An experiment in building macOS Python (.so) extensions containing Swift code
This is an Xcode project which builds a compiled Python extension on macOS containing Swift code.
Open spam.xcodeproj in Xcode and build; then, in a terminal, change to the directory containing the built product spam.so (type cd , drag spam.so from the Xcode Products folder to the terminal, and backspace over spam.so leaving only its directory); then start Python:
% python
Python 2.7.16 (default, Jan 27 2020, 04:46:15)
[GCC 4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.37.14)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import spam
>>> spam.add(3,4)
This is in Swift
7
>>> c = spam.Counter()
>>> c
<spam.Counter; count:0>
>>> c.increment()
>>> c.count
1
>>>
The code is a minimal Hello-World-style example, intended as a test of the process of building a Python extension. As such, it produces a file named spam.so implementing a module which is named, as per Pythonic tradition, spam. This module contains one standalone function: add, which takes two integers and returns their sum, and a class, Counter, which implements a simple integer counter, with a read-only count attribute and increment() and reset() methods. The business logic (the addition function and the counter class) is implemented in a Swift source file.
It appears to not be currently possible to compile Swift code to binaries complying with C calling conventions, and thus the foreign function interfaces of languages such as Python which expect dynamic libraries with C-style code. (While Swift has a @convention(c) attribute for specifying this calling convention, it is not available for use in function declarations, but only for callbacks passed to C functions called from Swift.) It also does not appear to be possible to define self-contained functions in Swift to be accessible to C code within the same project; C->Swift interoperability only covers Objective C classes.
As such, the approach used here is to define an Objective C-based class named Glue, with a number of class methods to act as relays between the C code exposed to Python and the Swift code containing the business logic. This, going through Objective C's dynamic dispatch mechanism, incurs a minor performance penalty, though this is not unreasonable, particularly in the context of communicating with a Python interpreter.
Attempting to load the compiled module after copying it to /tmp may fail with a code signing error,
This code currently only supports Python 2.7, as it was developed on macOS 10.14 where this is the system Python version. Adapting it to Python 3 should not be difficult.