What Does _All_ Mean in Python

What does __all__ mean in Python?

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.

What's the python __all__ module level variable for?

It has two purposes:

1. Anybody who reads the source will know what the exposed public API is. It doesn't prevent them from poking around in private declarations, but does provide a good warning not to.

2. When using from mod import *, only names listed in __all__ will be imported. This is not as important, in my opinion, because importing everything is a really bad idea.

Why does __all__ work differently in packages than in modules?

The documentation is a bit hard to parse because it does not mention that packages generally also have the behavior of modules, including their __all__ attribute. The behavior of packages is necessarily a superset of the behavior of modules, because packages, unlike modules, can have sub-packages and sub-modules. Behaviors not related to that feature are identical between the two as far as the end-user is concerned.

The python docs can be minimalistic at times. They did not bother to mention that

1. Package __init__ performs all the module-like code for a package, including support for star-import for direct attributes via __all__, just like a module does.
2. Modules support all the features of a package __init__.py, except that they can't have a sub-package or sub-module.

It goes without saying that to make a name refer to a sub-module, it has to be imported, hence the apparent, but not really double-standard.

Is __all__ in __init__.py, needed when accessing class or method?

The tutorial about modules explains that __all__ is actually used to restrict what is imported when you use import *.

If __all__ is defined then when you import * from your package, only the names defined in __all__ are imported in the current namespace.

So it can be redundant, but only if you put in __all__ everything you import and define in your package.

Minimal working example:

testing/__init__.py:

from .moda import A
from .modb import B

__all__ = ['A']

testing/moda.py:

class A:
    pass

testing/modb.py:

class B:
    pass

Then when I import * from testing, I've got only A imported in current namespace, not B:

>>> from testing import *
>>> dir()
['A', '__annotations__', '__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__']
>>> 'B' in dir()
False
>>> 'A' in dir()
True

What is __init__.py for?

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.

What does if __name__ == __main__: do?

Short Answer

It's boilerplate code that protects users from accidentally invoking the script when they didn't intend to. Here are some common problems when the guard is omitted from a script:

• If you import the guardless script in another script (e.g. import my_script_without_a_name_eq_main_guard), then the latter script will trigger the former to run at import time and using the second script's command line arguments. This is almost always a mistake.

• If you have a custom class in the guardless script and save it to a pickle file, then unpickling it in another script will trigger an import of the guardless script, with the same problems outlined in the previous bullet.

Long Answer

To better understand why and how this matters, we need to take a step back to understand how Python initializes scripts and how this interacts with its module import mechanism.

Whenever the Python interpreter reads a source file, it does two things:

• it sets a few special variables like __name__, and then

• it executes all of the code found in the file.

Let's see how this works and how it relates to your question about the __name__ checks we always see in Python scripts.

Code Sample

Let's use a slightly different code sample to explore how imports and scripts work. Suppose the following is in a file called foo.py.

# Suppose this is foo.py.

print("before import")
import math

print("before function_a")
def function_a():
    print("Function A")

print("before function_b")
def function_b():
    print("Function B {}".format(math.sqrt(100)))

print("before __name__ guard")
if __name__ == '__main__':
    function_a()
    function_b()
print("after __name__ guard")

Special Variables

When the Python interpreter reads a source file, it first defines a few special variables. In this case, we care about the __name__ variable.

When Your Module Is the Main Program

If you are running your module (the source file) as the main program, e.g.

python foo.py

the interpreter will assign the hard-coded string "__main__" to the __name__ variable, i.e.

# It's as if the interpreter inserts this at the top
# of your module when run as the main program.
__name__ = "__main__" 

When Your Module Is Imported By Another

On the other hand, suppose some other module is the main program and it imports your module. This means there's a statement like this in the main program, or in some other module the main program imports:

# Suppose this is in some other main program.
import foo

The interpreter will search for your foo.py file (along with searching for a few other variants), and prior to executing that module, it will assign the name "foo" from the import statement to the __name__ variable, i.e.

# It's as if the interpreter inserts this at the top
# of your module when it's imported from another module.
__name__ = "foo"

Executing the Module's Code

After the special variables are set up, the interpreter executes all the code in the module, one statement at a time. You may want to open another window on the side with the code sample so you can follow along with this explanation.

Always

1. It prints the string "before import" (without quotes).

2. It loads the math module and assigns it to a variable called math. This is equivalent to replacing import math with the following (note that __import__ is a low-level function in Python that takes a string and triggers the actual import):

# Find and load a module given its string name, "math",
# then assign it to a local variable called math.
math = __import__("math")

3. It prints the string "before function_a".

4. It executes the def block, creating a function object, then assigning that function object to a variable called function_a.

5. It prints the string "before function_b".

6. It executes the second def block, creating another function object, then assigning it to a variable called function_b.

7. It prints the string "before __name__ guard".

Only When Your Module Is the Main Program

8. If your module is the main program, then it will see that __name__ was indeed set to "__main__" and it calls the two functions, printing the strings "Function A" and "Function B 10.0".

Only When Your Module Is Imported by Another

8. (instead) If your module is not the main program but was imported by another one, then __name__ will be "foo", not "__main__", and it'll skip the body of the if statement.

Always

9. It will print the string "after __name__ guard" in both situations.

Summary

In summary, here's what'd be printed in the two cases:

# What gets printed if foo is the main program
before import
before function_a
before function_b
before __name__ guard
Function A
Function B 10.0
after __name__ guard

# What gets printed if foo is imported as a regular module
before import
before function_a
before function_b
before __name__ guard
after __name__ guard

Why Does It Work This Way?

You might naturally wonder why anybody would want this. Well, sometimes you want to write a .py file that can be both used by other programs and/or modules as a module, and can also be run as the main program itself. Examples:

• Your module is a library, but you want to have a script mode where it runs some unit tests or a demo.

• Your module is only used as a main program, but it has some unit tests, and the testing framework works by importing .py files like your script and running special test functions. You don't want it to try running the script just because it's importing the module.

• Your module is mostly used as a main program, but it also provides a programmer-friendly API for advanced users.

Beyond those examples, it's elegant that running a script in Python is just setting up a few magic variables and importing the script. "Running" the script is a side effect of importing the script's module.

Food for Thought

• Question: Can I have multiple __name__ checking blocks? Answer: it's strange to do so, but the language won't stop you.

• Suppose the following is in foo2.py. What happens if you say python foo2.py on the command-line? Why?

# Suppose this is foo2.py.
import os, sys; sys.path.insert(0, os.path.dirname(__file__)) # needed for some interpreters

def function_a():
    print("a1")
    from foo2 import function_b
    print("a2")
    function_b()
    print("a3")

def function_b():
    print("b")

print("t1")
if __name__ == "__main__":
    print("m1")
    function_a()
    print("m2")
print("t2")
      

• Now, figure out what will happen if you remove the __name__ check in foo3.py:

# Suppose this is foo3.py.
import os, sys; sys.path.insert(0, os.path.dirname(__file__)) # needed for some interpreters

def function_a():
    print("a1")
    from foo3 import function_b
    print("a2")
    function_b()
    print("a3")

def function_b():
    print("b")

print("t1")
print("m1")
function_a()
print("m2")
print("t2")

• What will this do when used as a script? When imported as a module?

# Suppose this is in foo4.py
__name__ = "__main__"

def bar():
    print("bar")
    
print("before __name__ guard")
if __name__ == "__main__":
    bar()
print("after __name__ guard")

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