Limitations

PyMode runs upstream CPython 3.13 compiled to wasm32-wasi inside Cloudflare Workers Durable Objects. This works well for most Python code, but the WASM/WASI/DO environment has inherent constraints.

Platform Constraints

Constraint	Details
No filesystem persistence	The WASM linear memory is reset per request. Use KV/R2/D1 for storage.
No network sockets	Standard socket module is unavailable. Use pymode.tcp and pymode.http instead.
No threads	threading is disabled (--disable-threads). Use pymode.parallel for real parallelism via child DOs.
No subprocess	os.system, subprocess, os.fork are not available in WASI.
No signals	Signal handling is emulated (-lwasi-emulated-signal), limited functionality.
30s CPU limit	Per-request CPU time is capped at 30 seconds (CF Workers limit).
128MB memory	Per-DO memory limit. Large data processing should use pymode.parallel to distribute across DOs.
10MB bundle size	Total compressed worker size. PyMode itself uses ~2-3MB gzipped, leaving ~7MB for your code and dependencies.

Bundled Stdlib Modules

PyMode bundles a subset of CPython’s stdlib as string constants in the worker. These are the modules available in the WASM runtime:

Always Available (bundled in stdlib-fs)

Category	Modules
Encodings	encodings, encodings.aliases, encodings.utf_8, encodings.ascii, encodings.latin_1
JSON	json
Regex	re
Collections	collections, collections.abc, functools, operator
Types	enum, types, typing, abc
Core	warnings, contextlib, dataclasses, copy, copyreg, weakref, _weakrefset
Text	string, textwrap
Crypto	base64, hashlib, hmac, secrets
Numbers	random, bisect, heapq, numbers, fractions, decimal
Date/Time	datetime, calendar
Path	fnmatch, glob, pathlib
URL	urllib, urllib.parse, ipaddress
Import	importlib, importlib.abc, importlib.machinery, importlib.util
Serialization	pickle, _compat_pickle, struct
I/O	io, _pyio
Misc	keyword, reprlib, traceback, linecache, tokenize, token, csv, pprint

Built-in C Modules (compiled into python.wasm)

These are C extension modules linked directly into the WASM binary:

array, _asyncio, _bisect, _contextvars, _csv, _heapq, _json, _pickle, _random, _struct, _zoneinfo, math, cmath, _statistics, _datetime, _decimal, _md5, _sha1, _sha2, _sha3, _blake2, _codecs, _collections, errno, _io, itertools, _sre, _thread, time, _typing, _weakref, _abc, _functools, _locale, _operator, _stat, _symtable, unicodedata, pyexpat, _elementtree

Polyfilled Modules

Module	Implementation	Notes
binascii	Pure Python (lib/polyfills/binascii.py)	Replaces the C extension. Covers hex, base64, crc32, uu encoding.
socket	Pure Python (lib/polyfills/socket.py)	Routes through pymode.tcp host imports. Provides constants, getaddrinfo, create_connection.
threading	Pure Python (lib/polyfills/threading.py)	No-op locks within a DO. Thread.start() spawns child DOs via pymode.parallel for real parallelism.
logging	Pure Python (lib/polyfills/logging/)	Threading-free logging that writes to stderr. Full API: getLogger, basicConfig, handlers.
multiprocessing	Pure Python (lib/polyfills/multiprocessing/)	cpu_count() returns 1, exception classes. Use pymode.parallel for real parallelism.
ssl	Pure Python (lib/polyfills/ssl.py)	TLS termination handled by CF’s edge — Python code doesn’t need raw TLS.
zlib	Pure Python (lib/polyfills/zlib.py)	Minimal compression polyfill for packages that import zlib at module level.

Not Available

These stdlib modules require OS features that WASI doesn’t provide:

Module	Reason
socket	Polyfilled — routes through pymode.tcp host imports.
ssl	Polyfilled — TLS termination handled by CF’s edge.
subprocess	No process spawning in WASI.
multiprocessing	Polyfilled — cpu_count() returns 1. Use pymode.parallel for real parallelism via child DOs.
threading	Polyfilled — no-op locks within a DO. Thread.start() spawns child DOs via pymode.parallel.
sqlite3	C extension not compiled for WASM. Use D1 binding instead.
ctypes	No dynamic linking for arbitrary shared libraries.
tkinter	No GUI in Workers.
curses	No terminal in Workers.
readline	No interactive terminal.
mmap	Not available in WASI.
fcntl, termios, grp, pwd	Unix-specific, not in WASI.
select, selectors	Polyfilled — selectors bundled in stdlib.
signal	Emulated only, limited functionality.
resource	No resource limits API in WASI.

