poltergeist

Rust-like error handling in Python, with type-safety in mind.

Installation

pip install poltergeist

Examples

Use the @catch decorator on any function:

from poltergeist import catch

# Handle an exception type potentially raised within the function
@catch(OSError)
def read_text(path: str) -> str:
    with open(path) as f:
        return f.read()

# Returns an object of type Result[str, OSError]
result = read_text("my_file.txt")

Or wrap errors manually:

from poltergeist import Result, Ok, Err

# Equivalent to the decorated function above
def read_text(path: str) -> Result[str, OSError]:
    try:
        with open(path) as f:
            return Ok(f.read())
    except OSError as e:
        return Err(e)

Then handle the result in a type-safe way:

# Get the Ok value or re-raise the Err exception
content: str = result.unwrap()

# Get the Ok value or a default if it's an Err
content: str = result.unwrap_or("lorem ipsum")
content: str | None = result.unwrap_or()

# Get the Ok value or compute it from the exception
content: str = result.unwrap_or_else(lambda e: f"The exception was: {e}")

# Get the Err exception or None if it's an Ok
err: OSError | None = result.err()

# Handle the result using structural pattern matching
match result:
    case Ok(content):
        print("Text in upper:", content.upper())
    case Err(FileNotFoundError() as e):
        print("File not found:", e.filename)

It's also possible to wrap multiple exception types with the decorator:

@catch(OSError, UnicodeDecodeError)
def read_text(path: str) -> str:
    with open(path) as f:
        return f.read()

Or manually:

def read_text(path: str) -> Result[str, OSError | UnicodeDecodeError]:
    try:
        with open(path) as f:
            return Ok(f.read())
    except (OSError, UnicodeDecodeError) as e:
        return Err(e)

There is also an async-compatible decorator:

from poltergeist import catch_async

@catch_async(OSError)
async def read_text(path: str) -> str:
    with open(path) as f:
        return f.read()

# Returns an object of type Result[str, OSError]
result = await read_text("my_file.txt")

Contributing

Set up the project using Poetry:

poetry install

Format the code:

make lint

Run tests:

make test

Check for typing and format issues:

make check