mpeqs

Exact answers for the arithmetic a language model should not be doing itself.

pip install mpeqs

import mpeqs

mpeqs.solve({"solver": "arith", "answer": "(17/100)*250"})   # Fraction(85, 2)

The model names a solver and fills its slots. This evaluates the spec exactly, or refuses by name — it never guesses, and it never returns a number it could not derive.

The refusal is the point. A solver that cannot evaluate a spec raises Refusal with a reason, and that reason is a routing signal: it tells the caller to ask the model, rather than quietly handing back a wrong number.

try:
    mpeqs.solve({"solver": "arith", "answer": "1/0"})
except mpeqs.Refusal as why:
    ...   # ask the model; the record declined this one

Shape disposes over name. A model that picks the wrong solver but fills the right slots has still described the problem correctly, so a spec carrying an answer key is arithmetic whatever it calls itself. Pass repair=False to turn that off.

What is exact and what is not

Results come back as fractions.Fraction, so nothing is lost on the way out. The exactness is in the arithmetic, not in the parsing: a ratio written as a ratio stays exact, while a decimal literal is read as a float first.

mpeqs.solve({"solver": "arith", "answer": "1/10+2/10"})   # Fraction(3, 10)  exact
mpeqs.solve({"solver": "arith", "answer": "0.1+0.2"})     # the binary expansion

Solvers

arith, geometry, iterate, modular, multisearch, polynomial, and the named families behind them — factorisation, gcd/lcm, linear systems, quadratics, remainders, combinatorics, rates, mixtures, ratios, base conversion.

mpeqs.solvers() lists what a given build dispatches to.

No dependencies

The whole library is standard library. One py3-none-any wheel serves Linux, macOS and Windows on every supported interpreter, the install is instant, and a test asserts that no third-party import creeps in.

PySpell: when an expression is not enough

solve() evaluates expressions, and is safe by restriction — the source is parsed to an AST, walked against a whitelist, and evaluated with __builtins__ emptied. Nothing runs that was not understood first.

What it cannot express is a procedure. There is no expression for "how many steps does the Collatz sequence take from 27" — that needs a loop, a branch and an accumulator. The usual answer is to let the model emit Python and exec it, which trades a whitelist for a sandbox and hope.

mpeqs.pyspell is the third option. PySpell is mpedb's stored-function language: a small deterministic subset of Python with no imports, no clock, no randomness, no file or network I/O, and a fixed instruction budget so a runaway loop fails identically everywhere. A model can write a loop. It still cannot open a socket.

import mpeqs.pyspell as spell

spell.call("""
def col(n):
    if n < 1:
        return 1 // 0
    c = 0
    while n != 1:
        if n % 2 == 0:
            n = n // 2
        else:
            n = 3 * n + 1
        c = c + 1
    return c
""", 27)
# 111

And when the model claims a relationship between two functions, the claim is checked against a probe corpus rather than believed:

spell.check_bijective(
    "def dbl(x):\n    return x * 2\n",
    "def hlv(x):\n    return x // 2\n")
# mpeqs.Refusal: dbl/hlv is not bijective: ... forward(Float(1e308)) = Float(inf) ...

That refusal is correct. x*2 then x//2 is genuinely not bijective over the values PySpell admits — a denormal floors to zero, and 1e308 is an integral float whose double is infinity. The verifier names the input that breaks it, every time. Guard the domain to bounded positive integers and the same pair is accepted, reporting how many probe values actually round-tripped rather than a bare "verified".

It caught the author of that example twice while it was being written. That is the argument for "declare and check" over "generate and hope".

Optional, because it is not free: mpedb ships as a compiled wheel needing CPython 3.12 or newer, while mpeqs itself runs from 3.10 with one artefact for every platform.

pip install mpeqs[pyspell]

Without it, mpeqs.pyspell.available() is False and every entry point raises Refusal naming the extra, rather than failing at import. Nothing else in mpeqs changes.

Licence

The mpedb License 1.0 — free of charge for every person and every organization, except that a group over five billion dollars in revenue or valuation owes seven US cents per device, once. Not an OSI-approved licence.