为什么即使未定义__all__，pkg import *最终导入模块？

Question

I have created a package pkg in this manner.

$ tree
.
└── pkg
    ├── foo.py
    └── __init__.py

1 directory, 2 files
susam@debian1:~/so$ cat pkg/__init__.py
susam@debian1:~/so$ cat pkg/foo.py
print('executing module foo ...')

def bar():
    print('bar')

All the Python shell snippets below are from a single interactive session with the Python interpreter. I have split them up into multiple blocks to add my own commentary in between.

Here is my Python version.

Python 3.4.2 (default, Oct  8 2014, 10:45:20)
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

The following import does not import foo because __all__ is not defined in __init__.py .

>>> from pkg import *
>>> foo.bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined
>>>

The above behaviour has been described in the Python tutorial at https://docs.python.org/3/tutorial/modules.html#importing-from-a-package .

If __all__ is not defined, the statement from sound.effects import * does not import all submodules from the package sound.effects into the current namespace; it only ensures that the package sound.effects has been imported (possibly running any initialization code in __init__.py ) and then imports whatever names are defined in the package.

The following imports only bar() . It does not import foo .

>>> from pkg.foo import bar
executing module foo ...
>>> bar()
bar
>>> foo.bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined

But strangely after the previous import, the following import ends up importing foo even if __all__ is not defined in __init__.py .

>>> from pkg import *
>>> foo.bar()
bar

Why does this happen?

Answer 1

Based off of the Python documentation that you cited, when you use from pkg import * you are importing only the module foo as well as __init__.py and not the functions of foo .

I suspect the last code snippet you provided works because you have already imported bar() directly from foo and reimporting foo after you've strictly imported the bar function establishes the connection between foo and bar() .

Do you run into this same issue when specifying __all__ ?

Answer 2

foo is already imported in pkg namespace as pkg.foo when you do from pkg.foo import bar . So when you then import all names from pkg namespace to current scope you also import foo . See:

>>> import pkg
>>> dir(pkg)
['__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__']
>>> from pkg.foo import bar
executing module foo ...
>>> dir(pkg)
['__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', 'foo']

(I am on python 2 but the logic for this is the same)