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 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 interpeter 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

In this case, the interpreter will look at the filename of your module, foo.py, strip off the .py, and assign that string to your module's __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 program
before import
before functionA
before functionB
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 functionA
before functionB
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 .pyfile that can be both used by other programs and modules as a module and can be run as the main program. 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.