Migrating from sparse-ho¶

sparho is a clean-break successor to sparse-ho, not a drop-in replacement. There is no compat module — porting code is a manual rewrite. This page is the translation table.

The shape of the rewrite¶

sparse-ho’s outer call took four objects:

sparse_ho.grad_search(algo, criterion, model, optimizer, X, y, alpha0, monitor)

sparho rolls algo (hypergradient) and optimizer (outer step rule) into the search function itself, and folds model (which estimator) into the Solver adapter you pass:

sparho.hoag_search(
    problem,         # ← Problem(datafit, penalty, X, y)
    hp0=alpha0,      # ← in log α space internally; pass α > 0
    solver=...,      # ← was `model` + adapter choice
    criterion=...,   # ← was `criterion`
    hypergrad=...,   # ← was `algo`, default `implicit_forward`
)

monitor becomes the return value: result.history is an immutable tuple of IterationRecords. Use grad_search instead of hoag_search for the naive fixed-lr outer loop (sparse-ho’s GradientDescent optimizer).

Translation table¶

Models → `Problem` + adapter¶

sparse-ho	sparho	Notes
`Lasso(X, y, estimator=None)`	`Problem(SquaredLoss(), L1(), X, y)` + `SklearnLasso()`	Default. Use `CelerLasso()` from the `[celer]` extra for sparse-X.
`Lasso(..., estimator=celer.Lasso(...))`	`Problem(SquaredLoss(), L1(), X, y)` + `CelerLasso(tol=...)`
`ElasticNet(X, y, estimator=...)`	`Problem(SquaredLoss(), ElasticNet(rho), X, y)` + `SklearnElasticNet()`	sparse-ho’s `l1_ratio` is sparho’s `rho`.
`WeightedLasso(X, y, ...)`	`Problem(SquaredLoss(), WeightedL1(), X, y)` + `SklearnWeightedLasso()`	Per-feature α.
`WeightedElasticNet(X, y, ...)`	not yet at v0.1	Open a discussion if you need it.
`SparseLogreg(X, y, ...)`	`Problem(LogisticLoss(), L1(), X, y)` + `SklearnLogisticRegression()`	sparse-ho’s `C = 1/α`; sparho’s `α` matches the math directly.
`SVM(...)`, `SVR(...)`, `SimplexSVR(...)`	not at v0.1	`SmoothHinge` deferred.
`Ridge(...)`	not at v0.1	The hypergradient is closed-form; out of scope.

Algorithms → `hypergrad=`¶

sparse-ho `algo`	sparho `hypergrad=`	Notes
`ImplicitForward(...)`	`implicit_forward` (the default)	The only mode at v0.1.
`Implicit(...)`, `ImplicitVariational(...)`	not at v0.1	Deferred.
`Forward(...)`, `Backward(...)`	not at v0.1	Unrolled modes deferred.

Criteria¶

sparse-ho	sparho	Notes
`HeldOutMSE(idx_train, idx_val)`	`HeldOutMSE(idx_train, idx_val)`	int32 indices required.
`HeldOutLogistic(idx_train, idx_val)`	`HeldOutLogistic(idx_train, idx_val)`	`y ∈ {−1, +1}` (same convention).
`CrossVal(cv, criterion=HeldOutMSE)`	`CrossVal.kfold(n_samples, k=...)` or `CrossVal(folds=..., base=HeldOutMSE)`	sparho’s `CrossVal` is a frozen dataclass; build it once and reuse. Opt-in `warm_start=True` reuses per-fold `β*` across outer iters.
`HeldOutSmoothedHinge(...)`	not at v0.1	SVM/SVR family deferred.
`FiniteDiffMonteCarloSure(...)`	`Sure(sigma=..., epsilon=..., random_state=...)`	Landed in v0.3 §2 (FDMC after Deledalle 2014). Refuses non-`SquaredLoss` problems.

