fiasto-py

fiasto-py

---

Pronouned like fiasco, but with a t instead of a c

---

(F)ormulas (I)n (AST) (O)ut

Python bindings for fiasto - A language-agnostic modern Wilkinson's formula parser and lexer.

🎯 Features

• Parse Wilkinson's Formulas: Convert formula strings into structured JSON metadata
• Tokenize Formulas: Break down formulas into individual tokens with detailed information
• Python Dictionaries: Returns native Python dictionaries for easy integration

🎯 Simple API

• parse_formula() - Takes a Wilkinson’s formula string and returns a Python dictionary
• lex_formula() - Tokenizes a formula string and returns a Python dictionary

🚀 Quick Start

Installation

Install from PyPI (recommended):

pip install fiasto-py

Usage

Usage: Parse Formula

import fiasto_py
from pprint import pprint
# Parse a formula into structured metadata
print("="*30)
print("Parse Formula")
print("="*30)
result = fiasto_py.parse_formula("y ~ x1 + x2 + (1|group)")
pprint(result, compact = True)

Output:

==============================
Parse Formula
==============================
{'all_generated_columns': ['y', 'x1', 'x2', 'group'],
 'columns': {'group': {'generated_columns': ['group'],
                       'id': 4,
                       'interactions': [],
                       'random_effects': [{'correlated': True,
                                           'grouping_variable': 'group',
                                           'has_intercept': True,
                                           'includes_interactions': [],
                                           'kind': 'grouping',
                                           'variables': []}],
                       'roles': ['GroupingVariable'],
                       'transformations': []},
             'x1': {'generated_columns': ['x1'],
                    'id': 2,
                    'interactions': [],
                    'random_effects': [],
                    'roles': ['FixedEffect'],
                    'transformations': []},
             'x2': {'generated_columns': ['x2'],
                    'id': 3,
                    'interactions': [],
                    'random_effects': [],
                    'roles': ['FixedEffect'],
                    'transformations': []},
             'y': {'generated_columns': ['y'],
                   'id': 1,
                   'interactions': [],
                   'random_effects': [],
                   'roles': ['Response'],
                   'transformations': []}},
 'formula': 'y ~ x1 + x2 + (1|group)',
 'metadata': {'family': None,
              'has_intercept': True,
              'has_uncorrelated_slopes_and_intercepts': False,
              'is_random_effects_model': True}}

Usage: Lex Formula

import fiasto_py
from pprint import pprint
print("="*30)
print("Lex Formula")
print("="*30)
tokens = fiasto_py.lex_formula("y ~ x1 + x2 + (1|group)")
pprint(tokens, compact = True)

Output:

==============================
Lex Formula
==============================
[{'lexeme': 'y', 'token': 'ColumnName'},
 {'lexeme': '~', 'token': 'Tilde'},
 {'lexeme': 'x1', 'token': 'ColumnName'},
 {'lexeme': '+', 'token': 'Plus'},
 {'lexeme': 'x2', 'token': 'ColumnName'},
 {'lexeme': '+', 'token': 'Plus'},
 {'lexeme': '(', 'token': 'FunctionStart'},
 {'lexeme': '1', 'token': 'One'},
 {'lexeme': '|', 'token': 'Pipe'},
 {'lexeme': 'group', 'token': 'ColumnName'},
 {'lexeme': ')', 'token': 'FunctionEnd'}]

Simple OLS Regression

import fiasto_py
import polars as pl
import numpy as np
from pprint import pprint

# Load data
mtcars_path = "https://gist.githubusercontent.com/seankross/a412dfbd88b3db70b74b/raw/5f23f993cd87c283ce766e7ac6b329ee7cc2e1d1/mtcars.csv"
df = pl.read_csv(mtcars_path)

# Parse formula
formula = "mpg ~ wt + cyl"
result = fiasto_py.parse_formula(formula)

pprint(result)

# Find the response column(s)
response_cols = [
    col for col, details in result["columns"].items()
    if "Response" in details["roles"]
]

# Find non-response columns
preds = [
    col for col, details in result["columns"].items()
    if "Response" not in details["roles"]
]

# Has intercept
has_intercept = result["metadata"]["has_intercept"]

# Prepare data matrices
X = df.select(preds).to_numpy()
y = df.select(response_cols).to_numpy().ravel()

# Add intercept if metadata says so
if has_intercept:
    X_with_intercept = np.column_stack([np.ones(X.shape[0]), X])
else:
    X_with_intercept = X

