlersBase:
         received a thread_id yet and returns the session that we last sent a
         message to
         """
-        sessions = self.sessions[jid].values()
+        sessions = list(self.sessions[jid].values())
 
         # sessions that we haven't received a thread ID in
         idless = [s for s in sessions if not s.received_thread_id]
@@ -1178,7 +1179,7 @@ class ConnectionHandlersBase:
         Find an active session that doesn't have a control attached
         """
         try:
-            sessions = self.sessions[jid].values()
+            sessions = list(self.sessions[jid].values())
 
             # filter out everything except the default session type
             chat_sessions = [s for s in sessions if isinstance(s,
@@ -1549,9 +1550,8 @@ ConnectionJingle, ConnectionIBBytestream):
             iq_obj = obj.stanza.buildReply('result')
             qp = iq_obj.setQuery()
             qp.setTagData('utc', strftime('%Y%m%dT%H:%M:%S', gmtime()))
-            qp.setTagData('tz', helpers.decode_string(tzname[daylight]))
-            qp.setTagData('display', helpers.decode_string(strftime('%c',
-                localtime())))
+            qp.setTagData('tz', tzname[daylight])
+            qp.setTagData('display', strftime('%c', localtime()))
         else:
             iq_obj = obj.stanza.buildReply('error')
             err = nbxmpp.ErrorNode(name=nbxmpp.NS_STANZAS + \
diff --git a/src/common/file_props.py b/src/common/file_props.py
index 808db80fd..1d9684dcb 100644
--- a/src/common/file_props.py
+++ b/src/common/file_props.py
@@ -64,7 +64,7 @@ class FilesProp:
 
     @classmethod
     def getAllFileProp(cls):
-        return cls._files_props.values()
+        return list(cls._files_props.values())
 
     @classmethod
     def setFileProp(cls, fp, account, sid):
diff --git a/src/common/helpers.py b/src/common/helpers.py
index 32c922400..59ffe634b 100644
--- a/src/common/helpers.py
+++ b/src/common/helpers.py
@@ -633,8 +633,6 @@ def datetime_tuple(timestamp):
     from time import strptime
     return strptime(timestamp, '%Y%m%dT%H:%M:%S')
 
-# import gajim only when needed (after decode_string is defined) see #4764
-
 from common import gajim
 if gajim.HAVE_PYCURL:
     import pycurl
@@ -904,7 +902,7 @@ def get_full_jid_from_iq(iq_obj):
     """
     Return the full jid (with resource) from an iq
     """
-    return parse_jid(iq_obj.getFrom())
+    return parse_jid(str(iq_obj.getFrom()))
 
 def get_jid_from_iq(iq_obj):
     """
diff --git a/src/common/protocol/bytestream.py b/src/common/protocol/bytestream.py
index f46fb4342..4077dbe18 100644
--- a/src/common/protocol/bytestream.py
+++ b/src/common/protocol/bytestream.py
@@ -852,7 +852,8 @@ class ConnectionIBBytestream(ConnectionBytestream):
                 chunk = file_props.fp.read(file_props.block_size)
                 if chunk:
                     datanode = nbxmpp.Node(nbxmpp.NS_IBB + ' data', {'sid': sid,
-                        'seq': file_props.seq}, base64.encodestring(chunk))
+                        'seq': file_props.seq}, base64.b64encode(chunk.encode(
+                        'utf-8')).decode('utf-8'))
                     file_props.seq += 1
                     file_props.started = True
                     if file_props.seq == 65536:
@@ -887,7 +888,7 @@ class ConnectionIBBytestream(ConnectionBytestream):
         log.debug('ReceiveHandler called sid->%s seq->%s' % (sid, seq))
         try:
             seq = int(seq)
-            data = base64.decodestring(data)
+            data = base64.b64decode(data.encode('utf-8')).decode('utf-8')
         except Exception:
             seq = ''
             data = ''
diff --git a/src/common/resolver.py b/src/common/resolver.py
index 8d52cba51..6f1200d54 100644
--- a/src/common/resolver.py
+++ b/src/common/resolver.py
@@ -70,33 +70,33 @@ class CommonResolver():
             # empty host, return empty list of srv records
             on_ready([])
             return
-        if self.resolved_hosts.has_key(host+type):
+        if host + type in self.resolved_hosts:
             # host is already resolved, return cached values
             log.debug('%s already resolved: %s' % (host,
-                self.resolved_hosts[host+type]))
-            on_ready(host, self.resolved_hosts[host+type])
+                self.resolved_hosts[host + type]))
+            on_ready(host, self.resolved_hosts[host + type])
             return
-        if self.handlers.has_key(host+type):
+        if host + type in self.handlers:
             # host is about to be resolved by another connection,
             # attach our callback
             log.debug('already resolving %s' % host)
-            self.handlers[host+type].append(on_ready)
+            self.handlers[host + type].append(on_ready)
         else:
             # host has never been resolved, start now
             log.debug('Starting to resolve %s using %s' % (host, self))
-            self.handlers[host+type] = [on_ready]
+            self.handlers[host + type] = [on_ready]
             self.start_resolve(host, type)
 
