Comparators

Comparators are the algorithms that determine how similar two field values are. Each comparator is optimized for a different data type or comparison strategy, returning a similarity score between 0.0 (completely different) and 1.0 (identical). When you define a StructuredModel, you assign a comparator to each field so Stickler knows how to evaluate that field.

Which Comparator Should I Use?

Comparator	Best For	Speed	Needs AWS?	Score Type
ExactComparator	IDs, codes, booleans	Instant	No	Binary (0.0 or 1.0)
LevenshteinComparator	Names, addresses, text with typos	Instant	No	Continuous (0.0--1.0)
NumericComparator	Prices, quantities, measurements	Instant	No	Binary (0.0 or 1.0)
FuzzyComparator	Flexible text, descriptions, reordered tokens	Fast	No	Continuous (0.0--1.0)
SemanticComparator	Meaning-based text similarity	Moderate	Yes (Bedrock)	Continuous (0.0--1.0)
BERTComparator	Contextual semantic similarity	Moderate	No (runs locally)	Continuous (0.0--1.0)
LLMComparator	Complex semantic evaluation with reasoning	Slow	Yes (Bedrock)	Binary (0.0 or 1.0)

Comparator Details

ExactComparator

Checks for exact string matching after normalizing whitespace, punctuation, and (by default) case. Returns 1.0 for exact matches and 0.0 otherwise.

When to use: Critical identifiers, status codes, booleans, or any field where partial matches are meaningless.

from stickler import StructuredModel, ComparableField
from stickler.comparators import ExactComparator

class Order(StructuredModel):
    order_id: str = ComparableField(
        comparator=ExactComparator(),
        threshold=1.0,
        weight=3.0
    )

Key parameters:

Parameter	Default	Description
`threshold`	`1.0`	Similarity threshold for binary classification
`case_sensitive`	`False`	Whether comparison is case-sensitive

LevenshteinComparator

Calculates the Levenshtein edit distance between two strings and returns a normalized similarity score: 1.0 - (edit_distance / max_length). Optionally normalizes input by stripping whitespace and lowercasing.

When to use: Names, addresses, free-form text where typos and minor variations are expected. This is the default comparator for string fields.

from stickler import StructuredModel, ComparableField
from stickler.comparators import LevenshteinComparator

class Contact(StructuredModel):
    name: str = ComparableField(
        comparator=LevenshteinComparator(threshold=0.8),
        weight=1.5
    )

Key parameters:

Parameter	Default	Description
`threshold`	`0.7`	Similarity threshold for binary classification
`normalize`	`True`	Strip whitespace and lowercase before comparison

NumericComparator

Extracts numeric values from strings or numbers and compares them with configurable tolerance. Handles currency symbols, commas, and accounting notation (e.g., (123) for negative values). Returns 1.0 if the numbers match within tolerance, 0.0 otherwise.

When to use: Prices, quantities, measurements, or any numeric field where small differences are acceptable.

from stickler import StructuredModel, ComparableField
from stickler.comparators import NumericComparator

class Invoice(StructuredModel):
    amount: float = ComparableField(
        comparator=NumericComparator(relative_tolerance=0.05),
        weight=2.0
    )

Key parameters:

Parameter	Default	Description
`threshold`	`1.0`	Similarity threshold for binary classification
`relative_tolerance`	`0.0`	Relative tolerance (e.g., `0.1` = 10%)
`absolute_tolerance`	`0.0`	Absolute tolerance (e.g., `0.01` for cents)
`tolerance`	`None`	Alias for `absolute_tolerance` (backward compatibility)

FuzzyComparator

Uses the rapidfuzz library for advanced fuzzy string matching. Supports multiple matching methods including standard ratio, partial matching, and token-based matching that is order-independent.

When to use: Descriptions, product names, or text where word order may vary or partial matches are valuable.

from stickler import StructuredModel, ComparableField
from stickler.comparators import FuzzyComparator

class Product(StructuredModel):
    description: str = ComparableField(
        comparator=FuzzyComparator(method="token_sort_ratio"),
        threshold=0.7
    )

Key parameters:

Parameter	Default	Description
`threshold`	`0.7`	Similarity threshold for binary classification
`method`	`"ratio"`	Matching method: `"ratio"`, `"partial_ratio"`, `"token_sort_ratio"`, or `"token_set_ratio"`
`normalize`	`True`	Strip whitespace and lowercase before comparison

Matching methods explained:

ratio -- Standard Levenshtein distance ratio (similar to LevenshteinComparator but using rapidfuzz's optimized implementation).
partial_ratio -- Finds the best partial match within the longer string. Good when one value is a substring of the other.
token_sort_ratio -- Splits strings into tokens, sorts them, then compares. Handles reordered words (e.g., "John Smith" vs "Smith John").
token_set_ratio -- Splits into token sets, comparing the intersection and remainder. Handles extra or missing words.

Dependency

FuzzyComparator requires the rapidfuzz package. Install it with: pip install rapidfuzz

SemanticComparator

Uses AWS Bedrock Titan embeddings to generate vector representations of text, then computes cosine similarity. Captures meaning rather than surface-level string similarity.

