Is __init__.py not required for packages in Python 3.3+
Solution 1
Python 3.3+ has Implicit Namespace Packages that allow it to create a packages without an __init__.py
file.
Allowing implicit namespace packages means that the requirement to provide an
__init__.py
file can be dropped completely, and affected ... .
The old way with __init__.py
files still works as in Python 2.
Solution 2
Overview
@Mike's answer is correct but too imprecise. It is true that Python 3.3+ supports Implicit Namespace Packages that allows it to create a package without an __init__.py
file. This is called a namespace package in contrast to a regular package which does have an __init__.py
file (empty or not empty).
However, creating a namespace package should ONLY be done if there is a need for it. For most use cases and developers out there, this doesn't apply so you should stick with EMPTY __init__.py
files regardless.
Namespace package use case
To demonstrate the difference between the two types of python packages, lets look at the following example:
google_pubsub/ <- Package 1
google/ <- Namespace package (there is no __init__.py)
cloud/ <- Namespace package (there is no __init__.py)
pubsub/ <- Regular package (with __init__.py)
__init__.py <- Required to make the package a regular package
foo.py
google_storage/ <- Package 2
google/ <- Namespace package (there is no __init__.py)
cloud/ <- Namespace package (there is no __init__.py)
storage/ <- Regular package (with __init__.py)
__init__.py <- Required to make the package a regular package
bar.py
google_pubsub
and google_storage
are separate packages but they share the same namespace google/cloud
. In order to share the same namespace, it is required to make each directory of the common path a namespace package, i.e. google/
and cloud/
. This should be the only use case for creating namespace packages, otherwise, there is no need for it.
It's crucial that there are no __init__py
files in the google
and google/cloud
directories so that both directories can be interpreted as namespace packages. In Python 3.3+ any directory on the sys.path
with a name that matches the package name being looked for will be recognized as contributing modules and subpackages to that package. As a result, when you import both from google_pubsub
and google_storage
, the Python interpreter will be able to find them.
This is different from regular packages which are self-contained meaning all parts live in the same directory hierarchy. When importing a package and the Python interpreter encounters a subdirectory on the sys.path
with an __init__.py
file, then it will create a single directory package containing only modules from that directory, rather than finding all appropriately named subdirectories outside that directory. This is perfectly fine for packages that don't want to share a namespace. I highly recommend taking a look at Traps for the Unwary in Python’s Import System to get a better understanding of how Python importing behaves with regular and namespace package and what __init__.py
traps to watch out for.
Summary
- Only skip
__init__.py
files if you want to create namespace packages. Only create namespace packages if you have different libraries that reside in different locations and you want them each to contribute a subpackage to the parent package, i.e. the namespace package. - Keep on adding empty
__init__.py
to your directories because 99% of the time you just want to create regular packages. Also, Python tools out there such asmypy
andpytest
require empty__init__.py
files to interpret the code structure accordingly. This can lead to weird errors if not done with care.
Resources
My answer only touches the surface of how regular packages and namespace packages work, so take a look at the following resources for further information:
- PEP 420 -- Implicit Namespace Packages
- The import system - Regular packages
- The import system - Namespace packages
- Traps for the Unwary in Python’s Import System
Solution 3
If you have setup.py
in your project and you use find_packages()
within it, it is necessary to have an __init__.py
file in every directory for packages to be automatically found.
Packages are only recognized if they include an
__init__.py
file
UPD: If you want to use implicit namespace packages without __init__.py
you just have to use find_namespace_packages()
instead
Solution 4
I would say that one should omit the __init__.py
only if one wants to have the implicit namespace package. If you don't know what it means, you probably don't want it and therefore you should continue to use the __init__.py
even in Python 3.
Solution 5
Based on my experience, even with python 3.3+, an empty __init__.py
is still needed sometimes. One situation is when you want to refer a subfolder as a package. For example, when I ran python -m test.foo
, it didn't work until I created an empty __init__.py
under the test folder. And I'm talking about 3.6.6 version here which is pretty recent.
Apart from that, even for reasons of compatibility with existing source code or project guidelines, its nice to have an empty __init__.py
in your package folder.
Related videos on Youtube
wujek
Updated on October 27, 2021Comments
-
wujek over 2 years
I am using Python 3.5.1. I read the document and the package section here: https://docs.python.org/3/tutorial/modules.html#packages
Now, I have the following structure:
/home/wujek/Playground/a/b/module.py
module.py
:class Foo: def __init__(self): print('initializing Foo')
Now, while in
/home/wujek/Playground
:~/Playground $ python3 >>> import a.b.module >>> a.b.module.Foo() initializing Foo <a.b.module.Foo object at 0x100a8f0b8>
Similarly, now in home, superfolder of
Playground
:~ $ PYTHONPATH=Playground python3 >>> import a.b.module >>> a.b.module.Foo() initializing Foo <a.b.module.Foo object at 0x10a5fee10>
Actually, I can do all kinds of stuff:
~ $ PYTHONPATH=Playground python3 >>> import a >>> import a.b >>> import Playground.a.b
Why does this work? I though there needed to be
__init__.py
files (empty ones would work) in botha
andb
formodule.py
to be importable when the Python path points to thePlayground
folder?This seems to have changed from Python 2.7:
~ $ PYTHONPATH=Playground python >>> import a ImportError: No module named a >>> import a.b ImportError: No module named a.b >>> import a.b.module ImportError: No module named a.b.module
With
__init__.py
in both~/Playground/a
and~/Playground/a/b
it works fine. -
wujek almost 8 yearsI will read the document, but it's a bit long. Is it possible to quickly summarize? Could you just tell me: does it still support init.py, or completely ignores them? If it does support them, what is the difference in functionality and why this duality?