     def _on_ready(self, host, type, result_list):
         # practically it is impossible to be the opposite, but who knows :)
         host = host.lower()
         log.debug('Resolving result for %s: %s' % (host, result_list))
-        if not self.resolved_hosts.has_key(host+type):
-            self.resolved_hosts[host+type] = result_list
-        if self.handlers.has_key(host+type):
-            for callback in self.handlers[host+type]:
+        if host + type not in self.resolved_hosts:
+            self.resolved_hosts[host + type] = result_list
+        if host + type in self.handlers:
+            for callback in self.handlers[host + type]:
                 callback(host, result_list)
-            del(self.handlers[host+type])
+            del(self.handlers[host + type])
 
     def start_resolve(self, host, type):
         pass
@@ -251,8 +251,7 @@ class NSLookupResolver(CommonResolver):
             domain = None
             if line.startswith(fqdn):
                 domain = fqdn # For nslookup 9.5
-            elif helpers.decode_string(line).startswith(ufqdn):
-                line = helpers.decode_string(line)
+            elif line.startswith(ufqdn):
                 domain = ufqdn # For nslookup 9.6
             if domain:
                 rest = line[len(domain):].split('=')
diff --git a/src/common/zeroconf/client_zeroconf.py b/src/common/zeroconf/client_zeroconf.py
index 69f1a4e6f..3d5b43f43 100644
--- a/src/common/zeroconf/client_zeroconf.py
+++ b/src/common/zeroconf/client_zeroconf.py
@@ -173,7 +173,7 @@ class P2PClient(IdleObject):
                 id_ = stanza.getID()
                 if not id_:
                     id_ = self.Dispatcher.getAnID()
-                if self.conn_holder.ids_of_awaiting_messages.has_key(self.fd):
+                if self.fd in self.conn_holder.ids_of_awaiting_messages:
                     self.conn_holder.ids_of_awaiting_messages[self.fd].append((
                         id_, thread_id))
                 else:
@@ -195,7 +195,7 @@ class P2PClient(IdleObject):
             id_ = stanza.getID()
             if not id_:
                 id_ = self.Dispatcher.getAnID()
-            if self.conn_holder.ids_of_awaiting_messages.has_key(self.fd):
+            if self.fd in self.conn_holder.ids_of_awaiting_messages:
                 self.conn_holder.ids_of_awaiting_messages[self.fd].append((id_,
                     thread_id))
             else:
@@ -253,10 +253,10 @@ class P2PClient(IdleObject):
                     'Incorrect answer from server.')
             return
         if self.sock_type == TYPE_SERVER:
-            if attrs.has_key('from'):
+            if 'from' in attrs:
                 self.to = attrs['from']
             self.send_stream_header()
-            if attrs.has_key('version') and attrs['version'] == '1.0':
+            if 'version' in attrs and attrs['version'] == '1.0':
                 # other part supports stream features
                 features = Node('stream:features')
                 self.Dispatcher.send(features)
@@ -270,12 +270,12 @@ class P2PClient(IdleObject):
 
     def on_disconnect(self):
         if self.conn_holder:
-            if self.conn_holder.ids_of_awaiting_messages.has_key(self.fd):
+            if self.fd in self.conn_holder.ids_of_awaiting_messages:
                 del self.conn_holder.ids_of_awaiting_messages[self.fd]
             self.conn_holder.remove_connection(self.sock_hash)
-        if self.__dict__.has_key('Dispatcher'):
+        if 'Dispatcher' in self.__dict__:
             self.Dispatcher.PlugOut()
-        if self.__dict__.has_key('P2PConnection'):
+        if 'P2PConnection' in self.__dict__:
             self.P2PConnection.PlugOut()
         self.Connection = None
         self._caller = None
@@ -294,7 +294,7 @@ class P2PClient(IdleObject):
                 self.Dispatcher.Stream._document_attrs is None:
             return
         self.onreceive(None)
-        if self.Dispatcher.Stream._document_attrs.has_key('version') and \
+        if 'version' in self.Dispatcher.Stream._document_attrs and \
         self.Dispatcher.Stream._document_attrs['version'] == '1.0':
                 #~ self.onreceive(self._on_receive_stream_features)
                 #XXX continue with TLS
@@ -710,7 +710,7 @@ class ClientZeroconf:
             if self.ip_to_hash[i] == sock_hash:
                 del self.ip_to_hash[i]
                 break
-        if self.hash_to_port.has_key(sock_hash):
+        if sock_hash in self.hash_to_port:
             del self.hash_to_port[sock_hash]
 
