Python: What's the difference between __builtin__ and __builtins__?

问题

I was coding today and noticed something. If I open a new interpreter session (IDLE) and check what\'s defined with the dir function I get this:

$ python
>>> dir()
[\'__builtins__\', \'__doc__\', \'__name__\', \'__package__\']
>>> dir(__builtins__)
[\'ArithmeticError\', \'AssertionError\', \'AttributeError\', \'BaseException\', \'BufferError\', \'BytesWarning\', \'DeprecationWarning\', \'EOFError\', \'Ellipsis\', \'EnvironmentError\', \'Exception\', \'False\', \'FloatingPointError\', \'FutureWarning\', \'GeneratorExit\', \'IOError\', \'ImportError\', \'ImportWarning\', \'IndentationError\', \'IndexError\', \'KeyError\', \'KeyboardInterrupt\', \'LookupError\', \'MemoryError\', \'NameError\', \'None\', \'NotImplemented\', \'NotImplementedError\', \'OSError\', \'OverflowError\', \'PendingDeprecationWarning\', \'ReferenceError\', \'RuntimeError\', \'RuntimeWarning\', \'StandardError\', \'StopIteration\', \'SyntaxError\', \'SyntaxWarning\', \'SystemError\', \'SystemExit\', \'TabError\', \'True\', \'TypeError\', \'UnboundLocalError\', \'UnicodeDecodeError\', \'UnicodeEncodeError\', \'UnicodeError\', \'UnicodeTranslateError\', \'UnicodeWarning\', \'UserWarning\', \'ValueError\', \'Warning\', \'ZeroDivisionError\', \'_\', \'__debug__\', \'__doc__\', \'__import__\', \'__name__\', \'__package__\', \'abs\', \'all\', \'any\', \'apply\', \'basestring\', \'bin\', \'bool\', \'buffer\', \'bytearray\', \'bytes\', \'callable\', \'chr\', \'classmethod\', \'cmp\', \'coerce\', \'compile\', \'complex\', \'copyright\', \'credits\', \'delattr\', \'dict\', \'dir\', \'divmod\', \'enumerate\', \'eval\', \'execfile\', \'exit\', \'file\', \'filter\', \'float\', \'format\', \'frozenset\', \'getattr\', \'globals\', \'hasattr\', \'hash\', \'help\', \'hex\', \'id\', \'input\', \'int\', \'intern\', \'isinstance\', \'issubclass\', \'iter\', \'len\', \'license\', \'list\', \'locals\', \'long\', \'map\', \'max\', \'memoryview\', \'min\', \'next\', \'object\', \'oct\', \'open\', \'ord\', \'pow\', \'print\', \'property\', \'quit\', \'range\', \'raw_input\', \'reduce\', \'reload\', \'repr\', \'reversed\', \'round\', \'set\', \'setattr\', \'slice\', \'sorted\', \'staticmethod\', \'str\', \'sum\', \'super\', \'tuple\', \'type\', \'unichr\', \'unicode\', \'vars\', \'xrange\', \'zip\']
>>> import __builtin__
[\'ArithmeticError\', \'AssertionError\', \'AttributeError\', \'BaseException\', \'BufferError\', \'BytesWarning\', \'DeprecationWarning\', \'EOFError\', \'Ellipsis\', \'EnvironmentError\', \'Exception\', \'False\', \'FloatingPointError\', \'FutureWarning\', \'GeneratorExit\', \'IOError\', \'ImportError\', \'ImportWarning\', \'IndentationError\', \'IndexError\', \'KeyError\', \'KeyboardInterrupt\', \'LookupError\', \'MemoryError\', \'NameError\', \'None\', \'NotImplemented\', \'NotImplementedError\', \'OSError\', \'OverflowError\', \'PendingDeprecationWarning\', \'ReferenceError\', \'RuntimeError\', \'RuntimeWarning\', \'StandardError\', \'StopIteration\', \'SyntaxError\', \'SyntaxWarning\', \'SystemError\', \'SystemExit\', \'TabError\', \'True\', \'TypeError\', \'UnboundLocalError\', \'UnicodeDecodeError\', \'UnicodeEncodeError\', \'UnicodeError\', \'UnicodeTranslateError\', \'UnicodeWarning\', \'UserWarning\', \'ValueError\', \'Warning\', \'ZeroDivisionError\', \'_\', \'__debug__\', \'__doc__\', \'__import__\', \'__name__\', \'__package__\', \'abs\', \'all\', \'any\', \'apply\', \'basestring\', \'bin\', \'bool\', \'buffer\', \'bytearray\', \'bytes\', \'callable\', \'chr\', \'classmethod\', \'cmp\', \'coerce\', \'compile\', \'complex\', \'copyright\', \'credits\', \'delattr\', \'dict\', \'dir\', \'divmod\', \'enumerate\', \'eval\', \'execfile\', \'exit\', \'file\', \'filter\', \'float\', \'format\', \'frozenset\', \'getattr\', \'globals\', \'hasattr\', \'hash\', \'help\', \'hex\', \'id\', \'input\', \'int\', \'intern\', \'isinstance\', \'issubclass\', \'iter\', \'len\', \'license\', \'list\', \'locals\', \'long\', \'map\', \'max\', \'memoryview\', \'min\', \'next\', \'object\', \'oct\', \'open\', \'ord\', \'pow\', \'print\', \'property\', \'quit\', \'range\', \'raw_input\', \'reduce\', \'reload\', \'repr\', \'reversed\', \'round\', \'set\', \'setattr\', \'slice\', \'sorted\', \'staticmethod\', \'str\', \'sum\', \'super\', \'tuple\', \'type\', \'unichr\', \'unicode\', \'vars\', \'xrange\', \'zip\']
>>> dir(__builtin__) == dir(__builtins__) # They seem to have the same things
True

