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diff --git a/resources/libraries/python/MLRsearch/target_spec.py b/resources/libraries/python/MLRsearch/target_spec.py
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+# Copyright (c) 2023 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Module defining TargetSpec class."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Optional
+
+from .discrete_width import DiscreteWidth
+
+
+@dataclass(frozen=True, eq=True)
+class TargetSpec:
+    """Composite object holding attributes specifying one search target.
+
+    Abstractly, this has several similar meanings.
+    With discrete_width attribute this specifies when a selector is Done.
+    With expansion_coefficient attribute it tells selector how quickly
+    should it expand interval in external search.
+    With "preceding" attribute it helps selector, so it does not need to point
+    to preceding target separately from its current target.
+    Without those three attributes this object is still sufficient
+    for LoadStats to classify loads as lower bound, upper bound, or unknown.
+    """
+
+    loss_ratio: float
+    """Target loss ratio. Equal and directly analogous to goal loss ratio,
+    but applicable also for non-final targets."""
+    exceed_ratio: float
+    """Target exceed ratio. Equal and directly analogous to goal exceed ratio,
+    but applicable also for non-final targets."""
+    discrete_width: DiscreteWidth
+    """Target relative width. Analogous to goal relative width,
+    but coarser for non-final targets."""
+    trial_duration: float
+    """Duration to use for trials for this target. Shorter trials have lesser
+    (and more complicated) impact when determining upper and lower bounds."""
+    duration_sum: float
+    """Sum of trial durations sufficient to classify a load
+    as an upper or lower bound.
+    For non-final targets, this is shorter than goal duration_sum."""
+    expansion_coefficient: int = field(repr=False)
+    """Equal and directly analogous to goal expansion coefficient,
+    but applicable also for non-final targets."""
+    fail_fast: bool = field(repr=False)
+    """Copied from goal. If true and min load is not an upper bound, raise."""
+    preceding: Optional[TargetSpec] = field(repr=False)
+    """Reference to next coarser target (if any) belonging to the same goal."""
+
+    # No conversions or validations, as this is an internal structure.
+
+    def __str__(self) -> str:
+        """Convert into a short human-readable string.
+
+        :returns: The short string.
+        :rtype: str
+        """
+        return (
+            f"lr={self.loss_ratio},er={self.exceed_ratio}"
+            f",ds={self.duration_sum}"
+        )
+
+    def with_preceding(self, preceding: Optional[TargetSpec]) -> TargetSpec:
+        """Create an equivalent instance but with different preceding field.
+
+        This is useful in initialization. Create semi-initialized targets
+        starting from final one, than add references in reversed order.
+
+        :param preceding: New value for preceding field, cannot be None.
+        :type preceding: Optional[TargetSpec]
+        :returns: Instance with the new value applied.
+        :rtype: TargetSpec
+        """
+        return TargetSpec(
+            loss_ratio=self.loss_ratio,
+            exceed_ratio=self.exceed_ratio,
+            discrete_width=self.discrete_width,
+            trial_duration=self.trial_duration,
+            duration_sum=self.duration_sum,
+            expansion_coefficient=self.expansion_coefficient,
+            fail_fast=self.fail_fast,
+            preceding=preceding,
+        )