Is it? I really don’t think so. What can you propose that’s better? I think if __name__ == __main__ works perfectly fine and can’t really think of anything that would be better.

And you don’t have to use it either if you don’t want to anyway, so no, I don’t think it’s that much of a hack. Especially when the comic compares C as an example, which makes no sense to me whatsoever.

aren’t most of not all conventions hacky anyways?

Tbh reserving “main” is just a hacky if not more so than checking __name__ if you actually understand language design.

Interesting observation. Can you give an example where this is relevant?

Why would you waste a function call on something so completely redundant?

~For real though, arg parsing goes in the if, then gets dispatched to whatever function call is needed to run the proper script.~

you can, no one stopping you

If the file is just a class I usually put example usage with some default arguments in that block by itself. There is no reason for a “main” function. It’s a nice obvious block that doesn’t run when someone imports the class but if they’re looking at the class there is a really obvious place to see the class usage. No confusion about what “main()” is meant to do.

if __name__ == '__main__':
    # MyClass example Usage
    my_object = MyClass()
    my_object.my_method()

I work in an academic / research environment. Depending who wrote it, even seeing a __name__ == "__main__" is a bit of a rare thing…

I definitely do for quick scripts, but I try to break this habit. The biggest advantage of def main() is that variables are local and not accessible to other functions defined in the same script, which can sometimes help catch bugs or typos.

Looks at all the Python scripts in my bin folder that I wrote.

Since Java 21, this has been shortened significantly. https://www.baeldung.com/java-21-unnamed-class-instance-main

I’m a little new to Python standards. Is this better or worse than putting the def main(): outside the if statement (but calling main() inside it)

I always use

if "__main__" == main:
    __main__()

…and earlier in the code:

def __main__():
    while True:
        pass
main = "__main__"

This helps to prevent people from arbitrarily running my code as a library or executable when I don’t went them to.

There are a lot of other helpful replies in this thread, so I won’t add much, but I did find this reference, which you could read if you have a lot of free time. But I particularly liked reading this summary:

• _start calls the libc __libc_start_main;
• __libc_start_main calls the executable __libc_csu_init (statically-linked part of the libc);
• __libc_csu_init calls the executable constructors (and other initialisatios);
• __libc_start_main calls the executable main();
• __libc_start_main calls the executable exit().

I haven’t done much low level stuff, but I think the ‘main’ function is something the compiler uses to establish an entry point for the compiled binary. The name ‘main’ would not exist in the compiled binary at all, but the function itself would still exist. Executable formats aren’t all the same, so they’ll have different ways of determining where this entry point function is expected to be. You can ‘run’ a binary library file by invoking a function contained therein, which is how DLL files work.

How does executing a program actually work?

Way too long an answer for a lemmy post

It has an executable flag, but what actually happens in the OS when it encounters a file with an executable file?

Depends on OS. Linux will look at the first bytes of the file, either see (ASCII) #! (called a shebang) or ELF magic, then call the appropriate interpreter with the executable as an argument. When executing e.g. python, it’s going to call /usr/bin/env with parameters python and the file name because the shebang was #!/usr/bin/env python.

How does it know to execute “main”?

Compiled C programs are ELF so it will go through the ELF header, figure out which ld.so to use, then start that so that it will find all the libraries, resolve all dynamic symbols, then do some bookkeeping, and jump to _start. That is, it doesn’t: main is a C thing.

Is it possible to have a library that can be called and also executed like a program?

Absolutely. ld.so is an example of that.. Actually, wait, I’m not so sure any more, I’m getting things mixed up with libdl.so. In any case ld.so is an executable with a file extension that makes it look like a library.

EDIT: It does work. My (GNU) libc spits out version info when executed as an executable.

If you want to start looking at the innards like that I would suggest starting here: Hello world in assembly. Note the absence of a main function, the symbol the kernel actually invokes is _start, the setup necessary to call a C main is done by libc.so. Don’t try to understand GNU’s libc it’s full of hystarical raisins I would suggest musl.

