Importing modules in Python and __init__.py

问题:

I have been reading about the function of __init__.py file. It is said that we need an empty __init__.py file in the folder which contains modules, so that these modules can be imported. However, I tried adding a folder path to PYTHONPATH (Environment Variable in Windows 7). Although this folder does not contain an __init__.py file, I can still import the modules from that folder. Could you please explain how these modules can be imported without the existence of an __init__.py?

Thanks,

Best regards

回答1:

__init__.py turns a folder into a package. This is useful to create a sort of hierarchy of modules, where you can have import-statements like this:

import mymodule.cool.stuff

This is not possible without packages.

回答2:

If a directory (folder) contains a __init__.py file then it becomes a package. What you thought you read was not strictly correct, as you found. A package can be imported as if it was a module by itself, and any code in __init__.py is run, although it is often empty. Packages are a way of grouping multiple modules together, and you can load them using:

import package-name.module-name

Packages can also be nested, and often are. Look in the Lib directory under your Python software directory for many examples.

回答3:

The difference between having _init_.py and not having one in your module directory is:

When you have __init__.py (blank one), you can import the module using

from dirname import MyModule

But when you dont have _init_.py at all, you cannot import the module without adding the path till that module to PYTHONPATH. In this case from dirname import MyModule FAILS, or reports error.

# Structure of directory/Module without _init_.py

[root@Machine python]# cd NoInitModule/
[root@Machine NoInitModule]# pwd
/python/NoInitModule
[root@Machine NoInitModule]# ls -l
total 4
-rw-r--r--. 1 root root 44 Jan 16 07:02 ModuleX.py

# Structure of directory/Module with _init_.py

[root@Machine NoInitModule]# cd ../InitModule/
[root@Machine InitModule]# ls -l
total 4
-rw-r--r--. 1 root root  0 Jan 16 07:13 __init__.py
-rw-r--r--. 1 root root 49 Jan 16 07:12 ModuleY.py
[root@Machine InitModule]#

Examples:

# NO _init_.py, from dir import module statement - DOESN'T WORK

[root@Machine python]# cat impomod.py
from NoInitModule import ModuleX

ModuleX.printBye()

[root@Machine python]# python impomod.py
Traceback (most recent call last):
  File "impomod.py", line 7, in <module>
    from NoInitModule import ModuleX
ImportError: No module named NoInitModule
[root@Machine python]#

# NO _init_.py, import statement - DOESN'T WORK

[root@Machine python]# vim impomod.py
[root@Machine python]# cat impomod.py
#from NoInitModule import ModuleX
import ModuleX

ModuleX.printBye()
[root@Machine python]# python impomod.py
Traceback (most recent call last):
  File "impomod.py", line 8, in <module>
    import ModuleX
ImportError: No module named ModuleX
[root@Machine python]#

# NO _init_.py, INSERT path in PYTHONPATH, import module statement after inserting path - WORKS

[root@Machine python]# vim impomod.py
[root@Machine python]# cat impomod.py
import sys
sys.path.append('/python/NoInitModule')
print sys.path

import ModuleX
ModuleX.printBye()

[root@Machine python]# python impomod.py
Bye from Module X with no __init__.py
[root@Machine python]#

# HAVING _init_.py, No INSERT of path, from dir import module statement - WORKS

[root@Machine python]# cat impXmod.py
import sys
from InitModule import ModuleY
ModuleY.printSeeU()
[root@Machine python]#

[root@Machine python]# python impXmod.py
See you from Module Y having __init__.py

# HAVING _init_.py, No INSERT of path, import module statement - DOESN'T WORK

[root@Machine python]# vim impXmod.py
[root@Machine python]# cat impXmod.py
import sys
#from InitModule import ModuleY
import ModuleY
ModuleY.printSeeU()

[root@Machine python]# python impXmod.py
Traceback (most recent call last):
  File "impXmod.py", line 3, in <module>
    import ModuleY
ImportError: No module named ModuleY

Ofcourse, subfolders issue can be added to this

回答4:

Yes, this works, as you can tell. The reason for the empty __init__.py file is to mark sub-folders as folders containing modules.

So a folder in PYTHONPATH is ok to have modules in it, but any subfolders of those folders have to have a __init__.py file in them to be able to import modules from them.