diff --git a/jedi/api/__init__.py b/jedi/api/__init__.py
index 515c523a..31463996 100644
--- a/jedi/api/__init__.py
+++ b/jedi/api/__init__.py
@@ -189,7 +189,7 @@ class Script(object):
                     completions.append((c, scope))
         else:
             completions = []
-            debug.dbg('possible scopes', scopes)
+            debug.dbg('possible completion scopes: %s', scopes)
             for s in scopes:
                 if s.isinstance(er.Function):
                     names = s.get_magic_function_names()
@@ -229,7 +229,7 @@ class Script(object):
         Base for completions/goto. Basically it returns the resolved scopes
         under cursor.
         """
-        debug.dbg('start: %s in %s' % (goto_path, self._parser.user_scope))
+        debug.dbg('start: %s in %s', goto_path, self._parser.user_scope)
 
         user_stmt = self._user_stmt(is_completion)
         if not user_stmt and len(goto_path.split('\n')) > 1:
diff --git a/jedi/cache.py b/jedi/cache.py
index 7b260d79..cb7dc405 100644
--- a/jedi/cache.py
+++ b/jedi/cache.py
@@ -257,7 +257,7 @@ class ParserPickling(object):
             finally:
                 gc.enable()
 
-        debug.dbg('pickle loaded', path)
+        debug.dbg('pickle loaded: %s', path)
         parser_cache[path] = parser_cache_item
         return parser_cache_item.parser
 
diff --git a/jedi/debug.py b/jedi/debug.py
index 6ab6ff6b..c29f5d09 100644
--- a/jedi/debug.py
+++ b/jedi/debug.py
@@ -49,20 +49,20 @@ def increase_indent(func):
     return wrapper
 
 
-def dbg(*args):
+def dbg(message, *args):
     """ Looks at the stack, to see if a debug message should be printed. """
     if debug_function and enable_notice:
         frm = inspect.stack()[1]
         mod = inspect.getmodule(frm[0])
         if not (mod.__name__ in ignored_modules):
             i = ' ' * debug_indent
-            debug_function(NOTICE, i + 'dbg: ' + ', '.join(u(a) for a in args))
+            debug_function(NOTICE, i + 'dbg: ' + message % args)
 
 
-def warning(*args):
+def warning(message, *args):
     if debug_function and enable_warning:
         i = ' ' * debug_indent
-        debug_function(WARNING, i + 'warning: ' + ', '.join(u(a) for a in args))
+        debug_function(WARNING, i + 'warning: ' + message % args)
 
 
 def speed(name):
diff --git a/jedi/evaluate/__init__.py b/jedi/evaluate/__init__.py
index 66ff9390..cacf591a 100644
--- a/jedi/evaluate/__init__.py
+++ b/jedi/evaluate/__init__.py
@@ -205,7 +205,7 @@ class Evaluator(object):
 
         :param stmt: A `pr.Statement`.
         """
-        debug.dbg('eval_statement %s (%s)' % (stmt, seek_name))
+        debug.dbg('eval_statement %s (%s)', stmt, seek_name)
         expression_list = stmt.expression_list()
 
         result = self.eval_expression_list(expression_list)
@@ -242,7 +242,7 @@ class Evaluator(object):
                 loop = evaluate_list_comprehension(nested_lc, loop)
             return loop
 
-        debug.dbg('eval_expression_list: %s' % expression_list)
+        debug.dbg('eval_expression_list: %s', expression_list)
         result = []
         calls_iterator = iter(expression_list)
         for call in calls_iterator:
@@ -354,7 +354,7 @@ class Evaluator(object):
             current = next(path)
         except StopIteration:
             return None
-        debug.dbg('_follow_path: %s in scope %s' % (current, typ))
+        debug.dbg('_follow_path: %s in scope %s', current, typ)
 
         result = []
         if isinstance(current, pr.Array):
@@ -367,7 +367,7 @@ class Evaluator(object):
                 result = self.execute(typ, current)
             else:
                 # Curly braces are not allowed, because they make no sense.
-                debug.warning('strange function call with {}', current, typ)
+                debug.warning('strange function call with {} %s %s', current, typ)
         else:
             # The function must not be decorated with something else.
             if typ.isinstance(er.Function):
@@ -385,7 +385,7 @@ class Evaluator(object):
         if obj.isinstance(er.Function):
             obj = obj.get_decorated_func()
 