When to use: Text fields where meaning matters more than exact wording. See LLM-as-a-Judge Comparators for a detailed guide.

from stickler import StructuredModel, ComparableField
from stickler.comparators import SemanticComparator

class Review(StructuredModel):
    summary: str = ComparableField(
        comparator=SemanticComparator(threshold=0.8),
        weight=1.0
    )

Key parameters:

Parameter	Default	Description
`threshold`	`0.7`	Similarity threshold for binary classification
`model_id`	`"amazon.titan-embed-text-v2:0"`	Bedrock embedding model ID
`sim_function`	`"cosine_similarity"`	Similarity function to use
`embedding_function`	`None`	Optional custom embedding function (bypasses Bedrock)

BERTComparator

Uses the BERTScore metric (via the evaluate library) to calculate contextual semantic similarity. Returns the F1 score component of BERTScore as the similarity measure. Runs entirely locally -- no API calls required.

When to use: Text fields where you need semantic understanding without cloud dependencies. See LLM-as-a-Judge Comparators for a detailed guide.

from stickler import StructuredModel, ComparableField
from stickler.comparators import BERTComparator

class Document(StructuredModel):
    summary: str = ComparableField(
        comparator=BERTComparator(threshold=0.85),
        weight=1.0
    )

Key parameters:

Parameter	Default	Description
`threshold`	`0.7`	Similarity threshold for binary classification

The default model is distilbert-base-uncased, loaded globally via the evaluate library.

LLMComparator

Uses a Large Language Model (via AWS Bedrock and the strands-agents library) to perform intelligent semantic comparisons. The LLM receives both values and optional evaluation guidelines, then returns a binary equivalence judgment. This is the most flexible comparator but also the most expensive.

When to use: Complex comparisons that require reasoning, domain-specific logic, or understanding of abbreviations and conventions. See LLM-as-a-Judge Comparators for a detailed guide.

from stickler import StructuredModel, ComparableField
from stickler.comparators import LLMComparator

class Address(StructuredModel):
    street: str = ComparableField(
        comparator=LLMComparator(
            model="us.amazon.nova-lite-v1:0",
            eval_guidelines="Consider street abbreviations equivalent (St=Street, Ave=Avenue)"
        ),
        threshold=0.8
    )

Key parameters:

Parameter	Default	Description
`model`	Required	Bedrock model ID string or a `strands.models.Model` instance
`eval_guidelines`	`None`	Custom guidelines for the LLM to follow during comparison

Dependency

LLMComparator requires the strands-agents package. Install it with: pip install stickler-eval[llm]

Default Comparators by Type

When you do not specify a comparator in ComparableField, Stickler assigns one based on the JSON schema type of the field:

JSON Schema Type	Default Comparator	Default Threshold	Rationale
`string`	LevenshteinComparator	0.5	Handles typos and minor variations
`number`	NumericComparator	0.5	Tolerates small numeric differences
`integer`	NumericComparator	0.5	Tolerates small numeric differences
`boolean`	ExactComparator	1.0	Must be exactly true or false
`array` (primitives)	Based on item type	Based on item type	Inherits from element type
`array` (objects)	Hungarian matching	0.7	Optimal pairing of list elements
`object`	Recursive comparison	0.7	Field-by-field nested comparison

Custom Comparators

You can create your own comparator by extending BaseComparator. The only requirement is implementing the compare method, which takes two values and returns a float between 0.0 and 1.0.

The BaseComparator Interface

from stickler.comparators.base import BaseComparator

class BaseComparator(ABC):
    def __init__(self, threshold: float = 0.7):
        self.threshold = threshold

    @abstractmethod
    def compare(self, str1: Any, str2: Any) -> float:
        """Compare two values and return a similarity score.

        Args:
            str1: First value
            str2: Second value

        Returns:
            Similarity score between 0.0 and 1.0
        """
        pass

BaseComparator also provides:

__call__ -- makes the comparator callable directly (delegates to compare).
binary_compare -- converts the continuous similarity score to a (tp, fp) tuple based on the threshold.

Example: Custom RegexComparator

import re
from typing import Any
from stickler.comparators.base import BaseComparator

class RegexComparator(BaseComparator):
    """Comparator that checks if a value matches a reference regex pattern."""

    def __init__(self, threshold: float = 1.0):
        super().__init__(threshold=threshold)

    def compare(self, pattern: Any, value: Any) -> float:
        if pattern is None or value is None:
            return 0.0
        try:
            return 1.0 if re.fullmatch(str(pattern), str(value)) else 0.0
        except re.error:
            return 0.0

Use it like any built-in comparator:

from stickler import StructuredModel, ComparableField

class PhoneRecord(StructuredModel):
    phone: str = ComparableField(
        comparator=RegexComparator(),
        threshold=1.0
    )

Next Steps

For a deep dive into the three AI-powered comparators (SemanticComparator, BERTComparator, and LLMComparator), see LLM-as-a-Judge Comparators.