Optimizers → search function¶

sparse-ho optimizer	sparho equivalent	Notes
`GradientDescent(n_outer, step_size=lr)`	`grad_search(..., n_iter=n_outer, lr=lr)`	Plain GD baseline.
`LineSearch(n_outer)`	`hoag_search(..., n_iter=n_outer)`	sparho’s recommended default — Lipschitz-adaptive steps + inner-tolerance scheduling. The original Armijo `LineSearch` is not a sparho v0.1 deliverable; HOAG subsumes it in practice.
`Adam(...)`, `TrustRegion(...)`, `NonMonotoneLineSearch(...)`	not at v0.1	Owner-paper features; depend on `ImplicitVariational`.

Monitor → `SearchResult.history`¶

sparse-ho’s Monitor mutated as the loop ran:

monitor = Monitor()
sparse_ho.grad_search(algo, crit, model, opt, X, y, alpha0, monitor)
alphas, mses = monitor.alphas, monitor.objs

sparho returns an immutable SearchResult:

result = sparho.hoag_search(problem, hp0=alpha0, solver=..., criterion=...)
alphas = [r.hyperparam for r in result.history]
mses = [r.value for r in result.history]
best_alpha = result.best_hyperparam
best_coef = result.best_coef        # refit on the full problem

A worked example¶

sparse-ho:

from celer import Lasso as CelerLasso
from sklearn.datasets import make_regression
from sparse_ho import grad_search
from sparse_ho.algo import ImplicitForward
from sparse_ho.criterion import HeldOutMSE
from sparse_ho.models import Lasso
from sparse_ho.optimizers import LineSearch
from sparse_ho.utils import Monitor

X, y = make_regression(n_samples=300, n_features=100, noise=1.0, random_state=0)
idx_train, idx_val = np.arange(200), np.arange(200, 300)

model = Lasso(X, y, estimator=CelerLasso(fit_intercept=False))
algo = ImplicitForward(criterion="HO")
criterion = HeldOutMSE(idx_train, idx_val)
optimizer = LineSearch(n_outer=30)
monitor = Monitor()

grad_search(algo, criterion, model, optimizer, X, y, alpha0=1e-2, monitor=monitor)
print(monitor.alphas[-1], monitor.objs[-1])

sparho:

import numpy as np
from sklearn.datasets import make_regression

from sparho import HeldOutMSE, L1, Problem, SquaredLoss, hoag_search
from sparho.adapters.celer import CelerLasso

X, y = make_regression(n_samples=300, n_features=100, noise=1.0, random_state=0)
idx_train = np.arange(200, dtype=np.int32)
idx_val = np.arange(200, 300, dtype=np.int32)

result = hoag_search(
    Problem(SquaredLoss(), L1(), X, y),
    hp0=1e-2,
    solver=CelerLasso(tol=1e-8),
    criterion=HeldOutMSE(idx_train, idx_val),
    n_iter=30,
)
print(result.best_hyperparam, result.history[-1].value)

Behavior differences to know about¶

α lives in log space. Both grad_search and hoag_search step in θ = log α; hp0 must be strictly positive. The chain rule dC/dθ = dC/dα · α is applied internally. sparse-ho left this to the user via log_alpha_max.
Full-data refit at the end. SearchResult.best_coef is the inner solver run on the full Problem at best_hyperparam. sparse-ho left monitor.alphas[-1] as the only output; a refit was on the user.
Sparse-X is CSC, not CSR. Convert before constructing Problem.
No Monitor. The history is immutable. If you need streaming observation, wrap the criterion (or the solver) yourself.
No sure criterion. Dropped at v0.1; the v0.1 audience tunes validation, not unsupervised SURE. Revisit if asked.
No grid-search fallback. sparse-ho had a grid ho.grid_search; sparho doesn’t ship one. Use sklearn.linear_model.LassoCV if you want a grid baseline.

What’s not yet ported¶

These were sparse-ho features that v0.1 deliberately leaves on the floor. Some are slated for v0.2 (skein adapter, more datafits/penalties); others are out of scope (SVM/SVR, imaging operators). See ROADMAP.md for the full picture.