Stories with Tag Type Checking

• Pyrefly vs. Ty: Comparing Python's two new Rust-based type checkers

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  Posted: 2025-05-27 15:01:55

  Verbose tl;dr

  Pyrefly and Ty are new Python type checkers implemented in Rust, aiming for improved performance compared to mypy. Pyrefly prioritizes speed and compatibility with existing mypy codebases, leveraging Rust's performance advantages without requiring significant changes for users already using mypy. Ty, while also faster than mypy, focuses more on a stricter type system with additional features and tighter integration with Rust, potentially requiring more code adaptations. Both projects are still in early stages but represent promising advancements for Python type checking, offering potentially faster and more powerful alternatives to existing tools.

  This blog post by Edward Li delves into a comparative analysis of Pyrefly and Ty, two nascent Python type checkers built upon the Rust programming language. The author's primary motivation stems from a desire for faster type checking within their Python projects, a common pain point for many developers. Both Pyrefly and Ty aim to address this performance bottleneck by leveraging Rust's efficiency.

  The post begins by establishing the context of existing Python type checkers, primarily MyPy, which, despite being widely adopted, can be slow, especially in larger codebases. The introduction then highlights the emergence of Rust-based checkers as a potential solution for improved performance. The core of the blog post is dedicated to a detailed comparison of Pyrefly and Ty across various key aspects.

  The first point of comparison is installation and setup. Both tools offer relatively straightforward installation processes, though Pyrefly's method is described as simpler due to its reliance on pip, a familiar tool for Python developers. Ty, on the other hand, requires a bit more manual setup, involving downloading binaries or building from source.

  Next, the post analyzes the features and compatibility offered by each checker. Pyrefly is characterized as a direct replacement for MyPy, aiming for seamless compatibility. This means it supports the same type annotations and configuration options. Ty, conversely, takes a different approach by introducing its own configuration format and focusing on specific Python versions and features. This difference in philosophy affects their respective levels of compatibility with existing MyPy configurations.

  The pivotal aspect of performance is then examined. The author conducts benchmarks on real-world projects and synthetic code samples. The results indicate that both Pyrefly and Ty deliver substantial performance improvements compared to MyPy. However, the benchmarks reveal that Ty generally outperforms Pyrefly, sometimes by a significant margin. The author speculates that this performance difference might be attributed to Ty's more focused approach and Rust implementation.

  The discussion then shifts towards error reporting. While both tools effectively identify type errors, their approaches to presenting these errors differ. Pyrefly's error messages are described as highly similar to MyPy's, providing a familiar experience for existing users. Ty's error messages, although different, are lauded for their clarity and conciseness.

  The post concludes by summarizing the strengths and weaknesses of each type checker. Pyrefly is praised for its ease of use and MyPy compatibility, making it an attractive option for projects already using MyPy. Ty, despite its steeper learning curve and distinct configuration, is recognized for its superior performance. The author's final verdict suggests that the best choice depends on individual project needs and priorities. If MyPy compatibility and minimal disruption are paramount, Pyrefly is recommended. However, if performance is the primary concern, Ty emerges as the more compelling option. The author also expresses anticipation for the continued development and maturation of both projects.

  • Python
  • Type Checking
  • Rust
  • Pyrefly
  • Ty
  • static analysis
  • performance
  • Programming Languages
  • Software Development
  • type systems
  • compiler design

  Summary of Comments ( 98 )
  https://news.ycombinator.com/item?id=44107655

  Verbose tl;dr

  Hacker News users discussed the relative merits of Pyrefly and Ty, two new Rust-based Python type checkers. Some found Pyrefly's approach of compiling to Rust more interesting than Ty's runtime checks, appreciating the potential performance benefits and the ability to catch errors earlier. Others expressed skepticism about the practical benefits of either, citing existing tools like MyPy and the general overhead of type checking. A few questioned the need for Rust in these projects specifically, suggesting that the performance gains might be negligible for Python codebases and the added complexity could be a barrier to adoption. Several commenters noted the difficulty of type checking dynamic features of Python, while others pointed out the lack of significant detail in the comparison, making a definitive judgment difficult. Overall, the discussion highlighted the ongoing exploration of improved type checking for Python and the various tradeoffs involved in different approaches.

  The Hacker News thread for "Pyrefly vs. Ty: Comparing Python's two new Rust-based type checkers" contains several comments discussing the relative merits and drawbacks of each project.

  One commenter points out that the performance comparison in the original blog post isn't entirely fair because Pyright is being run in "watch" mode, which incurs overhead for monitoring file changes. They suggest a fairer comparison would involve running both tools in a single-shot mode.

  Another commenter highlights the different design philosophies behind the two projects. Ty aims to be more conservative and focuses on compatibility with the existing Python ecosystem, while Pyrefly prioritizes performance and aims to eventually support more advanced type checking features. This difference in approach leads to trade-offs in terms of strictness and supported features.

  There's discussion about the practicality of type checkers for large, real-world Python codebases. One commenter expresses skepticism about the usefulness of strict type checking in dynamically typed languages like Python, arguing it can add significant overhead without proportional benefits, especially in larger projects where maintaining type annotations can become cumbersome.