# Solve normal equations: (X'X)^-1 X'y
XTX = X_with_intercept.T @ X_with_intercept
XTy = X_with_intercept.T @ y
coefficients = np.linalg.solve(XTX, XTy)

# Extract intercept and slopes
if has_intercept:
    intercept = coefficients[0]
    slopes = coefficients[1:]
else:
    intercept = 0.0
    slopes = coefficients

# Calculate R2
y_pred = X_with_intercept @ coefficients
ss_res = np.sum((y - y_pred) ** 2)
ss_tot = np.sum((y - np.mean(y)) ** 2)
r_squared = 1 - (ss_res / ss_tot)

# Prep Output
# Combine intercept and slopes into one dict
coef_dict = {"intercept": intercept} | dict(zip(preds, slopes))

# Create a tidy DataFrame
coef_df = pl.DataFrame(
    {
        "term": list(coef_dict.keys()),
        "estimate": list(coef_dict.values())
    }
)

# Print results
print(f"Formula: {formula}")
print(f"R² Score: {r_squared:.3f}")
print(coef_df)

Output:

{'all_generated_columns': ['mpg', 'intercept', 'wt', 'cyl'],
 'all_generated_columns_formula_order': {'1': 'mpg',
                                         '2': 'intercept',
                                         '3': 'wt',
                                         '4': 'cyl'},
 'columns': {'cyl': {'generated_columns': ['cyl'],
                     'id': 3,
                     'interactions': [],
                     'random_effects': [],
                     'roles': ['Identity'],
                     'transformations': []},
             'mpg': {'generated_columns': ['mpg'],
                     'id': 1,
                     'interactions': [],
                     'random_effects': [],
                     'roles': ['Response'],
                     'transformations': []},
             'wt': {'generated_columns': ['wt'],
                    'id': 2,
                    'interactions': [],
                    'random_effects': [],
                    'roles': ['Identity'],
                    'transformations': []}},
 'formula': 'mpg ~ wt + cyl',
 'metadata': {'family': None,
              'has_intercept': True,
              'has_uncorrelated_slopes_and_intercepts': False,
              'is_random_effects_model': False,
              'response_variable_count': 1}}
Formula: mpg ~ wt + cyl
R² Score: 0.830
shape: (3, 2)
┌───────────┬───────────┐
│ term      ┆ estimate  │
│ ---       ┆ ---       │
│ str       ┆ f64       │
╞═══════════╪═══════════╡
│ intercept ┆ 39.686261 │
│ cyl       ┆ -1.507795 │
│ wt        ┆ -3.190972 │
└───────────┴───────────┘

📋 Supported Formula Syntax

fiasto supports comprehensive Wilkinson's notation including:

• Basic formulas: y ~ x1 + x2
• Interactions: y ~ x1 * x2
• Smooth terms: y ~ s(z)
• Random effects: y ~ x + (1|group)
• Complex random effects: y ~ x + (1+x|group)

Supported Formulas (Coming Soon)

• Multivariate models: mvbind(y1, y2) ~ x + (1|g)
• Non-linear models: y ~ a1 - a2^x, a1 ~ 1, a2 ~ x + (x|g), nl = TRUE

For the complete reference, see the fiasto documentation.

📦 PyPI Package

The package is available on PyPI and can be installed with:

pip install fiasto-py

• PyPI Page: pypi.org/project/fiasto-py
• Source Code: github.com/alexhallam/fiasto-py
• Documentation: This README and inline docstrings

📚 API Reference

parse_formula(formula: str) -> dict

Parse a Wilkinson's formula string and return structured JSON metadata.

Parameters:

• formula (str): The formula string to parse

Returns:

• dict: Structured metadata describing the formula

Raises:

• ValueError: If the formula is invalid or parsing fails

lex_formula(formula: str) -> dict

Tokenize a formula string and return JSON describing each token.

Parameters:

• formula (str): The formula string to tokenize

Returns:

• dict: Token information for each element in the formula

Raises:

• ValueError: If the formula is invalid or lexing fails

🤝 Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

🙏 Acknowledgments

• fiasto - The underlying Rust library
• PyO3 - Python-Rust bindings
• maturin - Build system for Python extensions
• PyPI - Python Package Index for distribution

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.