Package Initialization

In this lesson, you’ll learn about package initialization. If a file named __init__.py is present in a package directory, it is invoked when the package or a module in the package is imported. You can use this to execute package initialization code, for example for the initialization of package-level data.

Consider the following __init__.py file:

print(f'Invoking __init__.py for {__name__}')
alist = ['spam', 'bacon', 'eggs']

Add this file to the pkg directory from the above example:

Now, when the package is imported, the global list alist is initialized:

>>> import pkg
Invoking __init__.py for pkg
>>> pkg.alist
['spam', 'bacon', 'eggs']
>>> pkg.mod1
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    pkg.mod1
AttributeError: module 'pkg' has no attribute 'mod1'

A module in the package can access the global by importing it in turn:

def load_data():
    print('loading data using mod1.load_data()')
    from pkg import alist
    print(f'This is from pkg - {alist}')

class Customer:
    pass

Here’s what you get:

>>> from pkg import mod1
Invoking __init__.py for pkg
>>> mod1.load_data()
loading data using mod1.load_data()
This is from pkg - ['spam', 'bacon', 'eggs']

Revert mod1.py to the previous version by changing load_data() to no longer use the import from pkg:

def load_data():
    print('loading data using mod1.load_data()')

class Customer:
    pass

You can also use __init__.py to effect automatic importing of modules from a package. For example, earlier you saw that the statement import pkg only places the name pkg in the caller’s local symbol table and doesn’t import any modules. Let’s say that __init__.py in the pkg directory contains the following:

print(f'Invoking __init__.py for {__name__}')
import pkg.mod1, pkg.mod2

Then, when you execute import pkg, modules mod1 and mod2 would be imported automatically:

>>> import pkg
Invoking __init__.py for pkg
>>> pkg.mod1.load_data()
loading data using mod1.load_data()
>>> x = pkg.mod2.Location()
>>> x
<pkg.mod2.Location object at 0x10e1c2dd8>