What is __init__.py for?

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What is __init__.py for?

What is __init__.py for in a Python source directory?

Asked By: Mat

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Answer #1:

It used to be a required part of a package (old, pre-3.3 “regular package”, not newer 3.3+ “namespace package”).

Here’s the documentation.

Python defines two types of packages, regular packages and namespace packages. Regular packages are traditional packages as they existed in Python 3.2 and earlier. A regular package is typically implemented as a directory containing an __init__.py file. When a regular package is imported, this __init__.py file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The __init__.py file can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.

But just click the link, it contains an example, more information, and an explanation of namespace packages, the kind of packages without __init__.py.

Answered By: Loki

Answer #2:

Files named __init__.py are used to mark directories on disk as Python package directories.
If you have the files

mydir/spam/__init__.py
mydir/spam/module.py

and mydir is on your path, you can import the code in module.py as

import spam.module

or

from spam import module

If you remove the __init__.py file, Python will no longer look for submodules inside that directory, so attempts to import the module will fail.

The __init__.py file is usually empty, but can be used to export selected portions of the package under more convenient name, hold convenience functions, etc.
Given the example above, the contents of the init module can be accessed as

import spam

based on this

Answered By: caritos

Answer #3:

In addition to labeling a directory as a Python package and defining __all__, __init__.py allows you to define any variable at the package level. Doing so is often convenient if a package defines something that will be imported frequently, in an API-like fashion. This pattern promotes adherence to the Pythonic “flat is better than nested” philosophy.

An example

Here is an example from one of my projects, in which I frequently import a sessionmaker called Session to interact with my database. I wrote a “database” package with a few modules:

database/
    __init__.py
    schema.py
    insertions.py
    queries.py

My __init__.py contains the following code:

import os
from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
engine = create_engine(os.environ['DATABASE_URL'])
Session = sessionmaker(bind=engine)

Since I define Session here, I can start a new session using the syntax below. This code would be the same executed from inside or outside of the “database” package directory.

from database import Session
session = Session()

Of course, this is a small convenience — the alternative would be to define Session in a new file like “create_session.py” in my database package, and start new sessions using:

from database.create_session import Session
session = Session()

Further reading

There is a pretty interesting reddit thread covering appropriate uses of __init__.py here:

http://www.reddit.com/r/Python/comments/1bbbwk/whats_your_opinion_on_what_to_include_in_init_py/

The majority opinion seems to be that __init__.py files should be very thin to avoid violating the “explicit is better than implicit” philosophy.

Answered By: Nathan Gould

Answer #4:

There are 2 main reasons for __init__.py

  1. For convenience: the other users will not need to know your functions’ exact location in your package hierarchy.

    your_package/
      __init__.py
      file1.py
      file2.py
        ...
      fileN.py
    
    # in __init__.py
    from file1 import *
    from file2 import *
    ...
    from fileN import *
    
    # in file1.py
    def add():
        pass
    

    then others can call add() by

    from your_package import add
    

    without knowing file1, like

    from your_package.file1 import add
    
  2. If you want something to be initialized; for example, logging (which should be put in the top level):

    import logging.config
    logging.config.dictConfig(Your_logging_config)
    
Answered By: flycee

Answer #5:

The __init__.py file makes Python treat directories containing it as modules.

Furthermore, this is the first file to be loaded in a module, so you can use it to execute code that you want to run each time a module is loaded, or specify the submodules to be exported.

Answered By: Can Berk Güder

Answer #6:

Since Python 3.3, __init__.py is no longer required to define directories as importable Python packages.

Check PEP 420: Implicit Namespace Packages:

Native support for package directories that don’t require __init__.py marker files and can automatically span multiple path segments (inspired by various third party approaches to namespace packages, as described in PEP 420)

Here’s the test:

$ mkdir -p /tmp/test_init
$ touch /tmp/test_init/module.py /tmp/test_init/__init__.py
$ tree -at /tmp/test_init
/tmp/test_init
??? module.py
??? __init__.py
$ python3
>>> import sys
>>> sys.path.insert(0, '/tmp')
>>> from test_init import module
>>> import test_init.module
$ rm -f /tmp/test_init/__init__.py
$ tree -at /tmp/test_init
/tmp/test_init
??? module.py
$ python3
>>> import sys
>>> sys.path.insert(0, '/tmp')
>>> from test_init import module
>>> import test_init.module

references:
https://docs.python.org/3/whatsnew/3.3.html#pep-420-implicit-namespace-packages
https://www.python.org/dev/peps/pep-0420/
Is __init__.py not required for packages in Python 3?

Answered By: zeekvfu

Answer #7:

Although Python works without an __init__.py file you should still include one.

It specifies that the directory should be treated as a package, so therefore include it (even if it is empty).

There is also a case where you may actually use an __init__.py file:

Imagine you had the following file structure:

main_methods
    |- methods.py

And methods.py contained this:

def foo():
    return 'foo'

To use foo() you would need one of the following:

from main_methods.methods import foo # Call with foo()
from main_methods import methods # Call with methods.foo()
import main_methods.methods # Call with main_methods.methods.foo()

Maybe there you need (or want) to keep methods.py inside main_methods (runtimes/dependencies for example) but you only want to import main_methods.


If you changed the name of methods.py to __init__.py then you could use foo() by just importing main_methods:

import main_methods
print(main_methods.foo()) # Prints 'foo'

This works because __init__.py is treated as part of the package.


Some Python packages actually do this. An example is with JSON, where running import json is actually importing __init__.py from the json package (see the package file structure here):

Source code: Lib/json/__init__.py

Answered By: Simon

Answer #8:

In Python the definition of package is very simple. Like Java the hierarchical structure and the directory structure are the same. But you have to have __init__.py in a package. I will explain the __init__.py file with the example below:

package_x/
|--  __init__.py
|--    subPackage_a/
|------  __init__.py
|------  module_m1.py
|--    subPackage_b/
|------  __init__.py
|------  module_n1.py
|------  module_n2.py
|------  module_n3.py

__init__.py can be empty, as long as it exists. It indicates that the directory should be regarded as a package. Of course, __init__.py can also set the appropriate content.

If we add a function in module_n1:

def function_X():
    print "function_X in module_n1"
    return

After running:

>>>from package_x.subPackage_b.module_n1 import function_X
>>>function_X()
function_X in module_n1

Then we followed the hierarchy package and called module_n1 the function. We can use __init__.py in subPackage_b like this:

__all__ = ['module_n2', 'module_n3']

After running:

>>>from package_x.subPackage_b import *
>>>module_n1.function_X()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named module_n1

Hence using * importing, module package is subject to __init__.py content.

Answered By: Marcus Thornton

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