Please note the last line.

So, my question is:

Is any an alias of the other one?
Are the Python guys planning to get rid of one of those?
What should I use for my own programs?
What about Python 3?
Any information is valuable!

Important:

I\'m using Python 2.7.2+ on Ubuntu.

回答1:

Straight from the python documentation: http://docs.python.org/reference/executionmodel.html

By default, when in the __main__ module, __builtins__ is the built-in module __builtin__ (note: no 's'); when in any other module, __builtins__ is an alias for the dictionary of the __builtin__ module itself.

__builtins__ can be set to a user-created dictionary to create a weak form of restricted execution.

CPython implementation detail: Users should not touch __builtins__; it is strictly an implementation detail. Users wanting to override values in the builtins namespace should import the __builtin__ (no 's') module and modify its attributes appropriately. The namespace for a module is automatically created the first time a module is imported.

Note that in Python3, the module __builtin__ has been renamed to builtins to avoid some of this confusion.

回答2:

You should use __builtin__ in your programs (in the rare cases that you need it), because __builtins__ is an implementation detail of CPython. It may either be identical to __builtin__, or to __builtin__.__dict__, depending on the context. As the documentation says:

Most modules have the name __builtins__ (note the 's') made available as part of their globals. The value of __builtins__ is normally either this module or the value of this modules’s __dict__ attribute. Since this is an implementation detail, it may not be used by alternate implementations of Python.

In Python 3, __builtin__ has been renamed to builtins, and __builtins__ remains the same (so you should only use builtins in Python 3).

Guido wanted to unite __builtin__ and __builtins__, as you can see here ("Having __builtins__ and __builtin__ both is clearly a bad idea.") , but apparently nothing came of it.

Apparently the use of __builtins__ is for performance - it gives direct access to __builtin__.__dict__ when used in a non-main module, and therefore removes one level of indirection.

回答3:

__builtin__ is a module containing the built-in functions and types. The fact that a name __builtins__ is available containing the same things is an implementation detail. In other words, if you need to use one of them, do import __builtin__ and then use __builtin__. See the documentation.