Violet is one of those Swift <-> Python interop thingies, except that this time we implement the whole language from scratch. Name comes from Violet Evergarden.

Many unwatched k-drama hours were put into this, so any ⭐ would be appreciated.

If something is not working, you have an interesting idea or maybe just a question, then you can start an issue or discussion. You can also contact us on twitter @itBrokeAgain (optimism, yay!).


  • 64 bit – for BigInt and (probably, maybe, I think) hash
  • Platform:
    • macOS – tested on 10.15.6 (Catalina) + Xcode 12.0 (Swift 5.3)
    • Ubuntu – tested on 21.04 + Swift 5.4.2
    • Docker – tested on swift:latest (5.4.2) on Ubuntu 21.04


We aim for compatibility with Python 3.7 feature set.

We are only interested in the language itself without additional modules. This means that importing anything except for most basic modules (sys, builtins and a few others) is not supported (although you can import other Python files).

See Documentation directory for a list of known unimplemented features. There is no list of unknown unimplemented features though…

Future plans

Our current goal was to ramp up the Python functionality coverage, which mostly meant passing as many Python tests (PyTests) as possible. This gives us us a safety net for any future regressions.

Next we will try to improve code-base by solving any shortcuts we took:

  • New object model (representation of a single Python object in a memory) – currently we are using Swift objects to represent Python instances, for example Swift PyInt object represents a Python int instance. There are better ways to do this, but this is a bit longer conversation in Swift. For details see this issue.

  • New method representation – currently we just wrap a Swift method in a PyBuiltinFunction and put it inside type __dict__. For example: int.add (implemented in Swift as PyInt.add(:_) with following signature: (PyInt) -> (PyObject) -> PyResult<PyObject>) is put inside int.__dict__. This can be simplified a bit, but it depends on the object model, so it has to wait.

  • Garbage collection and memory management – as we said: currently use Swift class instances to represent Python objects, which means that we are forced to use Swift ARC to manage object lifetime. Unfortunately this does not solve reference cycles (which we have, for example: object type has type type and type type is a subclass of object, not to mention that type type has type as its type), but for now we will ignore this… (how convenient!).

  • V8-style isolates – currently the Python context is represented as a global static Py (something like: Py.newInt(2) or Py.add(lhs, rhs)). This prevents us from having multiple VM instances running on the same thread (without using thread local storage), which in turn makes unit testing difficult.


Core modules

  • VioletCore — shared module imported by all of the other modules.
    • Contains things like NonEmptyArray, SourceLocation, SipHash, trap and unreachable.
  • BigInt — our implementation of unlimited integers
    • While it implements all of the operations expected of BigInt type, in reality it mostly focuses on performance of small integers — Python has only one int type and small numbers are most common.
    • Under the hood it is a union (via tagged pointer) of Int32 (called Smi, after V8) and a heap allocation (magnitude + sign representation) with ARC for garbage collection.
    • While the whole Violet tries to be as easy-to-read/accessible as possible, this does not apply to BigInt module. Numbers are hard, and for some reason humanity decided that “division” is a thing.
  • FileSystem — our version of Foundation.FileManager.
    • Main reason why we do not support other platforms (Windows etc.).
  • UnicodeData — apparently we also bundle our own Unicode database, because why not…


