REPL Session

This is script that runs a session on any REPL following a description in a JSON file. The output contains the commands entered and the results given. This can be useful to drive documentation tests or literate programming tasks. This way we decouple running the commands from rendering or presenting the corresponding results, leading to better reproducibility, caching output and modularity w.r.t. any other tools that you may use.

This is very similar to running a Jupyter notebook from the console, with the benefit that you don't need a Jupyter kernel available for the language you're using. The downside is that REPLs can be messy to interact with.

Is this for you?

This is only really useful if you're hacking together a literate programming environment similar to Entangled. Suppose you have your documentation written in Markdown, ready for rendering with MkDocs or Pandoc. You want to automatically evaluate some expressions in this document as if they're entered in a REPL and process the results for inclusion in the document generator. Here repl-session is a nicely confined command-line tool, so its easy to integrate into a build pipeline.

How does it work?

The REPL of your choice is started and interacted with through the pexpect library. All I/O is dealt with through msgspec.

The preferred way to solve this is by using jupyter-client in combination with an existing Jupyter kernel. However, not all languages have a Jupyter kernel available, and developing one takes a bit more than configuring pexpect for an existing REPL. I may still include Jupyter support in this application at a later stage.

Install

Install with,

pip install repl-session

Or equivalent Poetry, Astral Uv, Hatch or Conda commands.

Documentation

The full documentation is available at entangled.github.io/repl-session.

Examples

Here are some examples where we could interact with a REPL successfully. In general, the less intricate the REPL the better the results.

Chez Scheme

I like to work with Chez Scheme. Suppose I want to document an interactive session. This can be done:

#| file: test/scheme.yml
config:
  command: "scheme --eedisable"
  first_prompt: "> "
  change_prompt: '(waiter-prompt-string "{key}>")'
  prompt: "{key}> "
  continuation_prompt: ""
commands:
  - command: (* 6 7)
  - command: |
      (define (fac n init)
        (if (zero? n)
          init
          (fac (- n 1) (* init n)))))
  - command: (fac 10 1)

Passing this to repl-session, it will start the Chez Scheme interpreter, waiting for the > prompt to appear. It then changes the prompt to a generated uuid4 code, for instance 27e87a8a-742c-4501-b05d-b05814f5a010> . This will make sure that we can't accidentally match something else for an interactive prompt (imagine we're generating some XML!). Since commands are also echoed to standard out, we need to strip them from the resulting output. Running this should give:

repl-session < test/scheme.yml | jq '.commands.[].output'

"42"
"(define (fac n init)\n  (if (zero? n)\n    init\n    (fac (- n 1) (* init n)))))"
"3628800"

Lua

This looks very similar to the previous example:

#| file: test/lua.yml
config:
  command: "lua"
  first_prompt: "> "
  change_prompt: '_PROMPT = "{key}> "; _PROMPT2 = "{key}+ "'
  prompt: "{key}> "
  continuation_prompt: "{key}\\+ "
  strip_ansi: true
commands:
  - command: 6 * 7
  - command: '"Hello" .. ", " .. "World!"'
  - command: |
      function fac(n, m)
          if m == nil then
              return fac(n, 1)
          end
          if n == 0 then
              return m
          else
              return fac(n-1, m*n)
          end
      end
  - command: fac(10)

The Lua REPL is not so nice. It sends ANSI escape codes and those need to be filtered out.

repl-session < test/lua.yml | jq '.commands.[].output'

"42"
"Hello, World!"

Python

The Python REPL got a revision in version 3.13, with lots of colour and ANSI codes.

#| file: test/python.yml
config:
  command: python -q
  first_prompt: ">>>"
  change_prompt: 'import sys; sys.ps1 = "{key}>>> "; sys.ps2 = "{key}+++ "'
  prompt: "{key}>>> "
  continuation_prompt: "{key}\\+\\+\\+ "
  environment:
    NO_COLOR: "1"
commands:
  - command: print("Hello, World!")
  - command: 6 * 7
  - command: |
      def fac(n):
          for i in range(1, n):
              n *= i
          return n
  - command: fac(10)

Input/Output structure

The user can configure how the REPL is called and interpreted.

#| id: input-data
class ReplConfig(msgspec.Struct):
    """Configuration

    Attributes:
        command (str): Command to start the REPL
        first_prompt (str): Regex to match the first prompt
        change_prompt (str): Command to change prompt; should contain '{key}' as an
            argument.
        next_prompt (str): Regex to match the changed prompts; should contain '{key}'
            as an argument.
        append_newline (bool): Whether to append a newline to given commands.
        strip_command (bool): Whether to strip the original command from the gotten
            output; useful if the REPL echoes your input before answering.
        timeout (float): Command timeout for this session in seconds.
    """

    command: str
    first_prompt: str
    change_prompt: str
    prompt: str
    continuation_prompt: str | None = None
    strip_ansi: bool = False
    environment: dict[str, str] = msgspec.field(default_factory=dict)
    timeout: float = 5.0

Then, a session is a list of commands. Each command should be a UTF-8 string, and we allow to attach some meta-data like expected MIME type for the output. We can also pass an expected output in the case of a documentation test. If output was already given on the input, it is moved to expected. This way it becomes really easy to setup regression tests on your documentation. Just rerun on the generated output file.

#| id: input-data
class ReplCommand(msgspec.Struct):
    """A command to be sent to the REPL.

    Attributes:
        command (str): the command.
        output_type (str): MIME type of expected output.
        output (str | None): evaluated output.
        expected (str | None): expected output.
    """

    command: str
    output_type: str = "text/plain"
    output: str | None = None
    expected: str | None = None


class ReplSession(msgspec.Struct):
    """A REPL session.

    Attributes:
        config (ReplConfig): Config for setting up a REPL session.
        commands (list[ReplCommand]): List of commands in the session.
    """

    config: ReplConfig
    commands: list[ReplCommand]

License and contribution

Licensed under the Apache 2.0 license. Contributions are welcome: if you've succesfully applied repl-session to a REPL not listed in the documentation, consider contributing your configuration to the documentation. If your contribution fixes a bug, please first file an issue.