Python AST, parser, and quasiquotes for Scala


Parsel is a zero-dependency Scala library for working with Python code. You probably shouldn't use it.

It has:

  • AST data types. They try to align with Python's own AST data structures.
  • Visitor-ish base classes for defining AST transformations.
  • A parser for Python 3 programs. It's hand-written and is probably loaded with bugs.
  • Macro-based string interpolators for embedding Python 3 quasiquotes in your Scala code.

Currently, parsel only supports Scala 2. This is because the macros for parsel-quote are written with the Scala 2 macro API. It won't be that hard to port it, though – I just don't have a Scala 3 use case for this yet.


The AST data structures are in the parsel-ast module. They try to follow Python's own AST data structures, as mentioned above. Some things are even kept in these data structures that aren't used in the parser – e.g. type comments (which are currently never populated, because I don't know what they are. Because the grammar doesn't describe them at all.)

The common base trait of all the AST structures is parsel.ast.Tree (apologies that this will interfere with Scala macros's Tree). Every Tree has a no-args method .pretty which you can use to turn it back into a Python code string (say, if you want to evaluate it using jep or scalapy or even a subprocess).


Parsel's parser is hand-written, because I couldn't make a performant parser with fastparse while working from the Python official grammar. I later discovered that this is because the Python 3 grammar is left-recursive in several places, so it needs some not-quite-PEG tricks (which Python's own parser does, but arent't mentioned in the grammar specification) and so can be pretty frustrating to try and parse (this is probably why I couldn't find a pre-existing Python 3 parser). By the time I figured this out, I had already hand-written most of the parser, so I just went with it.

As a result of being hand-written, it's probably loaded with catastrophic bugs. But, at least it's pretty fast.

The parser is in the parsel-parser module. After adding a dependency on that, you can parse some python statements from a String using parsel.parse.Parser:

import parsel.ast.Module
import parsel.parse.Parser
val program: Module = Parser.parse(
  """def hooray():
    |  return "woo!"

This will throw a ParseError if it doesn't parse successfully. Did I mention that parsel is not functional? It mutates and throws exceptions all over the place. But, at least it's pretty fast.


Parsel's quasiquotes allow you to embed python ASTs using a string interpolator, and the content of the string will be checked (well, parsed, at least) at compile time, giving a compiler error if the Python code is invalid. This is similar to the quasiquotes of Scala 2 macros. You can use this feature by depending on the parsel-quotes module.

There are three flavors of quasiquote, which are accessed by importing parsel.quote.syntax._:

  • py quasiquotes contain Python statements. Quoted expressions are spliced eagerly as expression trees (see Quotable below), so this type of quasiquote results in a Module (which is just a container for a bunch of statements).
  • pyq quasiquotes also contain Python statements, but quoted expressions are suspended rather than being spliced eagerly. The idea here is that a Quotable instance could use side-effects when quoting (e.g. quote a large object or a DataFrame by writing it to a URI, and splice the necessary code for reading it in Python) and suspending it allows control over when those side effects happen. So this type of quasiquote results in a QuotedTree, which becomes a usable AST when its doQuote() method is called (triggering any side-effects).
  • pye quasiquotes contain a single Python expression, so this type of quasiquote results in an Expr.

In each of these, you can splice values which have a Quotable instance, or which themselves represent parsel ASTs (or Seqs of ASTs).

Here's what using them looks like:

import parsel.ast._
import parsel.quote.QuotedTree
import parsel.quote.syntax._

val myString = "hello quasiquotes!"
val mySuffix = " Yippee!"

// This will result in an Expr, where myString is spliced in as:
//     Constant(StringLiteral("hello quasiquotes!"))
val exampleExpr: Expr = pye"my_function($myString)"

// This will result in an AST, where mySuffix is spliced in as:
//     Constant(StringLiteral(" Yippee!"))
// in the same fashion, and exampleExpr is spliced in as an AST directly
val example1: Module = py"""
    def my_function(str):
      return str + $mySuffix

// This will result in a QuotedTree. When doQuote is called, the resulting
// AST will contain code to memoize the spliced value of myString by binding
// it to a fresh variable name, which is substituted in the quoted code.
// This probably seems kind of useless, but I needed it for my motivating use case.
val example2: QuotedTree = pyq"""

val example2Result: Module = example2.doQuote()


Splicing values is accomplished by the Quotable typeclass. Any expression of type T that's spliced into a quasiquote must have an instance of Quotable[T] in implicit scope where the quasiquote interpolater is used. This typeclass just has a method doQuote, which takes a value of type T and returns an Expr which represents that value. Instances are already defined for obvious constants, but I'm not sure how opinionated parsel ought to be about how things like collections and such ought to be quoted, so those aren't currently implemented (I'd be happy to hear feedback about this, but that would mean that you're using this library – and you probably shouldn't be).


These were mentioned earlier, but let's explicitly state them again:

  • The parser is probably loaded with bugs
  • Type comments are always None, because I don't know what those are
  • While I made an attempt to do ExprContext correctly, I have no idea what its utility is, nor what its semantics are. So don't rely on it for anything.

License & Copyright

Copyright 2022 Jeremy Smith

Licensed under the Apache License, Version 2.0 (the "License"); you may not use these files except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.