langchain/docs/extras/guides/evaluation/string/string_distance.ipynb

String Distance

One of the simplest ways to compare an LLM or chain's string output against a reference label is by using string distance measurements such as Levenshtein or postfix distance. This can be used alongside approximate/fuzzy matching criteria for very basic unit testing.

This can be accessed using the string_distance evaluator, which uses distance metric's from the rapidfuzz library.

Note: The returned scores are distances, meaning lower is typically "better".

For more information, check out the reference docs for the StringDistanceEvalChain for more info.

In [1]:

# %pip install rapidfuzz

In [2]:

from langchain.evaluation import load_evaluator

evaluator = load_evaluator("string_distance")

In [3]:

evaluator.evaluate_strings(
    prediction="The job is completely done.",
    reference="The job is done",
)

Out [3]:

{'score': 0.11555555555555552}

In [4]:

# The results purely character-based, so it's less useful when negation is concerned
evaluator.evaluate_strings(
    prediction="The job is done.",
    reference="The job isn't done",
)

Out [4]:

{'score': 0.0724999999999999}

Configure the String Distance Metric

By default, the StringDistanceEvalChain uses levenshtein distance, but it also supports other string distance algorithms. Configure using the distance argument.

In [5]:

from langchain.evaluation import StringDistance

list(StringDistance)

Out [5]:

[<StringDistance.DAMERAU_LEVENSHTEIN: 'damerau_levenshtein'>,
 <StringDistance.LEVENSHTEIN: 'levenshtein'>,
 <StringDistance.JARO: 'jaro'>,
 <StringDistance.JARO_WINKLER: 'jaro_winkler'>]

In [6]:

jaro_evaluator = load_evaluator(
    "string_distance", distance=StringDistance.JARO
)

In [7]:

jaro_evaluator.evaluate_strings(
    prediction="The job is completely done.",
    reference="The job is done",
)

Out [7]:

{'score': 0.19259259259259254}

In [8]:

jaro_evaluator.evaluate_strings(
    prediction="The job is done.",
    reference="The job isn't done",
)

Out [8]:

{'score': 0.12083333333333324}