  Another user questions the long-term viability of projects like Pyrefly and Ty, suggesting that mypy, with its established community and wider adoption, will likely remain the dominant type checker for Python. They also raise the concern that relying on Rust for core type checking functionality might introduce complexity and limit contributions from developers unfamiliar with Rust.

  Some commenters express interest in exploring the internals of Pyrefly and its use of Rust, seeing it as a potential model for other performance-sensitive Python tooling. They praise the project's architecture and its focus on leveraging Rust's performance benefits.

  The discussion also touches upon the challenges of parsing Python code, especially given its dynamic nature and the complexities of the Abstract Syntax Tree (AST). Commenters acknowledge the difficulty of this task and commend the efforts of both Pyrefly and Ty in tackling it.

  Finally, there's a brief exchange about the overall trend of leveraging Rust for performance-critical components in Python projects, with commenters expressing optimism about this approach and its potential to improve the performance of various Python tools.

• Pyrefly - A faster Python type checker written in Rust

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  Posted: 2025-04-29 12:13:31

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  Pyrefly is a new Python type checker built in Rust that prioritizes speed. Leveraging Rust's performance, it aims to be significantly faster than existing Python type checkers like MyPy, potentially by orders of magnitude. Pyrefly achieves this through a novel incremental checking architecture designed to minimize redundant work and maximize caching efficiency. It's compatible with Python 3.7+ and boasts features like gradual typing and support for popular type hinting libraries. While still under active development, Pyrefly shows promise as a high-performance alternative for type checking large Python codebases.

  Pyrefly introduces itself as a significantly faster type checker for Python, built using the Rust programming language. It aims to address the performance limitations often associated with existing Python type checkers, particularly MyPy, which can become a bottleneck in larger projects. Pyrefly achieves its speed improvements through several key strategies. Firstly, by leveraging Rust's inherent performance advantages, it executes type checking operations much more quickly than a comparable Python implementation. Secondly, Pyrefly employs a caching mechanism that stores and reuses previous type checking results, avoiding redundant computations and further accelerating the process.

  Pyrefly is designed to be fully compatible with MyPy, supporting the same configuration files and command-line options. This allows developers to seamlessly transition from MyPy to Pyrefly without needing to modify their existing type checking setup. It aims to be a drop-in replacement, offering the same functionality and output as MyPy, but with a considerable performance boost. The project emphasizes its focus on checking types quickly and accurately, minimizing the impact on development workflows.

  While still under active development, Pyrefly highlights its ability to check a substantial number of open-source Python projects successfully. This serves as a testament to its growing maturity and compatibility with real-world codebases. The project encourages community involvement and contributions to further refine and enhance its capabilities. Pyrefly positions itself as a promising alternative for developers seeking a faster and more efficient Python type checking solution, ultimately aiming to improve the developer experience by reducing the time spent waiting for type checks to complete.

  • Python
  • Rust
  • Type Checking
  • static analysis
  • performance
  • compiler
  • Programming Languages
  • Software Development
  • Pyrefly
  • Open Source

  Summary of Comments ( 11 )
  https://news.ycombinator.com/item?id=43831524

  Verbose tl;dr

  Hacker News users generally expressed excitement about Pyrefly, praising its speed and Rust implementation. Some questioned the practical benefits given existing type checkers like MyPy, with discussion revolving around performance comparisons and integration into developer workflows. Several commenters showed interest in the specific technical choices, asking about memory usage, incremental checking, and compatibility with MyPy stubs. The creator of Pyrefly also participated, responding to questions and clarifying design decisions. Overall, the comments reflected a cautious optimism about the project, acknowledging its potential while seeking more information on its real-world usability.

  The Hacker News post about Pyrefly, a faster Python type checker written in Rust, has generated a number of comments discussing its potential, implementation, and comparison to existing tools like MyPy.

  Several commenters express excitement about the performance improvements Pyrefly offers. One user highlights the impressive speed increase, seeing type checking times drop from minutes to mere seconds. This resonates with others who have experienced slow type checking as a bottleneck in their Python development workflows. The Rust implementation is frequently cited as the key to these gains, with commenters praising Rust's performance characteristics.

  Discussion also revolves around the practical implications of faster type checking. Some anticipate that this could lead to more widespread adoption of type hinting in Python, as the performance penalty becomes less of a deterrent. The potential for improved developer experience is mentioned, as faster feedback loops can make development more efficient and enjoyable.

  Comparison to MyPy, the established type checker for Python, is inevitable. Commenters acknowledge MyPy's maturity and comprehensive feature set, while also pointing out its performance limitations. Some suggest that Pyrefly could serve as a "drop-in replacement" for MyPy in certain scenarios, particularly those where speed is paramount. Others envision a future where projects might utilize both tools, leveraging MyPy's thoroughness for less frequent, comprehensive checks and Pyrefly's speed for more iterative development.

  A few comments delve into technical aspects of Pyrefly's implementation. One user questions the choice of using JSON for communication between the Python and Rust components, suggesting that a more efficient serialization method might further enhance performance. Another raises the issue of handling incremental type checking, an important feature for large projects where re-checking the entire codebase for every small change is impractical.

  Finally, some comments express interest in the project's future development and potential integration with other tools and IDEs. The overall sentiment appears to be positive, with many commenters eager to see how Pyrefly evolves and contributes to the Python type checking ecosystem.