Why would I put code in __init__.py files?

Libraries and frameworks usually use initialization code in __init__.py files to neatly hide internal structure and provide a uniform interface to the user.

Let’s take the example of Django forms module. Various functions and classes in forms module are defined in different files based on their classification.

forms/
  __init__.py
  extras/
    ...
  fields.py
  forms.py
  widgets.py
  ...

Now if you were to create a form, you would have to know in which file each function is defined and your code to create a contact form will have to look something like this (which is incovenient and ugly).

 class CommentForm(forms.forms.Form):
    name = forms.fields.CharField() 
    url = forms.fields.URLField()
    comment = forms.fields.CharField(widget=forms.widgets.Textarea) 

Instead, in Django you can just refer to various widgets, forms, fields etc. directly from the forms namespace.

from django import forms

class CommentForm(forms.Form):
    name = forms.CharField()
    url = forms.URLField()
    comment = forms.CharField(widget=forms.Textarea)

How is this possible? To make this possible, Django adds the following statement to forms/__init__.py file which import all the widgets, forms, fields etc. into the forms namespace.

from widgets import *
from fields import *
from forms import *
from models import *

As you can see, this simplifies your life when creating the forms because now you don’t have to worry about in where each function/class is defined and just use all of these directly from forms namespace. This is just one example but you can see examples like these in other frameworks and libraries.