-        debug.dbg('execute:', obj, params)
+        debug.dbg('execute: %s %s', obj, params)
         try:
             return stdlib.execute(self, obj, params)
         except stdlib.NotInStdLib:
@@ -410,11 +410,11 @@ class Evaluator(object):
                     try:
                         stmts = obj.execute_subscope_by_name('__call__', params)
                     except KeyError:
-                        debug.warning("no __call__ func available", obj)
+                        debug.warning("no __call__ func available %s", obj)
                 else:
-                    debug.warning("no execution possible", obj)
+                    debug.warning("no execution possible %s", obj)
 
-            debug.dbg('execute result: %s in %s' % (stmts, obj))
+            debug.dbg('execute result: %s in %s', stmts, obj)
             return imports.strip_imports(self, stmts)
 
     def goto(self, stmt, call_path=None):
@@ -482,8 +482,8 @@ def _assign_tuples(tup, results, seek_name):
             try:
                 func = r.get_exact_index_types
             except AttributeError:
-                debug.warning("invalid tuple lookup %s of result %s in %s"
-                              % (tup, results, seek_name))
+                debug.warning("invalid tuple lookup %s of result %s in %s",
+                              tup, results, seek_name)
             else:
                 with common.ignored(IndexError):
                     types += func(index)
diff --git a/jedi/evaluate/finder.py b/jedi/evaluate/finder.py
index 8fcdebbb..7c05acd3 100644
--- a/jedi/evaluate/finder.py
+++ b/jedi/evaluate/finder.py
@@ -22,7 +22,7 @@ class NameFinder(object):
     def find(self, scopes, resolve_decorator=True):
         names = self.filter_name(scopes)
         types = self._names_to_types(names, resolve_decorator)
-        debug.dbg('_names_to_types: %s, old: %s' % (names, types))
+        debug.dbg('_names_to_types: %s, old: %s', names, types)
         return self._resolve_descriptors(types)
 
     def scopes(self, search_global=False):
@@ -76,8 +76,8 @@ class NameFinder(object):
                     new_name.parent = r
                     result.append(new_name)
 
-        debug.dbg('sfn filter "%s" in (%s-%s): %s@%s'
-                  % (self.name_str, self.scope, nscope, u(result), self.position))
+        debug.dbg('sfn filter "%s" in (%s-%s): %s@%s', self.name_str,
+                  self.scope, nscope, u(result), self.position)
         return result
 
     def _check_getattr(self, inst):
diff --git a/jedi/evaluate/imports.py b/jedi/evaluate/imports.py
index 41c1a7dd..02f3cedc 100644
--- a/jedi/evaluate/imports.py
+++ b/jedi/evaluate/imports.py
@@ -94,7 +94,7 @@ class ImportPath(pr.Base):
         n = pr.Name(i._sub_module, names, zero, zero, self.import_stmt)
         new = pr.Import(i._sub_module, zero, zero, n)
         new.parent = parent
-        debug.dbg('Generated a nested import: %s' % new)
+        debug.dbg('Generated a nested import: %s', new)
         return new
 
     def get_defined_names(self, on_import_stmt=False):
@@ -193,7 +193,7 @@ class ImportPath(pr.Base):
             try:
                 scope, rest = self._follow_file_system()
             except ModuleNotFound:
-                debug.warning('Module not found: ' + str(self.import_stmt))
+                debug.warning('Module not found: %s', self.import_stmt)
                 self._evaluator.recursion_detector.pop_stmt()
                 return []
 
