bpo-30152: Reduce the number of imports for argparse. (#1269)

1 files changed, 26 insertions, 14 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 98bbed0e50..d8bbd352fd 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -84,15 +84,12 @@ __all__ = [
 
 
 import collections as _collections
-import copy as _copy
 import os as _os
 import re as _re
 import sys as _sys
-import textwrap as _textwrap
 
 from gettext import gettext as _, ngettext
 
-
 SUPPRESS = '==SUPPRESS=='
 
 OPTIONAL = '?'
@@ -137,10 +134,16 @@ class _AttributeHolder(object):
         return []
 
 
-def _ensure_value(namespace, name, value):
-    if getattr(namespace, name, None) is None:
-        setattr(namespace, name, value)
-    return getattr(namespace, name)
+def _copy_items(items):
+    if items is None:
+        return []
+    # The copy module is used only in the 'append' and 'append_const'
+    # actions, and it is needed only when the default value isn't a list.
+    # Delay its import for speeding up the common case.
+    if type(items) is list:
+        return items[:]
+    import copy
+    return copy.copy(items)
 
 
 # ===============
@@ -619,12 +622,17 @@ class HelpFormatter(object):
 
     def _split_lines(self, text, width):
         text = self._whitespace_matcher.sub(' ', text).strip()
-        return _textwrap.wrap(text, width)
+        # The textwrap module is used only for formatting help.
+        # Delay its import for speeding up the common usage of argparse.
+        import textwrap
+        return textwrap.wrap(text, width)
 
     def _fill_text(self, text, width, indent):
         text = self._whitespace_matcher.sub(' ', text).strip()
-        return _textwrap.fill(text, width, initial_indent=indent,
-                                           subsequent_indent=indent)
+        import textwrap
+        return textwrap.fill(text, width,
+                             initial_indent=indent,
+                             subsequent_indent=indent)
 
     def _get_help_string(self, action):
         return action.help
@@ -952,7 +960,8 @@ class _AppendAction(Action):
             metavar=metavar)
 
     def __call__(self, parser, namespace, values, option_string=None):
-        items = _copy.copy(_ensure_value(namespace, self.dest, []))
+        items = getattr(namespace, self.dest, None)
+        items = _copy_items(items)
         items.append(values)
         setattr(namespace, self.dest, items)
 
@@ -978,7 +987,8 @@ class _AppendConstAction(Action):
             metavar=metavar)
 
     def __call__(self, parser, namespace, values, option_string=None):
-        items = _copy.copy(_ensure_value(namespace, self.dest, []))
+        items = getattr(namespace, self.dest, None)
+        items = _copy_items(items)
         items.append(self.const)
         setattr(namespace, self.dest, items)
 
@@ -1000,8 +1010,10 @@ class _CountAction(Action):
             help=help)
 
     def __call__(self, parser, namespace, values, option_string=None):
-        new_count = _ensure_value(namespace, self.dest, 0) + 1
-        setattr(namespace, self.dest, new_count)
+        count = getattr(namespace, self.dest, None)
+        if count is None:
+            count = 0
+        setattr(namespace, self.dest, count + 1)
 
 
 class _HelpAction(Action):