比较候选股票池上的投资组合配置规则

# 17_portfolio_management.py


```py
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# %% [markdown]
# # US equities panel: the same names, sized differently
#
# [`16_backtest`](16_backtest.ipynb) put the same amount of money in every position. That is the
# plainest rule there is, and it embeds an assumption worth naming: that a stock the model ranked
# first and a stock it ranked fiftieth deserve the same capital, and that a quiet stock and a
# violent one do too.
#
# This notebook keeps the names and changes only the money. The model, the checkpoint, the
# rebalancing dates and which stocks are held are all held fixed; what varies is how much goes into
# each. Every allocator declared in `config/setup.yaml` is applied, and they answer the question
# in three different ways:
#
# - **From the prediction.** `score_weighted` gives more capital to the names the model was more
#   confident about, so it trusts the magnitude of a prediction and not only its order.
#   `conformal_weighted` reads the prediction's uncertainty rather than its size: it weights each
#   name by one over the width of its prediction interval, so a name the model is less sure about
#   gets less capital. The width is floored at the first percentile of that date's own
#   cross-section before the reciprocal is taken, which keeps an unusually confident name from
#   taking the whole leg and uses no width from a later date to do it. That is the same width whose
#   calibration [`15_model_analysis`](15_model_analysis.ipynb) checked, which is why the check
#   there matters here.
# - **From each stock's own volatility.** `inverse_vol` puts less into a stock that moves more, so
#   each position contributes a similar amount of variation rather than a similar amount of money.
#   `risk_parity` as implemented here is the same idea with a steeper exponent on volatility,
#   which approximates equal risk contribution without estimating how the stocks move together.
# - **From how the stocks move together.** `mvo_ledoit_wolf` and `hrp` read a covariance matrix, so
#   they alone can tell that two names which always move together are one bet held twice. That is
#   the property none of the rules above can see, and it is the one that has to be estimated. These
#   two need history before they can decide anything, and how much is declared per allocator rather
#   than assumed.
#
# **Equal weight is excluded here because its backtest already exists.** It is the baseline every
# row is measured against, and [`16_backtest`](16_backtest.ipynb) ran it.
#
# **A shortlist is taken first, and that is a real decision.** Applying every allocator to every
# member of the whole model population would multiply an already large grid by seven. So the
# highest validation Sharpe per distinct model configuration is carried forward, which means the
# allocator comparison is made on strategies the equal-weight rule already liked. An allocator that
# rescues a model equal weight buried is not something this design can find.
#
# **Learning objectives.** By the end of this notebook you will be able to:
#
# - Name the assumption an equal-weight book makes about its positions, and say what each family
#   of allocator replaces it with.
# - Say what a covariance-reading allocator can see that a per-stock one cannot, what it needs in
#   exchange, and which of the declared allocators actually read one.
# - Explain why a lookback window is declared per allocator rather than shared, and what a shared
#   one would silently do to the ones that need less.
# - State what a shortlist taken on baseline Sharpe makes it impossible for this comparison to
#   discover.
#
# **Book reference**: Chapter 17, Sections 17.2 to 17.8.
#
# **Prerequisites**: [`16_backtest`](16_backtest.ipynb) has frozen the equal-weight baseline sets
# this notebook draws from.
#
# **What it writes**: one validation backtest per surviving configuration and allocator, in
# `run_log/registry.db`, frozen as one named allocation set per label.
# [`18_risk_management`](18_risk_management.ipynb) reads them next.

# %%
"""Generate the US-equities allocation-stage validation population."""

import json
import os
from pathlib import Path

import matplotlib.pyplot as plt
import polars as pl

from case_studies.research import (
    CandidateSet,
    OfficialPopulation,
    candidate_set_supersedes,
    open_study,
    plan_backtests,
    population_supersedes,
    run_backtests,
)
from case_studies.research.strategy import strategy_warmup_periods
from case_studies.utils.backtest_loaders import (
    get_backtest_config,
    load_backtest_prices_for,
)
from case_studies.utils.notebook_contracts import degenerate_prediction_hashes
from case_studies.utils.sweep_config import (
    get_allocators,
    get_checkpoints_per_config,
    get_top_n_predictions,
    top_n_cap,
)
from utils.style import add_message_title, ml4t_palette, show_with_alt, zero_line

# %% tags=["parameters"]
CASE_STUDY_ID = "us_equities_panel"
BASELINE_SET_NAMES = [
    "us-equities-fwd-ret-1d-baseline-v1",
]
EXECUTION_TIER = "canonical"
POPULATION_NAME = ""
SUPERSEDES_POPULATION = ""
