Adding code to __init__.py

Question

I\'m taking a look at how the model system in django works and I noticed something that I don\'t understand.

I know that you create an empty __init__.py

Answer 1

All imports in __init__.py are made available when you import the package (directory) that contains it.

Example:

./dir/__init__.py:

import something

./test.py:

import dir
# can now use dir.something

EDIT: forgot to mention, the code in __init__.py runs the first time you import any module from that directory. So it's normally a good place to put any package-level initialisation code.

EDIT2: dgrant pointed out to a possible confusion in my example. In __init__.py import something can import any module, not necessary from the package. For example, we can replace it with import datetime, then in our top level test.py both of these snippets will work:

import dir
print dir.datetime.datetime.now()

and

import dir.some_module_in_dir
print dir.datetime.datetime.now()

The bottom line is: all names assigned in __init__.py, be it imported modules, functions or classes, are automatically available in the package namespace whenever you import the package or a module in the package.

Answer 2

It's just personal preference really, and has to do with the layout of your python modules.

Let's say you have a module called erikutils. There are two ways that it can be a module, either you have a file called erikutils.py on your sys.path or you have a directory called erikutils on your sys.path with an empty __init__.py file inside it. Then let's say you have a bunch of modules called fileutils, procutils, parseutils and you want those to be sub-modules under erikutils. So you make some .py files called fileutils.py, procutils.py, and parseutils.py:

erikutils
  __init__.py
  fileutils.py
  procutils.py
  parseutils.py

Maybe you have a few functions that just don't belong in the fileutils, procutils, or parseutils modules. And let's say you don't feel like creating a new module called miscutils. AND, you'd like to be able to call the function like so:

erikutils.foo()
erikutils.bar()

rather than doing

erikutils.miscutils.foo()
erikutils.miscutils.bar()

So because the erikutils module is a directory, not a file, we have to define it's functions inside the __init__.py file.

In django, the best example I can think of is django.db.models.fields. ALL the django *Field classes are defined in the __init__.py file in the django/db/models/fields directory. I guess they did this because they didn't want to cram everything into a hypothetical django/db/models/fields.py model, so they split it out into a few submodules (related.py, files.py, for example) and they stuck the made *Field definitions in the fields module itself (hence, __init__.py).

Answer 3

Using the __init__.py file allows you to make the internal package structure invisible from the outside. If the internal structure changes (e.g. because you split one fat module into two) you only have to adjust the __init__.py file, but not the code that depends on the package. You can also make parts of your package invisible, e.g. if they are not ready for general usage.

Note that you can use the del command, so a typical __init__.py may look like this:

from somemodule import some_function1, some_function2, SomeObject

del somemodule

Now if you decide to split somemodule the new __init__.py might be:

from somemodule1 import some_function1, some_function2
from somemodule2 import SomeObject

del somemodule1
del somemodule2

From the outside the package still looks exactly as before.