__init__.py files. What does it do?
It’s a list of public objects of that module, as interpreted by
import *. It overrides the default of hiding everything that begins with an underscore.
Linked to, but not explicitly mentioned here, is exactly when
__all__ is used. It is a list of strings defining what symbols in a module will be exported when
from <module> import * is used on the module.
For example, the following code in a
foo.py explicitly exports the symbols
__all__ = ['bar', 'baz'] waz = 5 bar = 10 def baz(): return 'baz'
These symbols can then be imported like so:
from foo import * print(bar) print(baz) # The following will trigger an exception, as "waz" is not exported by the module print(waz)
__all__ above is commented out, this code will then execute to completion, as the default behaviour of
import * is to import all symbols that do not begin with an underscore, from the given namespace.
__all__ affects the
from <module> import * behavior only. Members that are not mentioned in
__all__ are still accessible from outside the module and can be imported with
from <module> import <member>.
It also changes what pydoc will show:
a = "A" b = "B" c = "C"
__all__ = ['a', 'b'] a = "A" b = "B" c = "C"
$ pydoc module1
Help on module module1: NAME module1 FILE module1.py DATA a = 'A' b = 'B' c = 'C'
$ pydoc module2
Help on module module2: NAME module2 FILE module2.py DATA __all__ = ['a', 'b'] a = 'A' b = 'B'
__all__ in all my modules, as well as underscore internal details, these really help when using things you’ve never used before in live interpreter sessions.
I’m just adding this to be precise:
All other answers refer to modules. The original question explicitely mentioned
__init__.py files, so this is about python packages.
__all__ only comes into play when the
from xxx import * variant of the
import statement is used. This applies to packages as well as to modules.
The behaviour for modules is explained in the other answers. The exact behaviour for packages is described here in detail.
__all__ on package level does approximately the same thing as for modules, except it deals with modules within the package (in contrast to specifying names within the module). So
__all__ specifies all modules that shall be loaded and imported into the current namespace when us use
from package import *.
The big difference is, that when you omit the declaration of
__all__ in a package’s
__init__.py, the statement
from package import * will not import anything at all (with exceptions explained in the documentation, see link above).
On the other hand, if you omit
__all__ in a module, the “starred import” will import all names (not starting with an underscore) defined in the module.
Explain all in Python?
I keep seeing the variable
__all__set in different
What does this do?
It declares the semantically "public" names from a module. If there is a name in
__all__, users are expected to use it, and they can have the expectation that it will not change.
It also will have programmatic effects:
__all__ in a module, e.g.
__all__ = ['foo', 'Bar']
means that when you
import * from the module, only those names in the
__all__ are imported:
from module import * # imports foo and Bar
Documentation and code autocompletion tools may (in fact, should) also inspect the
__all__ to determine what names to show as available from a module.
__init__.pymakes a directory a Python package
From the docs:
__init__.pyfiles are required to make Python treat the directories as containing packages; this is done to prevent directories with a common name, such as string, from unintentionally hiding valid modules that occur later on the module search path.