SUPERSEDES_SETS: dict = {}
# Empty means this run writes to the case study's own store, which is what canonical
# production execution wants. Any other value routes the run's writes there instead, at
# either tier, and is how a rehearsal at full scale is compared against the published
# result without being able to damage it.
WORKSPACE = ""
PREVIEW_LABELS = []
PREVIEW_MAX_BASELINE_ROWS = 0
PREVIEW_MAX_ALLOCATORS = 0
MAX_SYMBOLS = 0
# None means the width `setup.yaml` declares; an int overrides it. Declared here because
# papermill only binds a name the parameters cell already holds - a run that passes
# TOP_N_PREDICTIONS to a notebook without it sweeps the declared width and exits 0.
TOP_N_PREDICTIONS = None

# %% [markdown]
# ## 2. The baseline this notebook varies
#
# The equal-weight sets are opened and checked complete. Everything below changes one thing about
# them, so a gap here would silently narrow what the allocator comparison is made over.

# %% [markdown]
# Both tiers resolve the study through `open_study`. It reads the labels and features in place and
# redirects only writes, so a preview run scores the same inputs a canonical one does and cannot
# publish over it.

# %%
workspace_override = os.environ.get("ML4T_OUTPUT_DIR") or WORKSPACE
if EXECUTION_TIER == "canonical":
    if PREVIEW_LABELS or PREVIEW_MAX_BASELINE_ROWS or PREVIEW_MAX_ALLOCATORS or MAX_SYMBOLS:
        raise ValueError("Canonical execution cannot declare preview reductions")
    if not BASELINE_SET_NAMES or len(BASELINE_SET_NAMES) != len(set(BASELINE_SET_NAMES)):
        raise ValueError("Canonical execution requires unique named baseline sets")
    study = open_study(
        CASE_STUDY_ID,
        execution_tier=EXECUTION_TIER,
        workspace=Path(workspace_override) if workspace_override else None,
    )
elif EXECUTION_TIER == "preview":
    if (
        not PREVIEW_LABELS
        or PREVIEW_MAX_BASELINE_ROWS < 1
        or PREVIEW_MAX_ALLOCATORS < 1
        or MAX_SYMBOLS < 1
    ):
        raise ValueError(
            "Preview execution requires labels and explicit row, allocator, and symbol limits"
        )
    study = open_study(
        CASE_STUDY_ID,
        execution_tier=EXECUTION_TIER,
        workspace=Path(workspace_override or "experiments"),
    )
else:
    raise ValueError(f"Unsupported execution tier: {EXECUTION_TIER!r}")

# %% [markdown]
# ## 3. Which baseline rows can be re-sized
#
# Complete, validation-split, and produced under this run's tier. A row failing any of those is
# refused rather than dropped, so the shortlist below is taken from a population that means what it
# says.

# %%
backtest_catalog = study.backtests.table(include_preview=True)
if EXECUTION_TIER == "canonical":
    baseline_sets = tuple(CandidateSet.one(study, name=name) for name in BASELINE_SET_NAMES)
    if any(result_set.member_kind != "backtest" for result_set in baseline_sets):
        raise ValueError("Every declared baseline set must contain backtests")
    baseline_members = tuple(
        member for result_set in baseline_sets for member in result_set.members
    )
    if len(baseline_members) != len(set(baseline_members)):
        raise ValueError("Declared baseline sets overlap")
    baseline = backtest_catalog.filter(pl.col("backtest_hash").is_in(baseline_members))
    if baseline.height != len(baseline_members):
        raise ValueError("The backtest catalog does not contain every baseline member")
else:
    baseline = (
        backtest_catalog.filter(
            (pl.col("execution_tier") == "preview")
            & (pl.col("stage") == "signal")
            & pl.col("label").is_in(PREVIEW_LABELS)
        )
        .sort("sharpe", "backtest_hash", descending=[True, False])
        .head(PREVIEW_MAX_BASELINE_ROWS)
    )

ineligible = baseline.filter(
    (pl.col("split") != "validation")
    | (pl.col("execution_tier") != EXECUTION_TIER)
    | (pl.col("stage") != "signal")
    | ~pl.col("complete")
    | pl.col("sharpe").is_null()
    | ~pl.col("sharpe").is_finite()
)
if baseline.is_empty() or not ineligible.is_empty():
    raise ValueError("Allocation requires complete finite equal-weight validation rows")