  • VioletLexer — transforms Python source code into a stream of tokens.
  • VioletParser — transforms a stream of tokens (from Lexer) into an abstract syntax tree (AST).
    • Yet Another Recursive Descent Parser with minor hacks for ambiguous grammar.
    • AST type definitions are generated by Elsa module from Elsa definitions/ast.letitgo.
  • VioletBytecode — instruction set of our VM.
    • 2-bytes per instruction.
    • No relative jumps, only absolute (via additional labels array).
    • Instruction enum is generated by Elsa module from Elsa definitions/opcodes.letitgo.
    • Use CodeObjectBuilder to create CodeObjects (whoa… what a surprise!).
    • Includes a tiny peephole optimizer, because sometimes the semantics depends on it (for example for short-circuit evaluation) .
  • VioletCompiler — responsible for transforming AST (from Parser) into CodeObjects (from Bytecode).
  • VioletObjects — contains all of the Python objects and modules.
    • Py represents a Python context. Common usage: Py.newInt(2) or Py.add(lhs, rhs).
    • Contains int, str, list and 100+ other Python types. Python object is represented as a Swift class instance (this will probably change in the future, but for now it is “ok”, since the whole subject is is a bit complicated in Swift). Read the docs in the Documentation directory!
    • Contains modules required to bootstrap Python: builtins, sys, _imp, _os and _warnings.
    • Does not contain importlib and importlib_external modules, because those are written in Python. They are a little bit different than CPython versions (we have 80% of the code, but only 20% of the functionality <great-success-meme.gif>).
    • PyResult<Wrapped> = Wrapped | PyBaseException is used for error handling.
  • VioletVM — manipulates Python objects according to the instructions from Bytecode.CodeObject, so that the output vaguely resembles what CPython does.
    • Mainly a massive switch over each possible Instruction (branch prediction ?).
  • Violet — main executable (duh…).
  • PyTests — runs tests written in Python from the PyTests directory.


  • Elsa — tiny DSL for code generation.
    • Uses .letitgo files from Elsa definitions directory.
    • Used for Parser.AST and Bytecode.Instruction types.
  • Rapunzel — pretty printer based on “A prettier printer” by Philip Wadler.
    • Used to print AST in digestible manner.
  • Ariel — prints module interface – all of the open/public declarations.


There are 2 types of tests in Violet:

  • Swift tests — standard Swift unit tests stored inside the ./Tests directory. You can run them by typing make test in repository root.

    You may want to disable unit tests for BigInt and UnicodeData if you are not touching those modules:

    • BigInt — we went with property based testing with means that we test millions of inputs to check if the general rule holds (for example: a+b=c -> c-a=b etc.). This takes time, but pays for itself by finding weird overflows in bit operations (we store “sign + magnitude”, so bit operations are a bit difficult to implement).
    • UnicodeData
      • In one of our tests we go through all of the Unicode code points and try to access various properties (crash -> fail). There are 0x11_0000 values to test, so… it is not fast.
      • We also have a few thousands of tests generated by Python. Things like: “is the COMBINING VERTICAL LINE ABOVE (U+030d) alpha-numeric?” (Answer: no, it is not. But you have to watch out because HANGUL CHOSEONG THIEUTH (U+1110) is).
  • Python tests — tests written in Python stored inside the ./PyTests directory. You can run them by typing make pytest in repository root (there is also make pytest-r for release mode).

    • Violet – tests written specially for “Violet”.
    • RustPython – tests taken from

    Those tests are executed when you run PyTests module.

Code style

  • 2-space indents and no tabs at all
  • 80 characters per line
    • You will get a SwiftLint warning if you go over 100.
    • Over 120 will result in a compilation error.
    • If 80 doesn’t give you enough room to code, your code is too complicated – consider using subroutines (advice from PEP-7).
  • Required self in methods and computed properties
    • All of the other method arguments are named, so we will require it for this one.
    • Self/type name for static methods is recommended, but not required.
    • I’m sure that they will depreciate the implicit self in the next major Swift version ?. All of that source breakage is completely justified.
  • No whitespace at the end of the line
    • Some editors may remove it as a matter of routine and we don’t want weird git diffs.
  • (pet peeve) Try to introduce a named variable for every if condition.
    • You can use a single logical operator – something like if !isPrincess or if isDisnepCharacter && isPrincess is allowed.
    • Do not use && and || in the same expression, create a variable for one of them.
    • If you need parens then it is already too complicated.

Anyway, just use SwiftLint and SwiftFormat with provided presets (see .swiftlint.yml and .swiftformat files).


“Violet” is licensed under the MIT License.
See LICENSE file for more information.