What's the difference between __builtin__ and __builtins__?

Question

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.

Campanulate

as far as my code goes, what seems to work is using import builtins see this for an example: (#61085416) – Seascape

Answer 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.

Gumboil

Great answer, thanks a lot. Do you know anything regarding Python 3? – Campanulate

As other answers have said it's the same for python 3 except the __builtin__ module is renamed to "builtins" (without underscores). docs.python.org/dev/reference/executionmodel.html – Gumboil

I don't care about the details. What will never make my code have a bug if I use it, modify it, etc? – Seascape

Answer 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.

Thereunder

What you mean by "non-main module"? Do you mean modules except builtins? – Ladyfinger

using __builtin__ gives me an error: NameError: name '__builtin__' is not defined. What is the right way to access this that doesn't break? – Seascape

I believe you're in the __main__ module is when __name__ == '__main__' is True. – Violaceous

Answer 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.

Chiapas

I don't care about the details. What will never make my code have a bug if I use it, modify it, etc? – Seascape

I think what you meant was import builtins e.g. see this code https://mcmap.net/q/37666/-how-does-one-make-an-already-opened-file-readable-e-g-sys-stdout – Seascape

Answer 4

You can understand these like following code. when cpython is started, cpython load __builtin__ modules into global namespace

import __builtin__as __builtins__

Toots

@Pinocchio why are you asking spamming this everywhere? maybe if you paid attention to the details or asked for more detail, you would understand why it would or would not cause a bug depending on how you use it. – Iraidairan

Answer 5

The __builtins__ module should not be confused with the __builtin__ module. The names, of course, are so similar that it tends to lead to some confusion among new Python programmers who have gotten this far. The __builtins__ module consists of a set of builtin names for the built-ins namespace. Most, if not all, of these names come from the __builtin__ module, which is a module of the built-in functions, exceptions, and other attributes. In standard Python execution, __builtins__ contains all the names from __builtin__. Python used to have a restricted execution model that allowed modification of __builtins__ where key pieces from __builtin__ were left out to create a sandbox environment.

However, due its security flaws and the difficulty involved with repairing it, restricted execution is no longer supported in Python (as of 2.3).

Otherworldly