# %% [markdown]
# ## 3b. The rows that rank but do not forecast
#
# A regularized linear model that shrinks every coefficient to zero on a fold predicts one
# constant for that fold. The backtest still runs: a constant score ranks nothing, so the
# top-k rule holds whichever names the tie-break leaves on top and the book turns into a slow
# buy-and-hold. That book has a *good*-looking Sharpe here, because it trades 5,761 times
# instead of 121,521 and so pays almost none of the costs that dominate every real member.
#
# **This is an exclusion, not a refusal.** The rows above are legitimate members of the
# baseline population and the sweep that produced them has no degeneracy filter of its own -
# the same gap `nasdaq100_microstructure/14_backtest` closes at the point of use. What must not
# happen is that they reach a leaderboard: `selectable_validation_candidates` already refuses
# them when it resolves the carrier, so without this the allocator comparison and the carrier
# pool would disagree about which configurations exist.

# %%
degenerate = degenerate_prediction_hashes(study.root)
excluded = baseline.filter(pl.col("prediction_hash").is_in(degenerate))
baseline = baseline.filter(~pl.col("prediction_hash").is_in(degenerate))
if baseline.is_empty():
    raise ValueError("Every baseline row is a constant-prediction set")
print(
    f"{excluded.height} of {excluded.height + baseline.height} baseline rows excluded as "
    f"constant-prediction sets, leaving {baseline.height}"
)
excluded.select("label", "family", "config_name", "prediction_hash", "sharpe", "max_drawdown")

# %% [markdown]
# ## 4. The shortlist, and what it costs
#
# One row per distinct model configuration, taken on baseline Sharpe. Without it every allocator
# would be applied to every member of the whole model population, multiplying an already large grid
# by the number of allocators.
#
# **What that makes invisible is worth stating plainly.** The allocators are compared only on
# strategies the equal-weight rule already ranked highly. An allocator whose value is precisely
# that it rescues a model equal weight buried cannot be discovered by this design, and no result
# below is evidence against one existing.

# %% tags=["results"]
if TOP_N_PREDICTIONS is None:
    TOP_N_PREDICTIONS = get_top_n_predictions(CASE_STUDY_ID, "allocation")
top_n = TOP_N_PREDICTIONS
label_cap = top_n_cap(top_n)
checkpoints_per_config = get_checkpoints_per_config(CASE_STUDY_ID)
if checkpoints_per_config != 1:
    raise ValueError(
        "backtest.sweep.checkpoints_per_config is "
        f"{checkpoints_per_config}; this notebook advances one checkpoint per "
        "model configuration"
    )
shortlist_parts = []
for label in baseline.get_column("label").unique().sort().to_list():
    ranked = baseline.filter(pl.col("label") == label).sort(
        "sharpe", "backtest_hash", descending=[True, False]
    )
    per_label = ranked.unique(
        subset=["family", "config_name"],
        keep="first",
        maintain_order=True,
    )
    # `top_n` of 0 asks for every configuration, as `top_n_predictions.signal` does in this
    # setup.yaml. Passed straight to `.head` it means the opposite, and the empty shortlist
    # then failed below as "the equal-weight baseline produced no allocation survivors",
    # blaming the baseline for a width the caller declared.
    if label_cap is not None:
        per_label = per_label.head(label_cap)
    shortlist_parts.append(per_label)
shortlist = pl.concat(shortlist_parts).sort("label", "sharpe", descending=[False, True])
if shortlist.is_empty():
    raise RuntimeError("The equal-weight baseline produced no allocation survivors")

shortlist.select(
    "label",
    "family",
    "config_name",
    "checkpoint_kind",
    "checkpoint_value",
    "prediction_hash",
    "backtest_hash",
    "sharpe",
)

