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Fatal RangeError: Maximum call stack size exceeded when combining partial with infinite recursion #5959
Description
The following code has infinite recursion. In python, this raises RecursionError.
from functools import partial
def f(obj, *, unrelated=1) -> object:
recur = partial(f, unrelated=1)
assert isinstance(obj, list)
for x in obj:
recur(x)
a = []
a.append(a)
# expect RecursionError, but get a fatal emscripten-level error
f(a)I would expect the same in pyodide, but instead, pyodide crashes fatally with:
File "/Users/tybug/Desktop/Liam/coding/hypothesis/test_pyodide/run.py", line 13 in f
...
RangeError: Maximum call stack size exceeded
at wasm://wasm/0268cb26:wasm-function[9549]:0x4bca27
at wasm://wasm/0268cb26:wasm-function[2322]:0x21266f
at wasm://wasm/0268cb26:wasm-function[2357]:0x213d21
at wasm://wasm/0268cb26:wasm-function[2551]:0x2204dd
at wasm://wasm/0268cb26:wasm-function[2552]:0x2205ff
at wasm://wasm/0268cb26:wasm-function[7366]:0x422758
at wasm://wasm/0268cb26:wasm-function[2590]:0x22553e
at wasm://wasm/0268cb26:wasm-function[4592]:0x328962
at wasm://wasm/0268cb26:wasm-function[1157]:0x1c47fb
at wasm://wasm/0268cb26:wasm-function[1166]:0x1c5006 {
pyodide_fatal_error: true
}(this reproduces for example in the online pyodide REPL: https://pyodide.org/en/stable/console.html.)
The critical piece is partial: if this is removed, for example setting recur = f, then we get a standard RecursionError instead of a fatal pyodide error.
Versions:
pyodide CLI version: 0.3.0
pyodide-build version: 0.30.0
pyodide-lock version: 0.1.0a7
auditwheel_emscripten version: 0.1.0
Activity
ryanking13 commentedon Oct 20, 2025
Thanks for the report! I can reproduce this in my Chrome (141.0), but not in firefox (144).
Unfortunately, this is not a bug IMO, and the workaround is to reduce the size of the max recursion in Python to, such as 700 using
(by default, we use 1000, which is the default value in CPython too, I think)
The maximum call stack size in JS differs per browser. We regularly measure the stack size in our CI:
Here, the
js_depth/py_depthmeans how much JS call stack is used for a single Python function on average, and thejs_depthis the maximum call stack size in the JS in the browser.Since the maximum call stack size is around 12,000 in JS, and each Python function call consumes around 6 JS call stack, the Python recursion limit 1000 would not hit the JS call stack maximum normally (1000 * 6 < 12000).
However, in your example, I think using
partialmakes us consume extra JS call stacks per the function call (I cannot explain exactly why though), which made 1000 python recursion consume more than the 12000 JS call stack size, resulting in theMaximum call stack size exceeded.We cannot dynamically calculate and update the Python recursion limit, so I think the user should adjust it to prevent such errors.
Liam-DeVoe commentedon Oct 20, 2025
That makes sense to me. As a bit of motivation: I ran into this while writing application-level code that handles
RecursionError(to fall back to an alternative algorithm when infinite recursion is detected). This won't catch the fatal pyodide error. Though I would like it to - If pyodide gave me a way to catch and handle javascript "max call stack size exceeded" errors, I would catch that here as well. I don't know if this can be done in a sane way though.I'll consider rewriting this code to avoid
partial, but it's such an ergonomic util that I wouldn't want to blanket avoid it just in case of running into this.ryanking13 commentedon Oct 21, 2025
That sounds good. Let me think about a way to do it, but probably @hoodmane has some experience with that.
hoodmane commentedon Oct 21, 2025
Just set recursion limit lower? @markshannon recently tried quite hard to make stack checks work on wasm and it didn't work out at all so I'm convinced there's no reasonable way to do it.