Merge pull request #1596 from python-discord/nedbat/if-name-main

if __name__ == '__main__':

Author: L3viathan

pydis_site/apps/content/resources/guides/python-guides/if-name-main.md

@@ -0,0 +1,67 @@
+---
+title: '`if __name__ == "__main__"`'
+description: 'What is `if __name__ == "__main__"`, and why would you use it?'
+---
+
+You might see this structure in a Python file:
+
+```py
+# prog.py
+
+def main_fn():
+    # Put the main part of your program here.
+
+if __name__ == "__main__":
+    main_fn()
+```
+
+If the file is run directly (with `python prog.py` or `py prog.py`), it's called the main file of your program. In that case, our `main_fn()` function will be run.  If instead the file is imported by another file, then `main_fn()` will not be run.
+
+When Python runs your module (your file), it automatically sets the `__name__` special global variable to `"__main__"`. The other way to run your file is to import it in a larger program. In that case, `__name__` is instead set to the filename of your module minus the `.py` extension.
+
+
+## Another Example
+
+```py
+# foo.py
+
+print("spam")
+
+if __name__ == "__main__":
+    print("eggs")
+```
+
+If you run the above module `foo.py` directly, `__name__` will be equal to `"__main__"`, so  both `spam`and `eggs` will be printed. Now consider this next example:
+
+```py
+# bar.py
+
+import foo
+```
+
+If you run this file, it will import foo, which executes the code in `foo.py`. Now in foo.py `__name__` will be equal to `"foo"`. First it will print `spam`, and then the `if` statement will be false, so `eggs` won't print.
+
+Often the code in the `if __name__` clause is a call to a main function, but it can be any code that you want.  Some files simply put their main code there even if its dozens of lines.
+
+If there is just a single function call in the `if __name__` clause, it's often called `main()`, but that name isn't special to Python. You can call any code you like. Many people use `main()` because it's a clear indication of what's going on.
+
+
+## Why would I do this?
+
+There are a few reasons for using this structure, mostly about having two different uses for the file, one if it's run directly, and a different one for when it's imported as part of a larger program.
+
+The most common reason is your module is a library, but also has a special case where it can be run directly. Sometimes libraries have small utility or demonstration scripts provided in the `__main__` clause.
+
+Even if you don't intend your file to be used in two different ways, the convention of `if __name__ == "__main__":` gives a clear indication to the reader that this is the main file of the program.
+
+
+## Why not have a `main` keyword?
+
+Python often uses so-called "dunder" names for special behavior. The global name `__name__` lets files know how they are being run: directly or imported. Other languages might have a special name for the main function. Python instead lets you write your own if statement using `__name__` to decide what code should run be as the main body.
+
+
+## Other resources
+
+- The Python docs have a section on [Idiomatic usage of `__main__`](https://docs.python.org/3/library/__main__.html#idiomatic-usage).
+
+- Real Python has a [longer tutorial on `__main__`](https://realpython.com/if-name-main-python/).