What Does ‘if name == “main”:’ in Python Mean?

Question
When programming in Python what does 'if name == "main":' mean?

Answer
To grasp this, realize that "main" is merely a quoted string. Now you must know how the built-in variable "name" is assigned. The name variable is set to main at run time by default.

From a command line or in a program, the value for name becomes "main". Here is an example:

>>> print(__name__)
__main__

Create two files foo.py and bar.py in the same directory and run foo.py:

# Here is the content of foo.py:
import bar
#import sys
#sys.path.insert(1, '/tmp/bar.py')
print("!!!!!!!!!!!!!!!!!")
print(__name__)
print("&&&&&&&&&&&&&&&&&") 

Here is the content of bar.py:

print ("Hello World from %s!" % __name__)

print("^^^^^^^^^^^^^^^^^^^^^^^^^^")
#print ("Hello World from %s!" % __main__)

if __name__ == "__main__":
    print("Hello World again from %s!" % __name__)

If you run foo.py (with bar.py in the same directory), you'll see this distinctive printout:

Hello World from bar!
^^^^^^^^^^^^^^^^^^^^^^^^^^
!!!!!!!!!!!!!!!!!
__main__
&&&&&&&&&&&&&&&&&

# You can see the first line was from bar.py (via "import bar").  The next lines were from foo.py.

Now, why would you want to do this 'if name == "main":'?

It allows you to repurpose a complex program. You may want to use a given module bar.py by itself: python bar.py

You may also want bar.py to be invoked via "import bar" from foo.py. You may want to leverage the program's functionality and have it enter an "if" clause when it is invoked via an import statement from a different program. Rather than develop separate programs, it can be very useful to ensure, via the "if" clause, certain preconditions are met before running a portion of the code.

It may be advisable to nest a program's main logic inside such a conditional if the program is going to be pushed to many different servers. Users will not accidentally execute the logic sequence in ways that were not intended. The .py file may be of value as a standalone program or via "import" statements in other programs. This equivalence logic facilitates reusing code and preventing execution that was not intended.

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