diff --git a/digdag-core/src/test/resources/io/digdag/core/workflow/multiple_operators.dig b/digdag-core/src/test/resources/io/digdag/core/workflow/multiple_operators.dig
index 70a4b89f59..45d0bf32fb 100644
--- a/digdag-core/src/test/resources/io/digdag/core/workflow/multiple_operators.dig
+++ b/digdag-core/src/test/resources/io/digdag/core/workflow/multiple_operators.dig
@@ -1,3 +1,4 @@
 +step1:
   sh>: echo this should fail
   py>: bar
+  python: python3
diff --git a/digdag-standards/src/main/resources/digdag/standards/py/runner.py b/digdag-standards/src/main/resources/digdag/standards/py/runner.py
index 3743cc9f1e..4227ba3b17 100644
--- a/digdag-standards/src/main/resources/digdag/standards/py/runner.py
+++ b/digdag-standards/src/main/resources/digdag/standards/py/runner.py
@@ -49,12 +49,7 @@ def store(self, params={}, **kwds):
 
     def add_subtask(self, function=None, **params):
         if function is not None and not isinstance(function, dict):
-            if hasattr(function, "im_class"):
-                # Python 2
-                command = ".".join([function.im_class.__module__, function.im_class.__name__, function.__name__])
-            else:
-                # Python 3
-                command = ".".join([function.__module__, function.__qualname__])
+            command = ".".join([function.__module__, function.__qualname__])
             config = params
             config["py>"] = command
         else:
@@ -66,7 +61,7 @@ def add_subtask(self, function=None, **params):
         try:
             json.dumps(config)
         except Exception as error:
-            raise TypeError("Parameters must be serializable using JSON: %s" % str(error))
+            raise TypeError(f"Parameters must be serializable using JSON: {str(error)}")
         self.subtask_config["+subtask" + str(self.subtask_index)] = config
         self.subtask_index += 1
 
@@ -87,14 +82,14 @@ def digdag_inspect_command(command):
         try:
             callable_type = getattr(mod, method_name)
         except AttributeError as error:
-            raise AttributeError("Module '%s' has no attribute '%s'" % (".".join(fragments), method_name))
+            raise AttributeError(f"Module '{'.'.join(fragments)}' has no attribute '{method_name}'")
     except ImportError as error:
         class_name = fragments.pop()
         mod = __import__(".".join(fragments), fromlist=[class_name])
         try:
             class_type = getattr(mod, class_name)
         except AttributeError as error:
-            raise AttributeError("Module '%s' has no attribute '%s'" % (".".join(fragments), method_name))
+            raise AttributeError(f"Module '{'.'.join(fragments)}' has no attribute '{method_name}'")
 
     if type(callable_type) == type:
         class_type = callable_type
@@ -109,12 +104,8 @@ def digdag_inspect_arguments(callable_type, exclude_self, params):
     if callable_type == object.__init__:
         # object.__init__ accepts *varargs and **keywords but it throws exception
         return {}
-    if hasattr(inspect, 'getfullargspec'): # Python3
-        spec = inspect.getfullargspec(callable_type)
-        keywords_ = spec.varkw
-    else: # Python 2
-        spec = inspect.getargspec(callable_type)
-        keywords_ = spec.keywords
+    spec = inspect.getfullargspec(callable_type)
+    keywords_ = spec.varkw
 
     args = {}
     for idx, key in enumerate(spec.args):
@@ -125,15 +116,8 @@ def digdag_inspect_arguments(callable_type, exclude_self, params):
         else:
             if spec.defaults is None or idx < len(spec.args) - len(spec.defaults):
                 # this keyword is required but not in params. raising an error.
-                if hasattr(callable_type, '__qualname__'):
-                    # Python 3
-                    name = callable_type.__qualname__
-                elif hasattr(callable_type, 'im_class'):
-                    # Python 2
-                    name = "%s.%s" % (callable_type.im_class.__name__, callable_type.__name__)
-                else:
-                    name = callable_type.__name__
-                raise TypeError("Method '%s' requires parameter '%s' but not set" % (name, key))
+                name = callable_type.__qualname__
+                raise TypeError(f"Method '{name}' requires parameter '{key}' but not set")
     if keywords_:
         # above code was only for validation
         return params
@@ -162,9 +146,9 @@ def digdag_inspect_arguments(callable_type, exclude_self, params):
 except SystemExit as e:
     # SystemExit only shows an exit code and it is not kind to users. So this block creates a specific error message.
     # This error will happen if called python module name and method name are equal to those of the standard library module. (e.g. tokenize.main)
-    error = Exception("Failed to call python command with code:%d" % e.code, "Possible cause: Invalid python module call, duplicate module name with standard library")
+    error = Exception(f"Failed to call python command with code:{str(e.code)}", "Possible cause: Ivalid python module call, duplicate module name with standard library")
     error_type, error_value, _tb = sys.exc_info()
-    error_message = "%s %s" % (error.args[0], error.args[1])
+    error_message = " ".join(error.args[:2])
     error_traceback = traceback.format_exception(error_type, error_value, _tb)
 except Exception as e:
     error = e
diff --git a/digdag-tests/src/test/java/acceptance/PyIT.java b/digdag-tests/src/test/java/acceptance/PyIT.java
index bfaa846c17..31d5dd152c 100644
--- a/digdag-tests/src/test/java/acceptance/PyIT.java
+++ b/digdag-tests/src/test/java/acceptance/PyIT.java
@@ -100,8 +100,8 @@ public void verifyConfigurationPythonOption()
         assertThat(attempt.getSuccess(), is(true));
 
