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Understanding Dunder Main (__main__)

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Conditional Check

4:31 with Ken Alger

We can utilize a conditional check to verify if our script is being run directly or being imported. Then we can handle those conditions differently based on our needs.

'__name__' is an special variable that holds the name of the program being executed.

We can use '__main__' to check if a script is running and gain more control over our programs

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    Let's start by creating a new file in our workspace called app.py. 0:00
    For our example here, I'll define the symbol method that just prints a little 0:08
    hello statement from our app. 0:12
    And then call the method to print that statement. 0:14
    We'll print 0:22
    Hello from app. 0:27
    Save our file. 0:36
    Go to the Console. 0:38
    And when we run our app, as expected, we get our output., great. 0:41
    Let's create a second file, we'll call it second_app.py. 0:47
    In here, we'll import app, And 0:57
    then we'll add another short print statement, Well, from second app. 1:01
    We can save that, And run it. 1:10
    Let's see what happens now with that import. 1:16
    Well, that's not entirely what we want. 1:20
    We really don't want to be calling print hello just by importing app.py. 1:23
    This is where we can utilize that conditional check with dunder name. 1:27
    Let's head back into app.py and add that. 1:32
    So right above our print_hello call, We'll add, 1:37
    if __name__, == __main__. 1:43
    Come over here, indent. 1:52
    Now Python will check to see if app.py is the main module being run, or 1:55
    if it is being imported. 2:00
    Let's double check this from the command line to make sure 2:03
    things are working as we have intended. 2:06
    We run python app.py again. 2:10
    Nice, that still works. 2:13
    And if we run second_app.py, 2:16
    That's better. 2:22
    And since we've imported app.py into second_app.py, 2:26
    we can call our print hello function when and if we need it. 2:29
    App_print_hello, 2:34
    Save it. 2:44
    Let's clear our console here, And run second_app.py again. 2:46
    Great, so we have access to print hello from our second app. 2:56
    Let's take a quick peek at dunder name by adding another print statement to app.py. 3:00
    Well print __name. 3:06
    Now, when we run our scripts, we can see the value of dunder name, 3:08
    based on if the module is run directly or imported. 3:16
    We'll run app.py and see dunder_name is equal to dunder_main. 3:24
    And if we run second_app.py, We see that dunder_name is equal to app. 3:33
    Notice here as well that by having the print dunder_name statement in app.py 3:41
    outside of a function, that command is run first inside second_app.py. 3:47
    This is another important reason to have statements inside methods or 3:53
    inside a conditional check for dunder name when using imports. 3:58
    Along your journey as a Pythonista, you'll find many recommended best practices. 4:02
    This conditional check is a common part of most Python programs and 4:08
    is used in all sorts of Python applications 4:13
    from trade Python scripts to Python frameworks like Flask and Django. 4:15
    With this best practice in place, you can provide for 4:20
    better management with what is happening in your scripts. 4:23
    I look forward to seeing what you build. 4:27
    Until next time, happy coding. 4:29

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