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 second script will trigger the first 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 mathprint("before functionA")def functionA():    print("Function A")print("before functionB")def functionB():    print("Function B {}".format(math.sqrt(100)))print("before __name__ guard")if __name__ == '__main__':    functionA()    functionB()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 functionA".

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

5. It prints the string "before functionB".

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

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 programbefore importbefore functionAbefore functionBbefore __name__ guardFunction AFunction B 10.0after __name__ guard

# What gets printed if foo is imported as a regular modulebefore importbefore functionAbefore functionBbefore __name__ guardafter __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 interpretersdef functionA():    print("a1")    from foo2 import functionB    print("a2")    functionB()    print("a3")def functionB():    print("b")print("t1")if __name__ == "__main__":    print("m1")    functionA()    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 interpretersdef functionA():    print("a1")    from foo3 import functionB    print("a2")    functionB()    print("a3")def functionB():    print("b")print("t1")print("m1")functionA()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")

When your script is run by passing it as a command to the Python interpreter,

python myscript.py

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.

In this case, the top-level code is an if block. __name__ is a built-in variable which evaluates to the name of the current module. However, if a module is being run directly (as in myscript.py above), then __name__ instead is set to the string "__main__". Thus, you can test whether your script is being run directly or being imported by something else by testing

if __name__ == "__main__":    ...

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.pydef 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.pyimport oneprint("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.pyone.py is being run directly

If you run two.py instead:

python two.py

You get

top-level in one.pyone.py is being imported into another moduletop-level in two.pyfunc() in one.pytwo.py is being run directly

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

The simplest explanation for the __name__ variable (imho) is the following:

Create the following files.

# a.pyimport b

and

# b.pyprint "Hello World from %s!" % __name__if __name__ == '__main__':    print "Hello World again from %s!" % __name__

Running them will get you this output:

$ python a.pyHello World from b!

As you can see, when a module is imported, Python sets globals()['__name__'] in this module to the module's name. Also, upon import all the code in the module is being run. As the if statement evaluates to False this part is not executed.

$ python b.pyHello World from __main__!Hello World again from __main__!

As you can see, when a file is executed, Python sets globals()['__name__'] in this file to "__main__". This time, the if statement evaluates to True and is being run.