         final String logs = getAttemptLogs(client, attempt.getId());
-        assertThat(logs, containsString("python python"));
-        assertThat(logs, containsString("python [u'python', u'-v']"));
+        assertThat(logs, containsString("python python3"));
+        assertThat(logs, containsString("python ['python3', '-v']"));
     }
 
     @Test
diff --git a/digdag-tests/src/test/resources/acceptance/echo_params/echo_params.dig b/digdag-tests/src/test/resources/acceptance/echo_params/echo_params.dig
index 342bdc32af..6ec6e82a7a 100644
--- a/digdag-tests/src/test/resources/acceptance/echo_params/echo_params.dig
+++ b/digdag-tests/src/test/resources/acceptance/echo_params/echo_params.dig
@@ -1,2 +1,3 @@
 +foo:
   py>: scripts.echo_params.echo_params
+  python: python3
diff --git a/digdag-tests/src/test/resources/acceptance/echo_params/scripts/echo_params.py b/digdag-tests/src/test/resources/acceptance/echo_params/scripts/echo_params.py
index 20d43e4aa6..f832415d26 100644
--- a/digdag-tests/src/test/resources/acceptance/echo_params/scripts/echo_params.py
+++ b/digdag-tests/src/test/resources/acceptance/echo_params/scripts/echo_params.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def echo_params():
diff --git a/digdag-tests/src/test/resources/acceptance/emr/pi.py b/digdag-tests/src/test/resources/acceptance/emr/pi.py
index 37029b7679..e646722533 100644
--- a/digdag-tests/src/test/resources/acceptance/emr/pi.py
+++ b/digdag-tests/src/test/resources/acceptance/emr/pi.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 #
 # Licensed to the Apache Software Foundation (ASF) under one or more
 # contributor license agreements.  See the NOTICE file distributed with
diff --git a/digdag-tests/src/test/resources/acceptance/params_visibility/dynamic_tasks_visibility.dig b/digdag-tests/src/test/resources/acceptance/params_visibility/dynamic_tasks_visibility.dig
index d2945f9aaf..78eca3e63a 100644
--- a/digdag-tests/src/test/resources/acceptance/params_visibility/dynamic_tasks_visibility.dig
+++ b/digdag-tests/src/test/resources/acceptance/params_visibility/dynamic_tasks_visibility.dig
@@ -2,6 +2,7 @@
 +initialize:
   +task:
     py>: helper.store_v
+    python: python3
     value: initial
 
 +loop:
@@ -20,6 +21,7 @@
         if>: true
         _do:
           py>: helper.store_v
+          python: python3
           value: override_${outer}_${inner}
 
       +dynamic_get_in_parallel:
diff --git a/digdag-tests/src/test/resources/acceptance/params_visibility/params_visibility.dig b/digdag-tests/src/test/resources/acceptance/params_visibility/params_visibility.dig
index 149ecba4f4..735cece70c 100644
--- a/digdag-tests/src/test/resources/acceptance/params_visibility/params_visibility.dig
+++ b/digdag-tests/src/test/resources/acceptance/params_visibility/params_visibility.dig
@@ -14,6 +14,7 @@
 +stores:
   +store:
     py>: helper.store_v
+    python: python3
     value: simple_store
 
   +simple_store:
@@ -21,6 +22,7 @@
 
   +overwrite:
     py>: helper.store_v
+    python: python3
     value: store_overwrite
 
   +store_overwrite:
@@ -29,6 +31,7 @@
 +parallel_store_fork:
   +prepare:
     py>: helper.store_v
+    python: python3
     value: prepare
 
   +fork:
@@ -37,6 +40,7 @@
     +a:
       +store:
         py>: helper.store_v
+        python: python3
         value: a
 
