python - __builtin__ 如何在运行时可用？

Question

为什么第一条语句返回NameError，而max可用

>>> __builtin__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__builtin__' is not defined
>>> max
<built-in function max>
>>> import __builtin__
>>> __builtin__.max
<built-in function max>

Answer 1

The builtins namespace associated with the execution of a code block is actually found by looking up the name __builtins__ in its global namespace; this should be a dictionary or a module (in the latter case the module’s dictionary is used). 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.

所以它实际上是在查找 __builtins__(因为你在主模块中)

>>> __builtins__.max
<built-in function max>

但如上所述，这只是 __builtin__ 的别名(它不是主模块命名空间的一部分，尽管它已被 __builtins__ 加载和引用).