python: __name__

simply say:

the __name__ always points to the name space of the currently running module. so, if it's running in an imported module, then calling __name__ would return the name of that module; if it's running in the main module, then it will return __main__.

exmaple:

# module1.py

print("the current module is: {}".format(__name__))

# module2.py
import module1
print("the current module is: {}".format(__name__))


>>> python3 module1.py
the current module is: __main__

>>> python3 module2.py
the current module is: module1.py
the current module is: __main__

When the Python interpreter reads a source file, it executes all of the code found in it.

Before executing the code, it will define a few special variables. For example, if the Python interpreter is running that module (the source file) as the main program, it sets the special __name__ variable to have a value "__main__". If this file is being imported from another module, __name__ will be set to the module's name.

In the case of your script, let's assume that it's executing as the main function, e.g. you said something like

>>> python threading_example.py

on the command line. After setting up the special variables, it will execute the import statement and load those modules. It will then evaluate the def block, creating a function object and creating a variable called myfunction that points to the function object. It will then read the ifstatement and see that __name__ does equal "__main__", so it will execute the block shown there.

One reason for doing this is that sometimes you write a module (a .py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.

See this page for some extra details.

Note (by Stainsor): If you put code before the function definitions, it will execute before main.

print("This code executes before main.") 

def functionA():
    print("Function A")

def functionB():
    print("Function B")

if __name__ == '__main__':
    functionA()
    functionB()

If this module is indeed main, this code results in:

This code executes before main. 
Function A 
Function B

If this module is not main, you get:

This code executes before main. 

all of the code that is at indentation level 0 gets executed. Functions and classes that are defined are, well, defined, but none of their code gets run. Unlike other languages, there's no main()function that gets run automatically - the main() function is implicitly all the code at the top level.

If your script is being imported into another module, its various function and class definitions will be imported and its top-level code will be executed, but the code in the then-body of the if clause above won't get run as the condition is not met. As a basic example, consider the following two scripts:

# file one.py
def func():
    print("func() in one.py")

print("top-level in one.py")

if __name__ == "__main__":
    print("one.py is being run directly")
else:
    print("one.py is being imported into another module")

# file two.py
import one

print("top-level in two.py")
one.func()

if __name__ == "__main__":
    print("two.py is being run directly")
else:
    print("two.py is being imported into another module")

Now, if you invoke the interpreter as

python one.py

The output will be

top-level in one.py
one.py is being run directly

If you run two.py instead:

python two.py

You get

top-level in one.py
one.py is being imported into another module
top-level in two.py
func() in one.py
two.py is being run directly

Thus, when module one gets loaded, its __name__ equals "one" instead of "__main__".

source: https://stackoverflow.com/questions/419163/what-does-if-name-main-do

Why do we need this?

Developing and Testing Your Code

Say you're writing a Python script designed to be used as a module:

def do_important():
    """This function does something very important"""

You could test the module by adding this call of the function to the bottom:

do_important()

and running it (on a command prompt) with something like:

>>> python important.py

The Problem

However, if you want to import the module to another script:

import important

On import, the do_important function would be called, so you'd probably comment out your function call, do_important(), at the bottom.

# do_important() # I must remember to uncomment to execute this!

And then you'll have to remember whether or not you've commented out your test function call. And this extra complexity would mean you're likely to forget, making your development process more troublesome.

A Better Way

The __name__ variable points to the namespace wherever the Python interpreter happens to be at the moment.

Inside an imported module, it's the name of that module.

But inside the primary module (or an interactive Python session, i.e. the interpreter's Read, Eval, Print Loop, or REPL) you are running everything from its "__main__".

So if you check before executing:

if __name__ == "__main__":
    do_important()

With the above, your code will only execute when you're running it as the primary module (or intentionally call it from another script).

An Even Better Way

There's a Pythonic way to improve on this, though.

What if we want to run this business process from outside the module?

If we put the code we want to exercise as we develop and test in a function like this and then do our check for '__main__' immediately after:

def main():
    """business logic for when running this module as the primary one!"""
    setup()
    foo = do_important()
    bar = do_even_more_important(foo)
    for baz in bar:
        do_super_important(baz)
    teardown()

# Here's our payoff idiom!
if __name__ == '__main__':
    main()

We now have a final function for the end of our module that will run if we run the module as the primary module.

It will allow the module and its functions and classes to be imported into other scripts without running the main function, and will also allow the module (and its functions and classes) to be called when running from a different '__main__' module, i.e.

import important
important.main()

This idiom can also be found in the Python documentation in an explanation of the __main__module. That text states:

This module represents the (otherwise anonymous) scope in which the interpreter’s main program executes — commands read either from standard input, from a script file, or from an interactive prompt. It is this environment in which the idiomatic “conditional script” stanza causes a script to run:

if __name__ == '__main__':
    main()