Fixed #23663 -- Initialize output streams for BaseCommand in __init__().

This helps with testability of management commands.

Thanks to trac username daveoncode for the report and to
Tim Graham and Claude Paroz for the reviews.

django/core/management/base.py

@@ -79,11 +79,20 @@ class OutputWrapper(object):
     """
     Wrapper around stdout/stderr
     """
+    @property
+    def style_func(self):
+        return self._style_func
+
+    @style_func.setter
+    def style_func(self, style_func):
+        if style_func and hasattr(self._out, 'isatty') and self._out.isatty():
+            self._style_func = style_func
+        else:
+            self._style_func = lambda x: x
+
     def __init__(self, out, style_func=None, ending='\n'):
         self._out = out
         self.style_func = None
-        if hasattr(out, 'isatty') and out.isatty():
-            self.style_func = style_func
         self.ending = ending
 
     def __getattr__(self, name):
@@ -93,8 +102,7 @@ class OutputWrapper(object):
         ending = self.ending if ending is None else ending
         if ending and not msg.endswith(ending):
             msg += ending
-        style_func = [f for f in (style_func, self.style_func, lambda x:x)
-                      if f is not None][0]
+        style_func = style_func or self.style_func
         self._out.write(force_str(style_func(msg)))
 
 
@@ -221,8 +229,14 @@ class BaseCommand(object):
     #
     # requires_system_checks = True
 
-    def __init__(self):
-        self.style = color_style()
+    def __init__(self, stdout=None, stderr=None, no_color=False):
+        self.stdout = OutputWrapper(stdout or sys.stdout)
+        self.stderr = OutputWrapper(stderr or sys.stderr)
+        if no_color:
+            self.style = no_style()
+        else:
+            self.style = color_style()
+            self.stderr.style_func = self.style.ERROR
 
         # `requires_model_validation` is deprecated in favor of
         # `requires_system_checks`. If both options are present, an error is
@@ -371,9 +385,7 @@ class BaseCommand(object):
             if options.traceback or not isinstance(e, CommandError):
                 raise
 
-            # self.stderr is not guaranteed to be set here
-            stderr = getattr(self, 'stderr', OutputWrapper(sys.stderr, self.style.ERROR))
-            stderr.write('%s: %s' % (e.__class__.__name__, e))
+            self.stderr.write('%s: %s' % (e.__class__.__name__, e))
             sys.exit(1)
 
     def execute(self, *args, **options):
@@ -382,12 +394,13 @@ class BaseCommand(object):
         controlled by attributes ``self.requires_system_checks`` and
         ``self.requires_model_validation``, except if force-skipped).
         """
-        self.stdout = OutputWrapper(options.get('stdout', sys.stdout))
         if options.get('no_color'):
             self.style = no_style()
-            self.stderr = OutputWrapper(options.get('stderr', sys.stderr))
-        else:
-            self.stderr = OutputWrapper(options.get('stderr', sys.stderr), self.style.ERROR)
+            self.stderr.style_func = None
+        if options.get('stdout'):
+            self.stdout = OutputWrapper(options['stdout'])
+        if options.get('stderr'):
+            self.stderr = OutputWrapper(options.get('stderr'), self.stderr.style_func)
 
         if self.can_import_settings:
             from django.conf import settings  # NOQA