Now Bundled (previously missing)

These modules have been added to the stdlib bundle since initial release:

Module	Added for
html, html.parser, html.entities	beautifulsoup4, web scraping
email (full package)	requests, urllib3, http.client
http, http.client, http.cookiejar	requests, httpx, urllib3
xml, xml.sax, xml.etree, xml.parsers	langchain, defusedxml
asyncio (full package)	pydantic, typing_extensions
argparse	numpy, click
locale	httpx, distro
tempfile, shutil	many packages
importlib.metadata, importlib.resources	package metadata discovery
concurrent.futures	tenacity, langsmith
zoneinfo	pydantic datetime
tomllib	pydantic, project configs
configparser	various packages
sysconfig	pydantic, setuptools
unittest	pyparsing

Requesting a module: If you need a stdlib module that’s not bundled, open an issue or add it to BOOT_FILES in scripts/generate-stdlib-fs.py.

Python Language Support

Full CPython 3.13 language support including:

• All syntax features (match/case, walrus operator, f-strings, etc.)
• Generators, async generators, coroutines
• Decorators, metaclasses, descriptors
• Exception groups, ExceptionGroup
• Type hints, typing module

Not supported:

• async/await at the handler level (use Asyncify for I/O instead)
• asyncio event loop (no I/O multiplexing in WASI)

Package Support

Tested & Working

These packages have conformance tests running in the actual workerd runtime:

Package	Type	Tests	Notes
pydantic	Rust ext (compiled to WASM)	4 passing	python-pydantic-core.wasm variant
fastapi	Pure Python (ASGI)	1 passing	Request validation, typed APIs
langchain-core	Pure Python	5 passing	Messages, documents, utils, serialization
langgraph	Pure Python	5 passing	StateGraph, compile, invoke, conditional edges
instructor	Pure Python	1 passing	Structured LLM output via pydantic
openai SDK	Pure Python	1 passing	Client types importable
numpy	C ext (compiled to WASM)	8 passing	python-numpy.wasm variant, arrays, FFT, random
jinja2	Pure Python	3 passing	Templates, loops, filters, inheritance
requests	Pure Python	1 passing	Session building, request construction
httpx	Pure Python	1 passing	Request building, URL parsing
beautifulsoup4	Pure Python	2 passing	HTML parsing, CSS selectors
pyyaml	Pure Python fallback	2 passing	YAML parse/dump
click	Pure Python	1 passing	CLI command creation
attrs	Pure Python	1 passing	Validators, asdict
starlette	Pure Python	1 passing	ASGI routing

Total: 261 tests across all test suites.

Polyfilled C/Rust Extensions

These packages have native extensions replaced with pure-Python polyfills:

Package	Original	Polyfill
uuid_utils	Rust (_uuid_utils)	stdlib uuid + time + os.urandom
xxhash	C (_xxhash)	hashlib.sha256 as stand-in
ormsgpack	Rust (ormsgpack)	Wraps pure-Python msgpack
jiter	Rust (jiter.jiter)	stdlib json
markupsafe	C (_speedups)	Pure Python fallback (built-in)
multiprocessing	C (process model)	Polyfill: cpu_count()=1, exception classes

Architecture: Each Package Gets Its Own DO

Memory constraints (128MB Worker / 256MB DO) apply per DO, not globally. Heavy packages run in dedicated DOs communicating via Worker RPC:

Orchestrator DO          Package DOs (each 256MB)
┌────────────┐          ┌─────────────┐
│  Your app  │───RPC───▶│  numpy DO   │
│  routing   │          └─────────────┘
│  logic     │───RPC───▶│  pandas DO  │
│            │          └─────────────┘
└────────────┘───RPC───▶│  langchain  │
                        │  DO         │
                        └─────────────┘

This means even pandas (~100MB) can work — it just needs its own DO. The threading.Thread polyfill already spawns child DOs via pymode.parallel.

Won’t Work in WASM (any DO)

Package	Reason
tensorflow	Needs GPU, ~500MB, C++/CUDA runtime
torch	Needs GPU, ~2GB, C++/CUDA runtime
scipy	Needs BLAS/LAPACK (Fortran), complex C

For ML inference, use Workers AI (env.AI.run()) instead.