{"id":1207,"date":"2019-01-09T19:40:19","date_gmt":"2019-01-09T19:40:19","guid":{"rendered":"http:\/\/www.afternerd.com\/blog\/?p=1207"},"modified":"2019-01-13T03:04:38","modified_gmt":"2019-01-13T03:04:38","slug":"python-__name__-__main__","status":"publish","type":"post","link":"https:\/\/www.afternerd.com\/blog\/python-__name__-__main__\/","title":{"rendered":"Python: __name__ == “__main__” Explained"},"content":{"rendered":"

You see it everywhere.<\/p>\n

If you have been coding in Python for a while, or just casually browsing Python Github repositories, you probably have come across this snippet of code.<\/p>\n

if __name__ == '__main__':\r\n  # do something<\/code><\/pre>\n
It is ubiquitous.<\/p>\n
In fact, this is how many times this above snippet appears in Github!<\/p>\n
\"\"<\/p>\n
18 millions+ times!<\/em><\/p>\n
So without further ado, let’s study what this snippet of code does exactly.<\/p>\n
What is __name__?<\/h2>\n
__name__ <\/strong>is simply a built-in variable in Python which evaluates to the name of the current module<\/em>.<\/p>\n
To fully understand what __name__<\/strong> is and how it used, let’s go through a series of examples.<\/p>\n
Example 1<\/h3>\n
Let’s create a simple Python script in a file called test.py<\/strong><\/p>\n
# test.py\r\nprint(\"__name__ in test.py is set to \" + __name__)<\/code><\/pre>\n
What this line does is that it is going to print out the variable __name__<\/strong> on the screen so that we can examine it.<\/p>\n
Let’s run this script from the terminal and see what we get.<\/p>\n
$ python test.py<\/code><\/pre>\n
Here is the output that you will see on the screen.<\/p>\n
$ python3 test.py\r\n__name__ in test.py is set to __main__<\/code><\/pre>\n
From this, we can conclude that in this example, the variable __name__<\/strong> was set to the string value __main__<\/span><\/p>\n
Now let’s look at a slightly different example.<\/p>\n
Example 2<\/h3>\n
Let’s create a new file test2.py\u00a0<\/strong>in the same directory as test.py<\/strong><\/p>\n
In this new file, let’s import test<\/strong> so that we can examine the __name__<\/strong> variable in test.py<\/strong> and let’s also print the __name__<\/strong> variable in test2.py<\/strong><\/p>\n
# test2.py\r\nimport test\r\n\r\nprint(\"__name__ in test2.py is set to \" + __name__)<\/code><\/pre>\n
If you run the test2.py<\/strong> script from the terminal, this is what you will see on your screen.<\/p>\n
$ python3 test2.py\r\n__name__ in test.py is set to test\r\n__name__ in test2.py is set to __main__<\/code><\/pre>\n
Hmm interesting, so what is going on?<\/p>\n
Basically, what’s happening is that\u00a0__name__<\/b>\u00a0 is set at the module level.<\/p>\n
It is set to the\u00a0name<\/b>\u00a0of the module. In other words, for every module in your code, __name__<\/strong> will be set to that module name.<\/p>\n
And for the execution entry point, Python’s main script, the __name__<\/strong> variable will be set to __main__<\/strong><\/p>\n
That’s cool and all but how is this special variable used in practice?<\/p>\n
How is __name__ used in real applications?<\/h2>\n
One reason for doing this is that if you want to write a module that can be executed directly or alternatively be imported and used in another module.<\/p>\n
For example, let’s create a dummy module hello.py<\/strong> that others can import to their own scripts, or they could directly execute it if they wish to.<\/p>\n
Here is what this module is supposed to do.<\/p>\n
If you directly execute the module, then it should print Hello, Afternerd<\/span> on the screen.<\/p>\n
But if you import it instead, it sould provide you a function hello()<\/span> where you can control who to greet.<\/p>\n
# hello.py\r\ndef hello(name):\r\n  print(f\"Hello, {name}!\")\r\n\r\nif __name__ == '__main__':\r\n  hello('Afternerd')<\/code><\/pre>\n
Now let’s create a main script main.py<\/strong> that will import the hello<\/strong> module.<\/p>\n
# main.py\r\nfrom hello import hello\r\n\r\nhello('Karim')<\/code><\/pre>\n
What do you think will happen if we directly execute hello.py<\/strong>?<\/em><\/h3>\n
We just get Hello, Afternerd!<\/strong> printed on the screen since the if condition inside the hello<\/strong> module will be satisfied!<\/p>\n
$ python3 hello.py\r\nHello, Afternerd!<\/code><\/pre>\n
How about if we directly execute main.py<\/strong> instead?<\/em><\/h3>\n
Now, we will only get Hello, Karim!<\/strong> printed on the screen.<\/p>\n
$ python3 main.py\r\nHello, Karim!<\/code><\/pre>\n
But what if we hadn’t included the if __name__ == ‘main’<\/span> condition in the hello<\/strong> module?<\/em><\/h3>\n
If you hadn’t included the if condition in your module, you would get this undesired output when you run the main script.<\/p>\n
$ python3 main.py\r\nHello, Afternerd!\r\nHello, Karim!<\/code><\/pre>\n
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