-
Mike Müller almost 8 yearsYes, it still works. So all your Python 2 packages with
__init__.py
files will work in terms of imports (other difference between Python 2 and 3 not considered here). -
Michel Samia over 6 yearsSo the tutorial should be probably updated. Is a documentation bug opened for it?
-
mrgloom almost 6 yearsSuppose I have
run_script.py
in same dir asparent_package
so can I just import likefrom parent_package.child_package import child1
without__init__.py
? -
JayRizzo almost 6 yearsI'm still upset that this defies the
Zen Of Python
line 2:Explicit is better than implicit.
.... -
Mike Müller almost 6 years@JayRizzo But: "Although practicality beats purity."
-
johnbakers almost 6 yearsIs the purpose of this so you can write child_package.some_function even if some_function is defined in childX.py? In another words it avoids requiring the user to know about the different files in child_package? ?
-
Paolo almost 6 years@JayRizzo IMO it is even more explicit. Sometimes it happens to do init stuff in
__init__.py
, sometimes not. In Python 3 when I need these stuff I create a new__init__.py
with specific code, otherwise I don't. This comes handy to know, visually, which packages have custom init. Instead in python 2 I always have to place an__init__.py
(often empty), making a great number of them and finally harder to remember where you placed your init code. This should also fit "There should be one-- and preferably only one --obvious way to do it.". -
JayRizzo almost 6 years@MikeMüller & @Paolo, This is why I love
SO
, Learn from the best! I Now understand better why this is a good thing, reverting my previous statement. =) . Thanks for the insight! -
binki over 5 yearsYeah, I don’t get why you would make
child1.py
,child2.py
instead of just putting their code together into__init__
.py directly. -
mrgloom over 5 yearsIs
__init__.py
is used in any case in Python 3? -
Mike Müller over 5 yearsYes, there are several uses. One of them would "flattening the name space", i.e. importing modules from sub-packages to make them available directly under package. For a large example look a NumPy github.com/numpy/numpy/blob/master/numpy/__init__.py Here
from .core import *
imports all names fromcore
and they can be accessed vianumpy.name
, where 'name' actually lives in 'numpy.core`. -
Halbeard about 5 yearsShouldn't the import statements in
__init__
be relative imports i.e.from . import child1
? The absolute import gives meModuleNotFoundError
(in Python 3.6) -
Prahlad Yeri almost 5 yearsIn my experience, even with python 3.3+, an empty
__init__.py
is still needed sometimes, like when you want to refer a subfolder as a package. For example, if I runpython -m test.foo
it didn't work until I created an empty__init__.py
under the test folder. And I'm talking about 3.6.6 version here! -
jciloa over 4 yearsThis is the actual documentation of how importing works and what built-in variables are available: docs.python.org/3/reference/import.html
-
mzjn almost 4 yearsAccording to dev.to/methane/don-t-omit-init-py-3hga, you should not omit
__init__.py
in your project unless you are 100% sure about what "namespace package" is. -
methane almost 4 yearsSee also packaging.python.org/guides/packaging-namespace-packages "Namespace packages allow you to split the sub-packages and modules within a single package across multiple, separate distribution packages (referred to as distributions in this document to avoid ambiguity)." Namespace package is not a regular package.
__init__.py
is not an old way. It is the only right way to create a regular package. -
ed22 almost 4 yearsIt seems that when I import a package without the init.py everything is imported. Even packages imported by the module I am importing seem to be imported to the end module.
-
NomNomNom over 3 yearsI think it's the other way around because there is a trap related to init with Python 3.3+. Wouldn't it be cleaner to just not have to init if it is empty? Especially if you have people coming from other languages. A question regarding why an empty init exists gets brought up often. If you have a specific need for init then one should use it imo.
-
Mi-La over 3 yearsI guess that the rules come from older versions of Python and changing it dramatically would break all the backward compatibility. I had also problems to understand how the
__init__.py
works and I don't really like it, but we must live with what we get :-). Also note that there are also still some tools that expect__init__.py
to be present in each package to work correctly. I remember that even pylint had some problems to implement implicit namespace packages logic correctly. -
Flimm over 3 years@PrahladYeri This deserves to be an answer.
-
iron9 over 3 yearsCan confirm. Using pytest 6.2.2 and python 3.8, I get a
AttributeError: 'NoneType' object has no attribute 'endswith'
when runningpytest . --doctest-modules
. The error disappeared after I added an empty__init__.py
to one of my directories containing a python file containing doctests. It's NOT necessary to add that file to a different directory also containing a python file containing doctests. I don't understand it at all.