If you want to have a library that can also be a standalone executable, just put the main function in an extra file and don’t compile that file when using the library as a library.
You could also use the preprocessor to do it similar to python but please don’t.

Just use any build tool, and have two targets, one library and one executable:

LIB_SOURCES = tools.c, stuff.c, more.c
EXE_SOURCES = main.c, $LIB_SOURCES

Edit: added example

You don’t. In C everything gets referenced by a symbol during the link stage of compilation. Libraries ultimately get treated like your source code during compilation and all items land in a symbol table. Two items with the same name result in a link failure and compilation aborts. So a library and a program with main is no bueno.

When Linux loads an executable they basically look at the program’s symbol table and search for “main” then start executing at that point

Windows behaves mostly the same way, as does MacOS. Most RTOS’s have their own special way of doing things, bare metal you’re at the mercy of your CPU vendor. The C standard specifies that “main” is the special symbol we all just happen to use

All code needs to have an entry point.

For Python and some other languages, this is the start of the file.

For other languages, this is a special function name reserved for this purpose - generally, “main”.

In the first kind of language, the thought process is basically: I have the flow of execution, starting at the top of the file. If I want to make a library, I should build the things I want to build, then get out of the way.

In the other kind of language, the thought process is basically: I am building a library. If I want to make an executable, I should create an entry point they the execution starts at.

The debate is honestly pretty dumb.

Python has a bunch of magic variables, like __name__. This one contains the name of the module you’re currently in (usually based on the file name), so if your file is called foo.py, it will have the value foo.

But that’s only if your module is being imported by another module. If it’s executed directly (e.g. python foo.py), it will instead have a __name__ of __main__. This is often used to add a standalone CLI section to modules - e.g. the module usually only defines functions that can be imported, but when executed it runs an example of those functions.

Basically, when you compile a program written in Rust or C/C++ (the first and second panels respectively), the compiler needs to know what’s supposed to be executed first when the program is run directly (i.e. when you click on the executable), which in these languages, is denoted by a special function called main(). Executable files can also contain functions and data structures that can be called by other programs, and when they are, you wouldn’t want to run an entire complex and resource intensive program if another program only needs to call a single function from it. In that case, the other program will call the function it wants but not main, so only that function executes and not the entire program.

However, Python is a scripting language that’s interpreted. So every Python source file is executable provided you have the Python runtime. Python also doesn’t have native support for main functions in the same way Rust and C/C++ does, and it will execute every line of code as it reads the source file. This is why a single line Python file that just calls print is valid, it doesn’t need to be wrapped in a main function to execute. However, what if your Python file is both meant to be executed directly and provides functions that other Python files can call? If you just put the main routine in the root of the file, it would be executed every time another program tries to import the file in order to call functions from it, since the import causes the file to be interpreted and executed in its entirety. You can still just have a main function in your file, but since Python doesn’t natively support it, your main function won’t do anything if you run the file directly because as far as Python is concerned, there is no executable code at the root of the file and you haven’t called any functions.

The workaround is to have a single if statement at the root of the file that looks like this:

if __name__ == '__main__':
    main()

It checks a special variable called __name__. If the Python file is directly executed, __name__ will have the value of the string '__main__', which satisfies the if statement so main() is called. If another Python file imports it, the value of __name__ will be the name of that file, so main() isn’t called. It’s clunky and not that efficient, but, 1, it works, and 2, if you cared about efficiency, you wouldn’t be writing it in Python.

Luckily Python is one step ahead:

Python 3.13.3 (main, Apr 22 2025, 00:00:00) [GCC 15.0.1 20250418 (Red Hat 15.0.1-0)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> if __name__ = "__main__":
... 
...    main()
...    
    File "<python-input-0>", line 1
    if __name__ = "__main__":
        ^^^^^^^^^^^^^^^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

Also TIL that := is a thing in Python.