BUG: Expr.__array_ufunc__ can't handle 0-dim array

[Zeroto521]

fix #1218

numpy automatically converts np.generic to a 0-dim array.

import numpy as np


class Expr:
    def __array_ufunc__(self, ufunc, method, *args, **kwargs):
        print("-" * 50)
        print(args)
        print([type(a) for a in args])
        print("-" * 50)

        return NotImplemented

    def __init__(self, value: int):
        self.value = value

    def __repr__(self):
        return f"Expr({self.value})"

    def __le__(self, other: int):
        return self.value <= other


if __name__ == "__main__":
    x = Expr(5)
    value = np.float64(5.0)
    print(value <= x)

    # --------------------------------------------------
    # (array(5.), Expr(5))  # <-- the first argument is a numpy array, not np.float64
    # [<class 'numpy.ndarray'>, <class '__main__.Expr'>]
    # --------------------------------------------------

[Copilot]

Pull request overview

Fixes NumPy scalar/0-d array interactions with PySCIPOpt expressions by preventing ExprLike.__array_ufunc__ from treating 0-d arrays as matrix inputs, resolving the AttributeError reported in #1218 when comparing/negating np.float64 values against expressions.

Changes:

• Update ExprLike.__array_ufunc__ to only route ndim >= 1 arrays through matrix-handling logic, and to coerce 0-d arrays (and np.generic) to native Python scalars via .item().
• Add a regression test covering comparisons between np.float64 (including negated) and expression variables in both operand orders.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_expr.py	Adds regression test for np.float64 comparison behavior with expressions (issue #1218).
src/pyscipopt/expr.pxi	Adjusts __array_ufunc__ handling to coerce 0-d arrays to scalars and avoid incorrect matrix-ufunc routing.

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[Joao-Dionisio]

Hey @Zeroto521 , thank you thank you thank you! Some comments from my Claude, please see if any make sense.

1. Gate the 0-d unwrap on dtype.kind for symmetry with the matrix path

The 1-d+ path returns NotImplemented for non-numeric dtypes, but the new 0-d unwrap calls .item() unconditionally. So np.array(some_object, dtype=object) <= x would now silently extract the Python
object instead of returning NotImplemented. Probably never hit in practice, but easy to keep consistent:

args = [
    a.item()
    if isinstance(a, np.generic)
    or (isinstance(a, np.ndarray) and a.ndim == 0 and a.dtype.kind in "fiub")
    else a
    for a in args
]

2. Broaden the regression test

Only <= is exercised, but the same code path serves >=, ==, and all the arithmetic ufuncs. Worth adding a couple more one-liners — == in particular is interesting because it produces a two-sided ExprCons:

  assert str(value >= x) == "ExprCons(Expr({Term(x): 1.0}), None, 5.0)"
  assert str(value == x) == "ExprCons(Expr({Term(x): 1.0}), 5.0, 5.0)"
  assert str(np.int64(5) <= x) == "ExprCons(Expr({Term(x): 1.0}), 5.0, None)"
  assert str(np.array(5) <= x) == "ExprCons(Expr({Term(x): 1.0}), 5.0, None)"

3. Comment is slightly off

The comment mentions np.generic + MatrixExpr, but the bug being fixed is comparison ufuncs hitting .view(MatrixExprCons) on a plain ExprCons, not + recursion. Something like:

  # Numpy scalars and 0-d arrays must take the scalar dispatch path below,
  # not the matrix path — the matrix path calls .view(MatrixExprCons) on
  # the result, which fails for scalar comparisons returning ExprCons (#1218).

Nitpick: stray space at tests/test_expr.py:339 — str(x <= value ).

[Zeroto521]

In this case, 1 <= np.array(x) gets recursion and crushes the Python kernel.

The 1-d+ path returns NotImplemented for non-numeric dtypes, but the new 0-d unwrap calls .item() unconditionally. So np.array(some_object, dtype=object) <= x would now silently extract the Python
object instead of returning NotImplemented. Probably never hit in practice, but easy to keep consistent:

args = [
    a.item()
    if isinstance(a, np.generic)
    or (isinstance(a, np.ndarray) and a.ndim == 0 and a.dtype.kind in "fiub")
    else a
    for a in args
]