--- title: WebAssembly as a Python extension platform | daily.dev description: WebAssembly can extend Python as an architecture-independent alternative to native C extensions, enabling two main use cases: accelerating pure Python hot... canonical: https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: WebAssembly as a Python extension platform | daily.dev og:description: WebAssembly can extend Python as an architecture-independent alternative to native C extensions, enabling two main use cases: accelerating pure Python hot... og:url: https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje og:image: https://api.daily.dev/og/posts/T5Ltc2gje.png og:image:alt: WebAssembly as a Python extension platform og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # WebAssembly as a Python extension platform **[null program](https://daily.dev/sources/nullprogram)** · 13 min read · 0 upvotes · 0 comments ## Summary WebAssembly can extend Python as an architecture-independent alternative to native C extensions, enabling two main use cases: accelerating pure Python hot spots (~10x speedup) and embedding capabilities from other languages without requiring a native toolchain on the host. The preferred runtime is wasmtime-py (version 40), which ships prebuilt binaries for Windows, macOS, and Linux on x86-64 and ARM64 but weighs ~18MiB and has a frequently breaking API. A critical pitfall is that Wasm runtimes treat all integers as signed, so pointers returned from Wasm malloc must be masked with `& 0xffffffff` to avoid silent writes to wrong addresses. The article also covers using a bump allocator instead of malloc for cleaner memory management, and demonstrates embedding the Monocypher cryptography library compiled to Wasm as a practical example of the embedded-capabilities pattern. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why do pointers returned from Wasm malloc come back as negative numbers in Python? Wasm runtimes interpret all integers as signed, and Wasm makes no distinction between pointers and integers. Because addresses start at 0 and the upper half of a 32-bit address space has the high bit set, those addresses appear negative when treated as signed. Every pointer coming out of Wasm must be masked: `pointer = malloc(...) & 0xffffffff` for wasm32, or `>>> 0` in JavaScript. _Developers shipping Wasm-backed Python libraries track footguns like this on daily.dev before they hit production._ ### What is the wasmtime-py package size and which platforms does it support? wasmtime-py (version 40) installs at approximately 18 MiB and ships prebuilt binaries for Windows, macOS, and Linux on both x86-64 and ARM64, covering nearly all Python installations. No native C toolchain is required on the host. The API breaks on a roughly monthly basis, so projects must stay current to avoid bitrot. _Teams evaluating Python packaging trade-offs for Wasm runtimes find comparisons like this on daily.dev._ ### How do I compile a C library like Monocypher to WebAssembly without libc for use in Python? Compile with Clang targeting wasm32, disabling the standard library and specifying no entry point: `clang --target=wasm32 -nostdlib -O2 -Wl,--no-entry -Wl,--export-all -o monocypher.wasm monocypher.c`. The `--export-all` flag exposes all externally-linked symbols as the Wasm interface. A bump allocator can be added in a wrapper translation unit to manage memory without shipping a full general-purpose allocator inside the Wasm binary. _Developers embedding C libraries into Python via Wasm find practical patterns like this on daily.dev._ ## Similar posts on daily.dev - [WebAssembly as a Python extension platform](https://daily.dev/posts/webassembly-as-a-python-extension-platform-uk45ymkeo) · Hacker News · 0 upvotes · 0 comments - [WebAssembly is everywhere. Here’s how it works](https://daily.dev/posts/webassembly-is-everywhere-here-s-how-it-works-d8uxjessm) · The New Stack · 2 upvotes · 0 comments - [Running Python code in a sandbox with MicroPython and WASM](https://daily.dev/posts/running-python-code-in-a-sandbox-with-micropython-and-wasm-zzzzwu8mf) · Simon Willison · 2 upvotes · 1 comments - [WebAssembly: an introduction](https://daily.dev/posts/webassembly-an-introduction-1iycvposp) · Flavio Copes · 3 upvotes · 0 comments - [The 7 Walls JavaScript Hits — and How WebAssembly Gets Past Them](https://daily.dev/posts/the-7-walls-javascript-hits-and-how-webassembly-gets-past-them-isxmm7isd) · DEV · 2 upvotes · 0 comments --- Tags: [#python](https://daily.dev/tags/python), [#c](https://daily.dev/tags/c), [#cryptography](https://daily.dev/tags/cryptography), [#webassembly](https://daily.dev/tags/webassembly) [View this post on daily.dev](https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"WebAssembly as a Python extension platform","url":"https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje"},"datePublished":"2026-08-11T08:51:44.900Z","dateModified":"2026-08-11T08:52:16.894Z","description":"WebAssembly can extend Python as an architecture-independent alternative to native C extensions, enabling two main use cases: accelerating pure Python hot...","image":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","thumbnailUrl":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","isAccessibleForFree":true,"articleSection":"null program","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"null program","logo":"https://media.daily.dev/image/upload/logos/placeholder.jpg","url":"https://daily.dev/sources/nullprogram"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"python,c,cryptography,webassembly","timeRequired":"PT13M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"null program","item":"https://daily.dev/sources/nullprogram"},{"@type":"ListItem","position":3,"name":"WebAssembly as a Python extension platform"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/webassembly-as-a-python-extension-platform-t5ltc2gje#faq","mainEntity":[{"@type":"Question","name":"Why do pointers returned from Wasm malloc come back as negative numbers in Python?","acceptedAnswer":{"@type":"Answer","text":"Wasm runtimes interpret all integers as signed, and Wasm makes no distinction between pointers and integers. 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