Speed up constant * Expr

[Zeroto521]

As the title. This pr is 1.47x faster than the master branch.

• Optimized before: 6.5767 seconds
• Optimized after: 4.4400 seconds

from timeit import timeit

from pyscipopt import Model


m = Model()

n = 1000
x = m.addMatrixVar(n)
e = x.sum()

number = 100_000
cost = timeit(lambda: e * -1.5, number=number)
print(
    f"Cost of multiplying an expression with a number over {number} runs: {cost:.4f} seconds"
)

[Copilot]

Pull request overview

This PR optimizes the multiplication of a constant with an Expr object by replacing Python-level dictionary comprehension with C-level PyDict_Next iteration, achieving a 1.47x speedup (from 6.58s to 4.44s for the benchmark).

Changes:

• Replaced dictionary comprehension with C-level API (PyDict_Next) for constant * Expr multiplication
• Renamed variable prod_v to coef for consistency across multiplication operations
• Added test coverage for Expr * number multiplication

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/pyscipopt/expr.pxi	Optimized __mul__ method to use C-level PyDict_Next API for scalar multiplication; renamed prod_v to coef for consistency
tests/test_expr.py	Added test case to verify Expr * number multiplication produces correct results
CHANGELOG.md	Documented the performance improvement for constant * Expr operations

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