Wandering Thoughts archives

An exploration of why Python doesn't require a 'main' function

Many languages start running your program by calling a function of yours that must have a specific name. In C (and many C derived languages), this is just called main(); in Go, it's main.main() (the main() function in the main package). Python famously doesn't require any such function, and won't automatically call a function called main() even if you create it. Recently I read Why doesn’t Python have a main function? (via), which puts forward one discussion for why this is so. However, I have a somewhat different way of explaining this situation.

The core reason that Python doesn't require a main() function is a combination of its execution model (specifically for what happens when you import something) and that under normal circumstances you start Python programs by (implicitly) importing a single file of Python code. So let's look at each of these parts.

In many languages things like functions, classes, and so on are created (defined) by the interpreter or compiler as it parses the source file. In Python, this is not quite the case; instead, def and class are executable statements, and they define classes and functions when they execute (among other things, this is part of why metaclasses work). When Python imports something, it simply executes everything in the file (or the import more generally). When what's executed is def and class statements, you get functions and classes. When what's executed is regular code, you get more complicated things happening, including conditional imports or calling functions on the fly under the right conditions. Or you can write an entire program that just runs inline, as the file is imported.

(This has some interesting consequences, including what reloading a Python module really does.)

However, Python is not quite as unique here as it might look. Many languages have some facility to run arbitrary code early on as the program is 'loading', before the program starts normal execution (Go has init() functions, for example). Where Python is different from these languages is that Python normally starts a program by loading and executing a specific single file. Because Python is only executing a single file, it's unambiguous what code is run in what order and it's straightforward for the code in that file to control what happens. In a sense, rather than picking an arbitrarily named function for where execution (nominally) starts, Python is able to sneakily pick an arbitrarily named file by having you provide it.

(Compiled languages traditionally have a model where code from a bunch of separate files is all sort of piled up together. In Python, you can't really aggregate multiple files together into a shared namespace this way; one way or another, you have to import them and everything starts from some initial file.)

Where this nice model breaks down and needs a workaround is if you run a package with 'python -m ...', where Python doesn't really have a single file that you're executing (or it'd have to make __init__.py serve double duty). As covered in the official documentation's __main__ — Top-level script environment (via), Python adopts the arbitrary convention of loading a __main__.py file from your package and declaring it more or less the point where execution starts.

(Under at least some situations, your package's __init__.py may also be executed.)

PS: contrary to the original article's views, I strongly suggest that you have a main() function, because there are significant benefits to keeping your program importable.