In-depth analysis of __builtins__ built-in objects in Python

If you have already learned about packages and modules.
Are you curious: Why can Python use some built-in functions directly without explicitly importing them, such as str() int() dir()...?
The reason is that when the Python interpreter is started for the first time, __builtins__ is already in the namespace (Note: there is s)

Enter Shell to see:

>>> globals()
{'__builtins__': <module '__builtin__' (built-in)>, 
'__name__': '__main__', '__doc__': None, '__package__': None}

You can import __builtin__ again (Note: no s):

import __builtin__
>>> globals()
{'__builtins__': <module '__builtin__' (built-in)>, 
'__name__': '__main__', '__doc__': None, '__builtin__': 
<module '__builtin__' (built-in)>, '__package__': None}

At this time, there is an additional __builtin__ object, and you can judge whether they are the same:

>>> __builtin__ is __builtins__
True
>>> type(__builtin__)
<type 'module'>
>>> type(__builtins__)
<type 'module'>

Now we import it from a file:

# file1.py
import __builtin__

print "module name __name__ : ", __name__
print "__builtin__ is __builtins__: ", __builtin__ is __builtins__
print "type(__builtin__): ", type(__builtin__)
print "type(__builtins__): ", type(__builtins__)
print "__builtins__ is __builtin__.__dict__", __builtins__ is __builtin__.__dict__


# file2.py
import file1

"""结果：
module name __name__ : file
__builtin__ is __builtins__: False
type(__builtin__): <type 'module'>
type(__builtins__): <type 'dict'>
__builtins__ is __builtin__.__dict__ True
"""

Conclusion:
__builtins__ is a reference to the built-in module __builtin__, and has the following two differences:

In the main module, that is, it is not imported by other files. __builtins__ is a reference to __builtin__ itself, the two are identical.

Conjecture via __builtins__ is __builtin__.__dict__:
In non-'__main__' modules, that is, after the module is imported, __builtins__ should be part of __builtin__.__dict__ and is a reference to __builtin__.__dict__, not builtin itself. It is visible everywhere. At this time, builtins Type is dictionary.

Decorate built-in functions
The official Python documentation explains how to decorate a built-in function:

import __builtin__

def open(path):
  f = __builtin__.open(path, 'r')
  return UpperCaser(f)

class UpperCaser:
  __metaclass__ = type

  def __init__(self, f):
    self._f = f

  def read(self):
    return self._f.read().upper()

print open('./a.txt').read()
# 将会全部转为大写输出

Note: In the Python3.X version, the built-in modules are renamed to builtins, which is different from Python2.X