     def start_listener(self, port):
diff --git a/src/dataforms_widget.py b/src/dataforms_widget.py
index 766e51477..7e2b8198b 100644
--- a/src/dataforms_widget.py
+++ b/src/dataforms_widget.py
@@ -566,7 +566,8 @@ class SingleForm(Gtk.Table, object):
                 for uri in field.media.uris:
                     if uri.type_.startswith('image/'):
                         try:
-                            img_data = base64.decodestring(uri.uri_data)
+                            img_data = base64.b64decode(uri.uri_data.encode(
+                                'utf-8')).decode('utf-8')
                             pixbuf_l = GdkPixbuf.PixbufLoader()
                             pixbuf_l.write(img_data)
                             pixbuf_l.close()
diff --git a/src/disco.py b/src/disco.py
index 12be18b34..9eb75e616 100644
--- a/src/disco.py
+++ b/src/disco.py
@@ -193,7 +193,6 @@ class CacheDictionary:
 
     def __contains__(self, key):
         return key in self.cache
-    has_key = __contains__
 
 _icon_cache = CacheDictionary(15)
 
diff --git a/src/gajim-remote.py b/src/gajim-remote.py
index 1df48a5ea..c01ac6f10 100644
--- a/src/gajim-remote.py
+++ b/src/gajim-remote.py
@@ -42,7 +42,7 @@ except Exception:
 
 def send_error(error_message):
     '''Writes error message to stderr and exits'''
-    print(error_message.encode(PREFERRED_ENCODING), file=sys.stderr)
+    print(error_message, file=sys.stderr)
     sys.exit(1)
 
 try:
diff --git a/src/gui_menu_builder.py b/src/gui_menu_builder.py
index 17364b08f..fabef8dc8 100644
--- a/src/gui_menu_builder.py
+++ b/src/gui_menu_builder.py
@@ -122,7 +122,7 @@ def build_invite_submenu(invite_menuitem, list_, ignore_rooms=[]):
     minimized_controls = []
     for account in connected_accounts:
         minimized_controls += \
-            gajim.interface.minimized_controls[account].values()
+            list(gajim.interface.minimized_controls[account].values())
     for gc_control in gajim.interface.msg_win_mgr.get_controls(
     message_control.TYPE_GC) + minimized_controls:
         acct = gc_control.account
diff --git a/src/profile_window.py b/src/profile_window.py
index 614af3add..696c66beb 100644
--- a/src/profile_window.py
+++ b/src/profile_window.py
@@ -171,7 +171,8 @@ class ProfileWindow:
             button.show()
             text_button = self.xml.get_object('NOPHOTO_button')
             text_button.hide()
-            self.avatar_encoded = base64.encodestring(data)
+            self.avatar_encoded = base64.b64encode(data.encode('utf-8')).decode(
+                'utf-8')
             # returns None if unknown type
             self.avatar_mime_type = mimetypes.guess_type(path_to_file)[0]
             if must_delete:
diff --git a/src/remote_control.py b/src/remote_control.py
index 60f22b7b1..26b8d7020 100644
--- a/src/remote_control.py
+++ b/src/remote_control.py
@@ -907,7 +907,7 @@ class SignalObject(dbus.service.Object):
         if not invalid_file and filesize < 16384:
             fd = open(picture, 'rb')
             data = fd.read()
-            avatar = base64.encodestring(data)
+            avatar = base64.b64encode(data.encode('utf-8')).decode('utf-8')
             avatar_mime_type = mimetypes.guess_type(picture)[0]
             vcard={}
             vcard['PHOTO'] = {'BINVAL': avatar}
diff --git a/src/roster_window.py b/src/roster_window.py
index 990df3cf5..fd6c6f462 100644
--- a/src/roster_window.py
+++ b/src/roster_window.py
@@ -2186,7 +2186,7 @@ class RosterWindow:
                 gajim.interface.status_sent_to_groups[account] = {}
             for gc_control in gajim.interface.msg_win_mgr.get_controls(
             message_control.TYPE_GC) + \
-            gajim.interface.minimized_controls[account].values():
+            list(gajim.interface.minimized_controls[account].values()):
                 if gc_control.account == account:
                     if gajim.gc_connected[account][gc_control.room_jid]:
                         gajim.connections[account].send_gc_status(
@@ -3923,7 +3923,7 @@ class RosterWindow:
                 )
 
     def on_plugins_menuitem_activate(self, widget):
-        if gajim.interface.instances.has_key('plugins'):
+        if 'plugins' in gajim.interface.instances:
             gajim.interface.instances['plugins'].window.present()
         else:
             gajim.interface.instances['plugins'] = plugins.gui.PluginsWindow()
diff --git a/src/vcard.py b/src/vcard.py
index 35c194f30..8fa203c10 100644
--- a/src/vcard.py
+++ b/src/vcard.py
@@ -61,7 +61,7 @@ def get_avatar_pixbuf_encoded_mime(photo):
         img_encoded = photo['BINVAL']
         avatar_encoded = img_encoded
         try:
-            img_decoded = base64.decodestring(img_encoded)
+            img_decoded = base64.b64decode(img_encoded.encode('utf-8')).decode('utf-8')
         except Exception:
             pass
     if img_decoded: