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The Main Function

All SPy programs that are run in the interpreter or compiled must have a main function. The main function is the entry point of the program, and it is where the execution of the program begins.

def main() -> None:
    print("Hello world")

Not every .spy module needs a main function, but the module invoked by, e.g. spy foo.spy spy build foo.spy must contain a main function.

Modules which are compiled as a library (e.g. spy build --target lib foo.spy or --target py-cffi) do not need a main function.

Return Codes

The main function may be typed to return an int (i32). If so, the return value of the main function will be the return value of the program:

#retcode.spy
def main() -> int
    return 123

$ uv run spy retcode.spy
$ echo $?
123

Accessing Command Line Arguments

If the main function accepts a list of strings as an argument, the SPy program will accept arguments from the command line, both when running in interpreted

#args.spy
def main(args: list[str]) -> None:
    print(args[1])

$ uv run spy args.spy 999
999

As with CPython, args[0] is the name of the string passed to the uv runtime. This is the equivalent of CPython's sys.argv:

#argname.spy
def main(argv: list[str]) -> None:
    print(args[0])

$ uv run spy argname.spy
foo.spy