@@ -225,7 +225,7 @@ class ImportPath(pr.Base):
                 scopes.append(self._get_nested_import(scope))
         else:
             scopes = [ImportPath.GlobalNamespace]
-        debug.dbg('after import', scopes)
+        debug.dbg('after import: %s', scopes)
 
         self._evaluator.recursion_detector.pop_stmt()
         return scopes
@@ -295,7 +295,7 @@ class ImportPath(pr.Base):
         Find a module with a path (of the module, like usb.backend.libusb10).
         """
         def follow_str(ns_path, string):
-            debug.dbg('follow_module', ns_path, string)
+            debug.dbg('follow_module %s %s', ns_path, string)
             path = None
             if ns_path:
                 path = ns_path
@@ -305,7 +305,7 @@ class ImportPath(pr.Base):
             if path is not None:
                 importing = find_module(string, [path])
             else:
-                debug.dbg('search_module', string, self.file_path)
+                debug.dbg('search_module %s %s', string, self.file_path)
                 # Override the sys.path. It works only good that way.
                 # Injecting the path directly into `find_module` did not work.
                 sys.path, temp = sys_path, sys.path
diff --git a/jedi/evaluate/iterable.py b/jedi/evaluate/iterable.py
index d53a5566..86b7cc70 100644
--- a/jedi/evaluate/iterable.py
+++ b/jedi/evaluate/iterable.py
@@ -28,7 +28,7 @@ class Generator(use_metaclass(CachedMetaClass, pr.Base)):
         for n in ('close', 'throw') + executes_generator:
             parent = self if n in executes_generator else compiled.builtin
             names.append(helpers.FakeName(n, parent))
-        debug.dbg('generator names', names)
+        debug.dbg('generator names: %s', names)
         return names
 
     def iter_content(self):
@@ -36,7 +36,7 @@ class Generator(use_metaclass(CachedMetaClass, pr.Base)):
         return self._evaluator.execute(self.func, self.var_args, True)
 
     def get_index_types(self, index=None):
-        debug.warning('Tried to get array access on a generator', self)
+        debug.warning('Tried to get array access on a generator: %s', self)
         return []
 
     def __getattr__(self, name):
@@ -183,7 +183,7 @@ def get_iterator_types(inputs):
             iterators.append(it)
         else:
             if not hasattr(it, 'execute_subscope_by_name'):
-                debug.warning('iterator/for loop input wrong', it)
+                debug.warning('iterator/for loop input wrong: %s', it)
                 continue
             try:
                 iterators += it.execute_subscope_by_name('__iter__')
@@ -204,7 +204,7 @@ def get_iterator_types(inputs):
             try:
                 result += gen.execute_subscope_by_name(name)
             except KeyError:
-                debug.warning('Instance has no __next__ function', gen)
+                debug.warning('Instance has no __next__ function in %s.', gen)
         else:
             # is a generator
             result += gen.iter_content()
@@ -363,9 +363,7 @@ class ArrayInstance(pr.Base):
                         if self.var_args.start_pos != array.var_args.start_pos:
                             items += array.iter_content()
                         else:
-                            debug.warning(
-                                'ArrayInstance recursion',
-                                self.var_args)
+                            debug.warning('ArrayInstance recursion %s', self.var_args)
                         continue
                 items += get_iterator_types([typ])
 
diff --git a/jedi/evaluate/recursion.py b/jedi/evaluate/recursion.py
index 620ed93e..15423d3d 100644
--- a/jedi/evaluate/recursion.py
+++ b/jedi/evaluate/recursion.py
@@ -40,8 +40,8 @@ class RecursionDetector(object):
         self.current = _RecursionNode(stmt, self.current)
         check = self._check_recursion()
         if check:  # TODO remove False!!!!
-            debug.warning('catched stmt recursion: %s against %s @%s'
-                          % (stmt, check.stmt, stmt.start_pos))
+            debug.warning('catched stmt recursion: %s against %s @%s', stmt,
+                          check.stmt, stmt.start_pos)
             self.pop_stmt()
             return True
         return False
@@ -121,7 +121,7 @@ class ExecutionRecursionDetector(object):
         self.execution_count = 0
 