# %% [markdown]
# ## 5. Planning one backtest per allocator
#
# Each surviving configuration crossed with each declared allocator, every identity written down
# before the first runs.
#
# **The history each allocator needs is declared per allocator, not shared.** The methods that read
# a covariance matrix cannot decide anything until they have enough bars to estimate one, and the
# amount differs between them - the mean-variance method here declares a longer window than the
# others because shrinkage on a matrix estimated from too few observations pulls it all the way to
# its target and hands back something close to equal weight under a different name. Each allocator
# therefore declares the history it needs, and is measured on that.
#
# `SUPERSEDES_POPULATION` and `SUPERSEDES_SETS` name the generation this run replaces. A population
# and a candidate set are both immutable, so a re-run that admits different members has to say
# which snapshot it supersedes or the registry refuses the write. Both default to empty, which is
# right for a first run and for a reader's clean clone; `population_supersedes` and
# `candidate_set_supersedes` withhold a declared hash wherever offering it would be refused.

# %% [markdown]
# **Prices are cached by label and warmup, not once per label.** Each allocator needs a different
# amount of history before it can decide anything - none for the ones that read only the
# predictions, a volatility window for the per-stock ones, a longer lookback for the ones that
# estimate a covariance matrix - and the price frame a member was handed is digested into that
# member's identity. So the frame has to be the one that member's own warmup implies, and the
# cache key is what keeps it that way while still loading each distinct frame once.

# %%
_price_cache: dict[tuple[str, int], object] = {}


def prices_for(label, warmup_periods):
    key = (str(label), int(warmup_periods))
    if key not in _price_cache:
        _price_cache[key] = load_backtest_prices_for(
            CASE_STUDY_ID,
            label,
            split="validation",
            max_symbols=MAX_SYMBOLS,
            warmup_periods=int(warmup_periods),
        )
    return _price_cache[key]


allocators = [
    config for config in get_allocators(CASE_STUDY_ID) if config["method"] != "equal_weight"
]
if EXECUTION_TIER == "preview":
    allocators = allocators[:PREVIEW_MAX_ALLOCATORS]
if not allocators or any(config["method"] == "equal_weight" for config in allocators):
    raise ValueError("Allocation requires at least one non-baseline sizing method")

prediction_catalog = study.predictions.table(include_preview=True)
backtest_config = get_backtest_config(CASE_STUDY_ID)
planned_requests = []
plan_rows = []


# %%
def plan_allocation_member(label, prices, allocation, baseline_row):
    selected_prediction = prediction_catalog.filter(
        pl.col("prediction_hash") == baseline_row["prediction_hash"]
    )
    if selected_prediction.height != 1:
        raise ValueError("A baseline survivor must resolve one prediction catalog row")
    baseline_spec = json.loads(baseline_row["spec_json"])
    signal = dict(baseline_spec["strategy"]["signal"])
    plan = plan_backtests(
        study,
        predictions=selected_prediction,
        signal=signal,
        allocation=allocation,
        prices=prices,
        chapter="ch17",
    )
    if len(plan.members) != 1:
        raise RuntimeError("One allocation request must plan one backtest")
    expected_hash = plan.expected_hashes[0]
    request = {
        "label": label,
        "selection": selected_prediction,
        "signal": signal,
        "allocation": allocation,
        "prediction_hash": baseline_row["prediction_hash"],
        "expected_hash": expected_hash,
    }
    row = {
        "label": label,
        "family": baseline_row["family"],
        "config_name": baseline_row["config_name"],
        "checkpoint_kind": baseline_row["checkpoint_kind"],
        "checkpoint_value": baseline_row["checkpoint_value"],
        "allocation": allocation["method"],
        "prediction_hash": baseline_row["prediction_hash"],
        "backtest_hash": expected_hash,
    }
    return request, row


# %%
for label in shortlist.get_column("label").unique().sort().to_list():
    for baseline_row in shortlist.filter(pl.col("label") == label).iter_rows(named=True):
        for allocation in allocators:
            prices = prices_for(
                label, strategy_warmup_periods({"strategy": {"allocation": allocation}})
            )
            request, row = plan_allocation_member(label, prices, allocation, baseline_row)
            planned_requests.append(request)
            plan_rows.append(row)

# %%
planned_population = pl.DataFrame(plan_rows).sort(
    "label", "family", "config_name", "checkpoint_value", "allocation", "backtest_hash"
)
if planned_population.get_column("backtest_hash").n_unique() != planned_population.height:
    raise ValueError("The allocation plan contains duplicate backtest identities")

official_population = None
if EXECUTION_TIER == "canonical":
    population_name = POPULATION_NAME or "us-equities-allocation-v1"
    official_population = OfficialPopulation.create(
        study,
        name=population_name,
        supersedes=population_supersedes(
            study, name=population_name, declared=SUPERSEDES_POPULATION
        ),
        member_kind="backtest",
        members=tuple(planned_population.get_column("backtest_hash")),
    )

planned_population