In the simplest case,
__init__.pycan just be an empty file, but it can also execute initialization code for the package or set the
__init__.py can declare the
__all__ for a package.
A package is typically made up of modules that may import one another, but that are necessarily tied together with an
__init__.py file. That file is what makes the directory an actual Python package. For example, say you have the following files in a package:
package ├── __init__.py ├── module_1.py └── module_2.py
Let’s create these files with Python so you can follow along – you could paste the following into a Python 3 shell:
from pathlib import Path package = Path('package') package.mkdir() (package / '__init__.py').write_text(""" from .module_1 import * from .module_2 import * """) package_module_1 = package / 'module_1.py' package_module_1.write_text(""" __all__ = ['foo'] imp_detail1 = imp_detail2 = imp_detail3 = None def foo(): pass """) package_module_2 = package / 'module_2.py' package_module_2.write_text(""" __all__ = ['Bar'] imp_detail1 = imp_detail2 = imp_detail3 = None class Bar: pass """)
And now you have presented a complete api that someone else can use when they import your package, like so:
import package package.foo() package.Bar()
And the package won’t have all the other implementation details you used when creating your modules cluttering up the
After more work, maybe you’ve decided that the modules are too big (like many thousands of lines?) and need to be split up. So you do the following:
package ├── __init__.py ├── module_1 │ ├── foo_implementation.py │ └── __init__.py └── module_2 ├── Bar_implementation.py └── __init__.py
First make the subpackage directories with the same names as the modules:
subpackage_1 = package / 'module_1' subpackage_1.mkdir() subpackage_2 = package / 'module_2' subpackage_2.mkdir()
Move the implementations:
package_module_1.rename(subpackage_1 / 'foo_implementation.py') package_module_2.rename(subpackage_2 / 'Bar_implementation.py')
__init__.pys for the subpackages that declare the
__all__ for each:
(subpackage_1 / '__init__.py').write_text(""" from .foo_implementation import * __all__ = ['foo'] """) (subpackage_2 / '__init__.py').write_text(""" from .Bar_implementation import * __all__ = ['Bar'] """)
And now you still have the api provisioned at the package level:
>>> import package >>> package.foo() >>> package.Bar() <package.module_2.Bar_implementation.Bar object at 0x7f0c2349d210>
And you can easily add things to your API that you can manage at the subpackage level instead of the subpackage’s module level. If you want to add a new name to the API, you simply update the
__init__.py, e.g. in module_2:
from .Bar_implementation import * from .Baz_implementation import * __all__ = ['Bar', 'Baz']
And if you’re not ready to publish
Baz in the top level API, in your top level
__init__.py you could have:
from .module_1 import * # also constrained by __all__'s from .module_2 import * # in the __init__.py's __all__ = ['foo', 'Bar'] # further constraining the names advertised
and if your users are aware of the availability of
Baz, they can use it:
import package package.Baz()
but if they don’t know about it, other tools (like pydoc) won’t inform them.
You can later change that when
Baz is ready for prime time:
from .module_1 import * from .module_2 import * __all__ = ['foo', 'Bar', 'Baz']
By default, Python will export all names that do not start with an
_ when imported with
import *. As demonstrated by the shell session here,
import * does not bring in the
_us_non_public name from the
$ cat us.py USALLCAPS = "all caps" us_snake_case = "snake_case" _us_non_public = "shouldn't import" $ python Python 3.10.0 (default, Oct 4 2021, 17:55:55) [GCC 10.3.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> from us import * >>> dir() ['USALLCAPS', '__annotations__', '__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__', 'us_snake_case']
You certainly could rely on this mechanism. Some packages in the Python standard library, in fact, do rely on this, but to do so, they alias their imports, for example, in
import os as _os, sys as _sys
_ convention can be more elegant because it removes the redundancy of naming the names again. But it adds the redundancy for imports (if you have a lot of them) and it is easy to forget to do this consistently – and the last thing you want is to have to indefinitely support something you intended to only be an implementation detail, just because you forgot to prefix an
_ when naming a function.
I personally write an
__all__ early in my development lifecycle for modules so that others who might use my code know what they should use and not use.
Most packages in the standard library also use
It makes sense to stick to the
_ prefix convention in lieu of
The downside of using
__all__ is that you have to write the names of functions and classes being exported twice – and the information is kept separate from the definitions. We could use a decorator to solve this problem.
I got the idea for such an export decorator from David Beazley’s talk on packaging. This implementation seems to work well in CPython’s traditional importer. If you have a special import hook or system, I do not guarantee it, but if you adopt it, it is fairly trivial to back out – you’ll just need to manually add the names back into the
So in, for example, a utility library, you would define the decorator:
import sys def export(fn): mod = sys.modules[fn.__module__] if hasattr(mod, '__all__'): mod.__all__.append(fn.__name__) else: mod.__all__ = [fn.__name__] return fn
and then, where you would define an
__all__, you do this:
$ cat > main.py from lib import export __all__ =  # optional - we create a list if __all__ is not there. @export def foo(): pass @export def bar(): 'bar' def main(): print('main') if __name__ == '__main__': main()
And this works fine whether run as main or imported by another function.
$ cat > run.py import main main.main() $ python run.py main
And API provisioning with
import * will work too:
$ cat > run.py from main import * foo() bar() main() # expected to error here, not exported $ python run.py Traceback (most recent call last): File "run.py", line 4, in <module> main() # expected to error here, not exported NameError: name 'main' is not defined
A module is a
.py file meant to be imported.
