cflowpy 0.4.0

Control flow constructs for python.

cflowpy - Control Flow Objects for Python

Features

Option Type

• Represent optional values, freeing None to be a usable value.

  from cflowpy import Option, Some, Nothing
  val: Option[int] = Some(42)
  empty: Option[int] = Nothing()
  

• Pattern Matching: Destructure option.

  match val:
      case Some(x):
          print(x)
      case Nothing():
          print("Empty")
  

• Functional Methods: Transform inner values.

  val.map(lambda x: x * 2)  # Some(84)
  empty.map_or(lambda x: x * 2, 0)  # 0
  

• Unwrapping: Extract inner value.

  x: int = val.unwrap()  # 42
  empty.unwrap()  # raises UnpackingException
  y: int = empty.unwrap_or(0)  # 0
  

• For Loop unpacking: Run code in a block if the value is some, using for loops

  some: Option[str] = Some("a")
  nothing: Option[str] = Nothing()
  
  for value in some:
      print("This messag will be printed.")
  
  for value in nothing:
      print("This message will not be printed.")
  

• Default Value Sentinel (nothing): Define default parameter values without type checker warnings.

  from cflowpy import Option, nothing
  
  def func(val: Option[int] = nothing):
      pass
  

Result Type

• Represent success or error values.

  from cflowpy import Result, Ok, Err
  success: Result[int, str] = Ok(42)
  failure: Result[int, str] = Err("Error message")
  

• Pattern Matching: Destructure result.

  match success:
      case Ok(x):
          print(x)
      case Err(err):
          print(err)
  

• Functional Methods: Transform inner values.

  success.map(lambda x: x * 2)  # Ok(84)
  failure.map_err(lambda err: f"Error: {err}")  # Err("Error: Error message")
  

• Unwrapping: Extract inner value.

  x: int = success.unwrap()  # 42
  failure.unwrap()  # raises UnpackingException
  failure.unwrap_or_raise()  # raises wrapped exception
  y: int = failure.unwrap_or(0)  # 0
  

• For Loop unpacking: Run code in a block if the value is ok, using for loops.

  success: Result[str, int] = Ok("a")
  failure: Result[str, int] = Err(1)
  
  for value in success:
      print("This message will be printed.")
  
  for value in failure:
      print("This message will not be printed.")
  

Architecture

• All classes / functions are re-exported to the packages __init__.py

--> There is only one place to import everything

from cflowpy import Option, Unit

• The actual definitions are done in private modules. F.e _options.py

Development Environment

Dependencies

• uv: Install from here: https://docs.astral.sh/uv/getting-started/installation/

Environment Setup

On every session start, call:

source env

• Adds relative directory scripts/ to PATH
• Calls scripts/setup and checks for system dependecies
• Creates .venv and syncs dependencies

On the first startup, this should setup the dev environment completely. Otherwise you can call scripts/setup manually.

Development Tools

• Linting: ruff
• Type-checking: basedpyright

Internal Dev Commands

Some Commands that might be useful:

• style-check: Runs linter and type checker
• style-format: Runs formatter
• test-all: Run the tests.
  • The tests contain calls to other typecheckers. This can be skipped by passing the --skip-typing flag.
  • Use the test-all --help command for more information.

Supported Type Checker

While ty is mainly used during the development, the tests also run additional typecheckers to see how the type-inference works.
The following table contains a summary of the support. Some type checkerr have additional information in a subsection below.

type checker	is tested for	functionality
ty	✓	Missing feature in ty for match value unpacking.
pyrefly	✓	Failing the exhaustiveness checks in the match statement.
pyright	✓	✓ + Non-speced feature: Complain on Nothing in a with statement.
basedpyright	✓	? + Non-speced feature: Complain on Nothing in a with statement.
mypy	✗	?

ty

• ty =< 0.0.16 currently does not infer the type of an unpacked value in a match statement

from typing import assert_type, assert_never
from cflowpy import Option

# snip...: some code that sets some_value

some_value: Option[int]

match some_value:
    case Some(unpacked_value):
        # ty will complain here
        # its infered type is @Todo
        assert_type(unpacked_value, int)
    case Nothing():
        pass
    case _:
        assert_never(some_value)

There is a test that will test this case. It will currently always fail.
A workaround requires the typing.cast function. Simply adding the typehint unpacked_value: int will not work.

from typing import assert_type, assert_never, cast
from cflowpy import Option

# snip...: some code that sets some_value

some_value: Option[int]

match some_value:
    case Some(unpacked_value):
        unpacked_value = cast(int, unpacked_value)
        assert_type(unpacked_value, int)
    case Nothing():
        pass
    case _:
        assert_never(some_value)

This has the drawback of not giving a typeerror when the some_value ever switches in type.