Python Pattern Matching¶
Python, I love you. But I’d like you to change. It’s not you, it’s me. Really.
See, you don’t have pattern matching. But, that’s not the root of it. Macros
are the root of it. You don’t have macros but that’s OK. Right now, I want
pattern matching. I know you offer me if/elif/else statements but I
need more. I’m going to abuse your functions. Guido, et al, I hope you can
forgive me. This will only hurt a little.
Python Pattern Matching is an Apache2 licensed Python module for pattern matching like that found in functional programming languages. Most projects that address Python pattern matching focus on syntax and simple cases. Operator overloading is often used to change the semantics of operators to support pattern matching. In other cases, function decorators are used to implement multiple dispatch, sometimes known as function overloading. Each of these syntaxes, operators, and decorators, is really more of a detail in the application of pattern matching.
A lot of people have tried to make this work before. Somehow it didn’t take. I should probably call this yet-another-python-pattern-matching-module but “yappmm” doesn’t roll off the tongue. Other people have tried overloading operators and changing codecs. This module started as a codec hack but those are hard because they need an ecosystem of emacs-modes, vim-modes and the like to really be convenient.
Python Pattern Matching focuses instead on the semantics of pattern matching in Python. The dynamic duck-typing behavior in Python is distinct from the tagged unions found in functional programming languages. Rather than trying to emulate the behavior of functional pattern matching, this project attempts to implement pattern matching that looks and feels native to Python. In doing so the traditional function call is used as syntax. There are no import hooks, no codecs, no AST transforms.
Todo
Python match function example.
Finally, pythonic pattern matching! If you’ve experienced the feature before in “functional” languages like Erlang, Haskell, Clojure, F#, OCaml, etc. then you can guess at the semantics.
Todo
Show the same code without patternmatching.
Features¶
- Pure-Python
- Developed on Python 3.7
- Tested on CPython 3.4, 3.5, 3.6, 3.7 and PyPy3
Todo
- Fully Documented
- 100% test coverage
- Hours of stress testing
Quickstart¶
Installing Python Pattern Matching is simple with pip:
$ pip install patternmatching
You can access documentation in the interpreter with Python’s built-in help function. The help works on modules, classes, and functions in pattern matching.
>>> from pypatt import match, bind, bound, like
>>> help(match)
Alternative Packages¶
- https://github.com/lihaoyi/macropy - module import, but similar design
- https://github.com/Suor/patterns - decorator with funky syntax - Shared at Python Brazil 2013
- https://github.com/mariusae/match - http://monkey.org/~marius/pattern-matching-in-python.html - operator overloading
- http://blog.chadselph.com/adding-functional-style-pattern-matching-to-python.html - multi-methods
- http://svn.colorstudy.com/home/ianb/recipes/patmatch.py - multi-methods
- http://www.artima.com/weblogs/viewpost.jsp?thread=101605 - the original multi-methods
- http://speak.codebunk.com/post/77084204957/pattern-matching-in-python - multi-methods supporting callables
- http://www.aclevername.com/projects/splarnektity/ - not sure how it works but the syntax leaves a lot to be desired
- https://github.com/martinblech/pyfpm - multi-dispatch with string parsing
- https://github.com/jldupont/pyfnc - multi-dispatch
- http://www.pyret.org/ - It’s own language
- https://pypi.python.org/pypi/PEAK-Rules - generic multi-dispatch style for business rules
- http://home.in.tum.de/~bayerj/patternmatch.py - Pattern-object idea (no binding)
- https://github.com/admk/patmat - multi-dispatch style
Other Languages¶
- https://msdn.microsoft.com/en-us/library/dd547125.aspx F#
- https://doc.rust-lang.org/book/patterns.html Rust
- https://www.haskell.org/tutorial/patterns.html Haskell
- http://erlang.org/doc/reference_manual/expressions.html#pattern Erlang
- https://ocaml.org/learn/tutorials/data_types_and_matching.html Ocaml
Developer Guide¶
Project Links¶
Python Pattern Matching License¶
Copyright 2015-2019, Grant Jenks
Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file 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.