A package is a directory with a
__init__.py file. A package usually contains modules.
""" cheese.py - an example module """ __all__ = ['swiss', 'cheddar'] swiss = 4.99 cheddar = 3.99 gouda = 10.99
cheddar are brought into the local namespace, but not
>>> from cheese import * >>> swiss, cheddar (4.99, 3.99) >>> gouda Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'gouda' is not defined
__all__, any symbol (that doesn’t start with an underscore) would have been available.
*are not affected by
>>> import cheese >>> cheese.swiss, cheese.cheddar, cheese.gouda (4.99, 3.99, 10.99)
>>> from cheese import swiss, cheddar, gouda >>> swiss, cheddar, gouda (4.99, 3.99, 10.99)
>>> import cheese as ch >>> ch.swiss, ch.cheddar, ch.gouda (4.99, 3.99, 10.99)
__init__.py file of a package
__all__ is a list of strings with the names of public modules or other objects. Those features are available to wildcard imports. As with modules,
__all__ customizes the
* when wildcard-importing from the package.[@MartinStettner]
Here’s an excerpt from the Python MySQL Connector
__all__ = [ 'MySQLConnection', 'Connect', 'custom_error_exception', # Some useful constants 'FieldType', 'FieldFlag', 'ClientFlag', 'CharacterSet', 'RefreshOption', 'HAVE_CEXT', # Error handling 'Error', 'Warning', ...etc... ]
The default case, asterisk with no
__all__ for a package, is complicated, because the obvious behavior would be expensive: to use the file system to search for all modules in the package. Instead, in my reading of the docs, only the objects defined in
__init__.py are imported:
__all__is not defined, the statement
from sound.effects import *does not import all submodules from the package
sound.effectsinto the current namespace; it only ensures that the package
sound.effectshas been imported (possibly running any initialization code in
__init__.py) and then imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by
__init__.py. It also includes any submodules of the package that were explicitly loaded by previous import statements.
And lastly, a venerated tradition for stack overflow answers, professors, and mansplainers everywhere, is the bon mot of reproach for asking a question in the first place:
Wildcard imports … should be avoided, as they [confuse] readers and many automated tools.
[PEP 8, @ToolmakerSteve]
__all__ is used to document the public API of a Python module. Although it is optional,
__all__ should be used.
Here is the relevant excerpt from the Python language reference:
The public names defined by a module are determined by checking the module’s namespace for a variable named
__all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in
__all__are all considered public and are required to exist. If
__all__is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (‘_’).
__all__should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).
PEP 8 uses similar wording, although it also makes it clear that imported names are not part of the public API when
__all__ is absent:
To better support introspection, modules should explicitly declare the names in their public API using the
__all__to an empty list indicates that the module has no public API.
Imported names should always be considered an implementation detail. Other modules must not rely on indirect access to such imported names unless they are an explicitly documented part of the containing module’s API, such as
os.pathor a package’s
__init__module that exposes functionality from submodules.
Furthermore, as pointed out in other answers,
__all__ is used to enable wildcard importing for packages:
The import statement uses the following convention: if a package’s
__init__.pycode defines a list named
__all__, it is taken to be the list of module names that should be imported when
from package import *is encountered.
from <module> import * statements.
Consider this example:
foo ├── bar.py └── __init__.py
(Implicit) If we don’t define
from foo import * will only import names defined in
(Explicit) If we define
__all__ = , then
from foo import * will import nothing.
(Explicit) If we define
__all__ = [ <name1>, ... ], then
from foo import * will only import those names.
Note that in the implicit case, python won’t import names starting with
_. However, you can force importing such names using
You can view the Python document here.
__all__ affects how
from foo import * works.
Code that is inside a module body (but not in the body of a function or class) may use an asterisk (
*) in a
from foo import *
* requests that all attributes of module
foo (except those beginning with underscores) be bound as global variables in the importing module. When
foo has an attribute
__all__, the attribute’s value is the list of the names that are bound by this type of
foo is a package and its
__init__.py defines a list named
__all__, it is taken to be the list of submodule names that should be imported when
from foo import * is encountered. If
__all__ is not defined, the statement
from foo import * imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by
__all__ doesn’t have to be a list. As per the documentation on the
import statement, if defined,
__all__ must be a sequence of strings which are names defined or imported by the module. So you may as well use a tuple to save some memory and CPU cycles. Just don’t forget a comma in case the module defines a single public name:
__all__ = ('some_name',)
See also Why is “import *” bad?
This is defined in PEP8 here:
Global Variable Names
(Let’s hope that these variables are meant for use inside one module only.) The conventions are about the same as those for functions.
Modules that are designed for use via
from M import *should use the
__all__mechanism to prevent exporting globals, or use the older convention of prefixing such globals with an underscore (which you might want to do to indicate these globals are "module non-public").
PEP8 provides coding conventions for the Python code comprising the standard library in the main Python distribution. The more you follow this, closer you are to the original intent.