# %% [markdown]
# ## 6. Running them
#
# Independent per member, so a failure costs that allocator on that configuration and leaves the
# rest usable.

# %%
execution_rows = []
failure_rows = []


def execute_allocation_member(prices, request):
    execution = run_backtests(
        study,
        predictions=request["selection"],
        signal=request["signal"],
        allocation=request["allocation"],
        prices=prices,
        chapter="ch17",
    )
    if len(execution.results) != 1 or execution.results[0].hash != request["expected_hash"]:
        raise RuntimeError("Allocation execution changed its planned identity")
    return {
        "label": request["label"],
        "prediction_hash": request["prediction_hash"],
        "allocation": request["allocation"]["method"],
        "backtest_hash": execution.results[0].hash,
        "status": execution.diagnostics[0]["status"],
    }


# %% tags=["results"]
for label in shortlist.get_column("label").unique().sort().to_list():
    for request in (item for item in planned_requests if item["label"] == label):
        try:
            prices = prices_for(
                label,
                strategy_warmup_periods({"strategy": {"allocation": request["allocation"]}}),
            )
            execution_rows.append(execute_allocation_member(prices, request))
        except Exception as error:
            failure_rows.append(
                {
                    "label": label,
                    "prediction_hash": request["prediction_hash"],
                    "allocation": request["allocation"]["method"],
                    "backtest_hash": request["expected_hash"],
                    "error_type": type(error).__name__,
                    "error": str(error),
                }
            )

# %% tags=["results"]
execution_diagnostics = pl.DataFrame(
    execution_rows,
    schema={
        "label": pl.String,
        "prediction_hash": pl.String,
        "allocation": pl.String,
        "backtest_hash": pl.String,
        "status": pl.String,
    },
)
failures = pl.DataFrame(
    failure_rows,
    schema={
        "label": pl.String,
        "prediction_hash": pl.String,
        "allocation": pl.String,
        "backtest_hash": pl.String,
        "error_type": pl.String,
        "error": pl.String,
    },
)
if not failures.is_empty():
    raise RuntimeError(f"Allocation population has {failures.height} unsuccessful members")

if official_population is not None:
    official_population.require_complete()

execution_diagnostics

# %% [markdown]
# ## 7. Naming the allocation sets
#
# One frozen set per label, published only by an unnarrowed canonical run, for the reason
# [`16_backtest`](16_backtest.ipynb) gives.
#
# **The freeze is also the comparability check.** Nothing is declared comparable, so
# `CandidateSet.create` requires every field of the protocol to be identical across the members:
# two rows that measured their Sharpe on different folds are not two rankings of one thing, and
# this is what refuses to freeze them together.

# %% tags=["results"]
set_rows = []
completed = study.backtests.table(include_preview=True).filter(
    pl.col("backtest_hash").is_in(planned_population.get_column("backtest_hash"))
)
if (
    completed.height != planned_population.height
    or completed.filter(~pl.col("complete")).height
    or completed.filter(pl.col("stage") != "allocation").height
    or completed.filter(pl.col("execution_tier") != EXECUTION_TIER).height
    or completed.filter(pl.col("sharpe").is_null() | ~pl.col("sharpe").is_finite()).height
):
    raise RuntimeError("The allocation catalog is incomplete or mis-staged")
if EXECUTION_TIER == "canonical":
    for label in completed.get_column("label").unique().sort().to_list():
        label_name = label.replace("_", "-")
        result_set_name = f"us-equities-{label_name}-allocation-v1"
        result_set = study.backtests.freeze(
            completed.filter(pl.col("label") == label),
            name=result_set_name,
            supersedes=candidate_set_supersedes(
                study, name=result_set_name, declared=SUPERSEDES_SETS.get(result_set_name, "")
            ),
        )
        set_rows.append(
            {"label": label, "set_name": result_set.name, "members": len(result_set.members)}
        )

compatible_sets = pl.DataFrame(
    set_rows,
    schema={"label": pl.String, "set_name": pl.String, "members": pl.Int64},
)
compatible_sets