     def __call__(self, execution, evaluate_generator=False):
-        debug.dbg('Execution recursions: %s' % execution, self.recursion_level,
+        debug.dbg('Execution recursions: %s', execution, self.recursion_level,
                   self.execution_count, len(self.execution_funcs))
         if self.check_recursion(execution, evaluate_generator):
             result = []
diff --git a/jedi/evaluate/representation.py b/jedi/evaluate/representation.py
index e917dd17..758fe8d7 100644
--- a/jedi/evaluate/representation.py
+++ b/jedi/evaluate/representation.py
@@ -334,16 +334,15 @@ class Function(use_metaclass(CachedMetaClass, pr.IsScope)):
         # Only enter it, if has not already been processed.
         if not self.is_decorated:
             for dec in reversed(self.base_func.decorators):
-                debug.dbg('decorator:', dec, f)
+                debug.dbg('decorator: %s %s', dec, f)
                 dec_results = set(self._evaluator.eval_statement(dec))
                 if not len(dec_results):
-                    debug.warning('decorator not found: %s on %s' %
-                                 (dec, self.base_func))
+                    debug.warning('decorator not found: %s on %s', dec, self.base_func)
                     return None
                 decorator = dec_results.pop()
                 if dec_results:
-                    debug.warning('multiple decorators found', self.base_func,
-                                  dec_results)
+                    debug.warning('multiple decorators found %s %s',
+                                  self.base_func, dec_results)
                 # Create param array.
                 old_func = Function(self._evaluator, f, is_decorated=True)
                 if instance is not None and decorator.isinstance(Function):
@@ -352,15 +351,15 @@ class Function(use_metaclass(CachedMetaClass, pr.IsScope)):
 
                 wrappers = self._evaluator.execute(decorator, (old_func,))
                 if not len(wrappers):
-                    debug.warning('no wrappers found', self.base_func)
+                    debug.warning('no wrappers found %s', self.base_func)
                     return None
                 if len(wrappers) > 1:
                     # TODO resolve issue with multiple wrappers -> multiple types
-                    debug.warning('multiple wrappers found', self.base_func,
-                                  wrappers)
+                    debug.warning('multiple wrappers found %s %s',
+                                  self.base_func, wrappers)
                 f = wrappers[0]
 
-                debug.dbg('decorator end', f)
+                debug.dbg('decorator end %s', f)
         if f != self.base_func and isinstance(f, pr.Function):
             f = Function(self._evaluator, f)
         return f
diff --git a/jedi/evaluate/sys_path.py b/jedi/evaluate/sys_path.py
index 7537a6dc..59ab1230 100644
--- a/jedi/evaluate/sys_path.py
+++ b/jedi/evaluate/sys_path.py
@@ -75,14 +75,14 @@ def sys_path_with_modifications(module):
                 res = execute_code(exe.get_code())
                 if res is not None:
                     sys_path.insert(0, res)
-                    debug.dbg('sys path inserted: %s' % res)
+                    debug.dbg('sys path inserted: %s', res)
                 exe.type = exe_type
                 exe.values.insert(0, exe_pop)
             elif array_cmd == 'append':
                 res = execute_code(exe.get_code())
                 if res is not None:
                     sys_path.append(res)
-                    debug.dbg('sys path added: %s' % res)
+                    debug.dbg('sys path added: %s', res)
         return sys_path
 
     if module.path is None:
@@ -115,6 +115,6 @@ def _detect_django_path(module_path):
 
         with common.ignored(IOError):
             with open(module_path + os.path.sep + 'manage.py'):
-                debug.dbg('Found django path: %s' % module_path)
+                debug.dbg('Found django path: %s', module_path)
                 result.append(module_path)
     return result
diff --git a/jedi/parser/__init__.py b/jedi/parser/__init__.py
index 54706c86..88821d5d 100644
--- a/jedi/parser/__init__.py
+++ b/jedi/parser/__init__.py
@@ -269,11 +269,8 @@ class Parser(object):
         first_pos = self.start_pos
         token_type, cname = self.next()
         if token_type != tokenize.NAME:
-            debug.warning(
-                "class: syntax err, token is not a name@%s (%s: %s)" % (
-                    self.start_pos[0], tokenize.tok_name[token_type], cname
-                )
-            )
+            debug.warning("class: syntax err, token is not a name@%s (%s: %s)",
+                          self.start_pos[0], tokenize.tok_name[token_type], cname)
             return None
 
         cname = pr.Name(self.module, [(cname, self.start_pos)], self.start_pos,
@@ -286,7 +283,7 @@ class Parser(object):
             token_type, _next = self.next()
 
         if _next != ':':
-            debug.warning("class syntax: %s@%s" % (cname, self.start_pos[0]))
+            debug.warning("class syntax: %s@%s", cname, self.start_pos[0])
             return None
 