       +dump:
@@ -45,6 +49,7 @@
     +b:
       +store:
         py>: helper.store_v
+        python: python3
         value: b
 
       +dump:
@@ -62,15 +67,18 @@
 
     +a:
       py>: helper.store_v
+      python: python3
       value: a
 
     +b:
       py>: helper.store_v
+      python: python3
       value: b
       _after: [+a]
 
     +c:
       py>: helper.store_v
+      python: python3
       value: c
 
   +dump:
diff --git a/digdag-tests/src/test/resources/acceptance/py/config_python.dig b/digdag-tests/src/test/resources/acceptance/py/config_python.dig
index bede81fa97..cb81ea86fd 100644
--- a/digdag-tests/src/test/resources/acceptance/py/config_python.dig
+++ b/digdag-tests/src/test/resources/acceptance/py/config_python.dig
@@ -2,8 +2,8 @@
 
 +python_string_option:
   py>: scripts.echo_params.echo_params
-  python: "python"
+  python: "python3"
 
 +python_array_option:
   py>: scripts.echo_params.echo_params
-  python: ["python", "-v"]
+  python: ["python3", "-v"]
diff --git a/digdag-tests/src/test/resources/acceptance/py/dup_module.dig b/digdag-tests/src/test/resources/acceptance/py/dup_module.dig
index 1bd6f218ab..72a6a21bed 100644
--- a/digdag-tests/src/test/resources/acceptance/py/dup_module.dig
+++ b/digdag-tests/src/test/resources/acceptance/py/dup_module.dig
@@ -1,2 +1,3 @@
 +task1:
   py>: tokenize.main
+  python: python3
diff --git a/digdag-tests/src/test/resources/acceptance/py/stacktrace_python.dig b/digdag-tests/src/test/resources/acceptance/py/stacktrace_python.dig
index 7acb000543..b243af3d0c 100644
--- a/digdag-tests/src/test/resources/acceptance/py/stacktrace_python.dig
+++ b/digdag-tests/src/test/resources/acceptance/py/stacktrace_python.dig
@@ -1,5 +1,6 @@
 +stacktrace_python:
   py>: scripts.stacktrace.run
+  python: python3
 
 _error:
   echo>: ERROR_MESSAGE_BEGIN ${error.message} ERROR_MESSAGE_END
diff --git a/digdag-tests/src/test/resources/acceptance/py/syntax_error_python.dig b/digdag-tests/src/test/resources/acceptance/py/syntax_error_python.dig
index f71d1d1ca7..460df4e376 100644
--- a/digdag-tests/src/test/resources/acceptance/py/syntax_error_python.dig
+++ b/digdag-tests/src/test/resources/acceptance/py/syntax_error_python.dig
@@ -1,2 +1,3 @@
 +task1:
   py>: scripts.syntax_error.func1
+  python: python3
diff --git a/examples/tasks/check_task.py b/examples/tasks/check_task.py
index 0cb645a170..b87aae9d57 100644
--- a/examples/tasks/check_task.py
+++ b/examples/tasks/check_task.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def generate():
diff --git a/examples/tasks/conditions.py b/examples/tasks/conditions.py
index 94099eada7..98a14ee3ec 100644
--- a/examples/tasks/conditions.py
+++ b/examples/tasks/conditions.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import re
 import digdag
 
diff --git a/examples/tasks/error_task.py b/examples/tasks/error_task.py
index 87ac8dc2b6..f842e6638b 100644
--- a/examples/tasks/error_task.py
+++ b/examples/tasks/error_task.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def fails():
diff --git a/examples/tasks/export_params.py b/examples/tasks/export_params.py
index 656c930125..eab91d4303 100644
--- a/examples/tasks/export_params.py
+++ b/examples/tasks/export_params.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def set_my_param():
diff --git a/examples/tasks/generate_subtasks.py b/examples/tasks/generate_subtasks.py
index 2f5f82282d..4d98004bac 100644
--- a/examples/tasks/generate_subtasks.py
+++ b/examples/tasks/generate_subtasks.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 class ParallelProcess(object):
diff --git a/examples/tasks/params.py b/examples/tasks/params.py
index 3f32adb0f5..a220d035ea 100644
--- a/examples/tasks/params.py
+++ b/examples/tasks/params.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def simple(data, number):
diff --git a/examples/tasks/python_args.py b/examples/tasks/python_args.py
index d0ee9a0fae..6ab9e4b8b3 100644
--- a/examples/tasks/python_args.py
+++ b/examples/tasks/python_args.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import digdag
 
 def required_arguments(required1, required2):