# %% [markdown]
# ## 8. What came out
#
# Each allocator against the equal-weight row it was built from. The comparison is like-for-like:
# same model, same checkpoint, same names, same dates, different money.
#
# **A small difference is a result.** Equal weight is a strong baseline on a broad cross-section
# precisely because it makes no estimate that can be wrong, and an allocator that reads a
# covariance matrix has to estimate one well enough to beat that. Where the differences are small,
# what that says is that the estimation was not worth its error here - not that sizing does not
# matter.
#
# **Still gross of costs.** The allocators differ in how much they trade, and turnover is charged
# in [`19_costs`](19_costs.ipynb), so an allocator that looks better here may not survive it.

# %% tags=["results"]
allocation_results = planned_population.select("label", "allocation", "backtest_hash").join(
    completed.select("backtest_hash", "sharpe"),
    on="backtest_hash",
    how="inner",
    validate="1:1",
)
if allocation_results.height != planned_population.height:
    raise RuntimeError("The plotted allocation population differs from the planned population")

fig, ax = plt.subplots(figsize=(10, 5))
allocator_order = allocation_results.get_column("allocation").unique().sort().to_list()
labels = allocation_results.get_column("label").unique().sort().to_list()
# `ml4t_palette` returns a list of that many colours, so it is called once and indexed.
palette = ml4t_palette(len(labels), categorical=True)
for index, label in enumerate(labels):
    label_rows = allocation_results.filter(pl.col("label") == label)
    positions = [allocator_order.index(name) for name in label_rows.get_column("allocation")]
    # A small fixed offset per label so three points on one allocator stay countable rather than
    # landing on top of each other; the horizontal position carries no meaning of its own.
    offset = (index - (len(labels) - 1) / 2) * 0.14
    ax.scatter(
        [position + offset for position in positions],
        label_rows["sharpe"],
        alpha=0.6,
        s=22,
        color=palette[index],
        edgecolors="none",
        label=label,
    )
zero_line(ax)
ax.set_xticks(range(len(allocator_order)), allocator_order, rotation=25, ha="right")
ax.set_xlim(-0.5, len(allocator_order) - 0.5)
ax.set_ylabel("Validation Sharpe")
add_message_title(
    ax,
    "Validation Sharpe by allocator, for the shortlisted configurations",
    subtitle="One point per shortlisted configuration and allocator, coloured by label",
)
ax.legend(fontsize=8, frameon=False)
# The alt text counts rather than asserts: how many allocators clear zero anywhere is a fact about
# the frame, and a panel described as beating the baseline when it does not is a claim the data
# refutes.
_above = allocation_results.group_by("allocation").agg(best=pl.col("sharpe").max())
_n_positive = int((_above.get_column("best") > 0).sum())
show_with_alt(
    fig,
    "A scatter plot with one column per allocator and a dashed line at zero. Each point is one "
    "shortlisted configuration re-sized by that allocator, placed at its validation Sharpe, with "
    "the three labels offset slightly from one another and coloured separately. Counted from the "
    f"underlying frame, {_n_positive} of {_above.height} allocators reach a positive Sharpe on at "
    "least one configuration.",
)

# %% [markdown]
# ## What to notice
#
# **Every row here differs from its baseline in exactly one thing.** Same model, same checkpoint,
# same names on the same dates, different money. That is what makes a difference attributable to
# the sizing rule.
#
# **Equal weight is hard to beat on a broad cross-section, and the reason is estimation.** The
# allocators that read a covariance matrix have to estimate one from a finite window, and a
# three-thousand-name cross-section gives far fewer observations per parameter than a small
# universe does. An allocator that does not beat equal weight here has not shown that sizing is
# irrelevant; it has shown that the estimate it needed was not accurate enough to pay for itself.
#
# **The shortlist bounds what this can find.** Allocators are compared only on strategies equal
# weight already ranked highly, so nothing here can discover one whose value is rescuing a model
# equal weight buried.
#
# **Still gross of costs, and the allocators differ in turnover.** A rule that reweights more
# aggressively trades more, so an ordering established here can change once
# [`19_costs`](19_costs.ipynb) charges for it.
#
# **Known limitations.** The covariance-reading allocators are sensitive to their lookback, and one
# window per allocator is declared rather than swept, so nothing here separates an allocator's
# method from its window. Validation folds have been read many times over by this point.
#
# **Next**: [`18_risk_management`](18_risk_management.ipynb) lays rules on top that can close a
# position before the next rebalance.

```