         # because of 2 line class initializations
@@ -446,8 +443,8 @@ class Parser(object):
             or self.user_scope is None
             and self.start_pos[0] >= self.user_position[0]
         ):
-            debug.dbg('user scope found [%s] = %s' %
-                     (self.parserline.replace('\n', ''), repr(self._scope)))
+            debug.dbg('user scope found [%s] = %s',
+                      self.parserline.replace('\n', ''), self._scope)
             self.user_scope = self._scope
 
         self._current = typ, tok
@@ -473,8 +470,8 @@ class Parser(object):
         # This iterator stuff is not intentional. It grew historically.
         for token_type, tok in self.iterator:
             self.module.temp_used_names = []
-            # debug.dbg('main: tok=[%s] type=[%s] indent=[%s]'\
-            #    % (tok, tokenize.tok_name[token_type], start_position[0]))
+            # debug.dbg('main: tok=[%s] type=[%s] indent=[%s]', \
+            #           tok, tokenize.tok_name[token_type], start_position[0])
 
             while token_type == tokenize.DEDENT and self._scope != self.module:
                 token_type, tok = self.next()
@@ -504,8 +501,7 @@ class Parser(object):
             if tok == 'def':
                 func = self._parse_function()
                 if func is None:
-                    debug.warning("function: syntax error@%s" %
-                                  self.start_pos[0])
+                    debug.warning("function: syntax error@%s", self.start_pos[0])
                     continue
                 self.freshscope = True
                 self._scope = self._scope.add_scope(func, self._decorators)
@@ -549,7 +545,7 @@ class Parser(object):
                     tok = 'import'
                     mod = None
                 if not mod and not relative_count or tok != "import":
-                    debug.warning("from: syntax error@%s" % self.start_pos[0])
+                    debug.warning("from: syntax error@%s", self.start_pos[0])
                     defunct = True
                     if tok != 'import':
                         self._gen.push_last_back()
diff --git a/jedi/parser/representation.py b/jedi/parser/representation.py
index 60973ade..64a54780 100644
--- a/jedi/parser/representation.py
+++ b/jedi/parser/representation.py
@@ -477,7 +477,7 @@ class Function(Scope):
             try:
                 n.append(p.get_name())
             except IndexError:
-                debug.warning("multiple names in param %s" % n)
+                debug.warning("multiple names in param %s", n)
         return n
 
     def get_call_signature(self, width=72, funcname=None):
@@ -1018,12 +1018,12 @@ class Statement(Simple):
 
             middle, tok = parse_stmt_or_arr(token_iterator, ['in'], True)
             if tok != 'in' or middle is None:
-                debug.warning('list comprehension middle @%s' % str(start_pos))
+                debug.warning('list comprehension middle @%s', start_pos)
                 return None, tok
 
             in_clause, tok = parse_stmt_or_arr(token_iterator)
             if in_clause is None:
-                debug.warning('list comprehension in @%s' % str(start_pos))
+                debug.warning('list comprehension in @%s', start_pos)
                 return None, tok
 
             return ListComprehension(st, middle, in_clause, self), tok
@@ -1156,7 +1156,7 @@ class Param(Statement):
         """ get the name of the param """
         n = self.get_set_vars()
         if len(n) > 1:
-            debug.warning("Multiple param names (%s)." % n)
+            debug.warning("Multiple param names (%s).", n)
         return n[0]
 
 
diff --git a/jedi/parser/user_context.py b/jedi/parser/user_context.py
index 6736ae28..31414606 100644
--- a/jedi/parser/user_context.py
+++ b/jedi/parser/user_context.py
@@ -99,7 +99,7 @@ class UserContext(object):
                 string += tok
                 last_type = token_type
         except tokenize.TokenError:
-            debug.warning("Tokenize couldn't finish", sys.exc_info)
+            debug.warning("Tokenize couldn't finish: %s", sys.exc_info)
 
         # string can still contain spaces at the end
         return string[::-1].strip(), start_cursor