How do i check if two strings are similar in python?

How do I get the probability of a string being similar to another string in Python?

I want to get a decimal value like 0.9 (meaning 90%) etc. Preferably with standard Python and library.

e.g.

similar("Apple","Appel") #would have a high prob.

similar("Apple","Mango") #would have a lower prob.

smci

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asked Jun 30, 2013 at 7:35

5

There is a built in.

from difflib import SequenceMatcher

def similar(a, b):
    return SequenceMatcher(None, a, b).ratio()

Using it:

>>> similar("Apple","Appel")
0.8
>>> similar("Apple","Mango")
0.0

answered Jun 30, 2013 at 8:18

Inbar RoseInbar Rose

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5

Solution #1: Python builtin

use SequenceMatcher from difflib

pros: native python library, no need extra package.
cons: too limited, there are so many other good algorithms for string similarity out there.

example :

>>> from difflib import SequenceMatcher
>>> s = SequenceMatcher(None, "abcd", "bcde")
>>> s.ratio()
0.75

Solution #2: jellyfish library

its a very good library with good coverage and few issues. it supports:
- Levenshtein Distance
- Damerau-Levenshtein Distance
- Jaro Distance
- Jaro-Winkler Distance
- Match Rating Approach Comparison
- Hamming Distance

pros: easy to use, gamut of supported algorithms, tested.
cons: not native library.

example:

>>> import jellyfish
>>> jellyfish.levenshtein_distance(u'jellyfish', u'smellyfish')
2
>>> jellyfish.jaro_distance(u'jellyfish', u'smellyfish')
0.89629629629629637
>>> jellyfish.damerau_levenshtein_distance(u'jellyfish', u'jellyfihs')
1

answered Sep 8, 2017 at 22:49

Iman MirzadehIman Mirzadeh

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1

TheFuzz is a package that implements Levenshtein distance in python, with some helper functions to help in certain situations where you may want two distinct strings to be considered identical. For example:

>>> fuzz.ratio("fuzzy wuzzy was a bear", "wuzzy fuzzy was a bear")
    91
>>> fuzz.token_sort_ratio("fuzzy wuzzy was a bear", "wuzzy fuzzy was a bear")
    100

answered Jan 18, 2017 at 22:26

BLTBLT

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3

You can create a function like:

def similar(w1, w2):
    w1 = w1 + ' ' * (len(w2) - len(w1))
    w2 = w2 + ' ' * (len(w1) - len(w2))
    return sum(1 if i == j else 0 for i, j in zip(w1, w2)) / float(len(w1))

answered Jun 30, 2013 at 7:41

3

Package distance includes Levenshtein distance:

import distance
distance.levenshtein("lenvestein", "levenshtein")
# 3

answered Apr 10, 2017 at 22:02

Note, difflib.SequenceMatcher only finds the longest contiguous matching subsequence, this is often not what is desired, for example:

>>> a1 = "Apple"
>>> a2 = "Appel"
>>> a1 *= 50
>>> a2 *= 50
>>> SequenceMatcher(None, a1, a2).ratio()
0.012  # very low
>>> SequenceMatcher(None, a1, a2).get_matching_blocks()
[Match(a=0, b=0, size=3), Match(a=250, b=250, size=0)]  # only the first block is recorded

Finding the similarity between two strings is closely related to the concept of pairwise sequence alignment in bioinformatics. There are many dedicated libraries for this including biopython. This example implements the Needleman Wunsch algorithm:

>>> from Bio.Align import PairwiseAligner
>>> aligner = PairwiseAligner()
>>> aligner.score(a1, a2)
200.0
>>> aligner.algorithm
'Needleman-Wunsch'

Using biopython or another bioinformatics package is more flexible than any part of the python standard library since many different scoring schemes and algorithms are available. Also, you can actually get the matching sequences to visualise what is happening:

>>> alignment = next(aligner.align(a1, a2))
>>> alignment.score
200.0
>>> print(alignment)
Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-Apple-
|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-|||-|-
App-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-elApp-el

answered Dec 4, 2019 at 9:36

Chris_RandsChris_Rands

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The builtin SequenceMatcher is very slow on large input, here's how it can be done with diff-match-patch:

from diff_match_patch import diff_match_patch

def compute_similarity_and_diff(text1, text2):
    dmp = diff_match_patch()
    dmp.Diff_Timeout = 0.0
    diff = dmp.diff_main(text1, text2, False)

    # similarity
    common_text = sum([len(txt) for op, txt in diff if op == 0])
    text_length = max(len(text1), len(text2))
    sim = common_text / text_length

    return sim, diff

answered Apr 30, 2018 at 14:24

damiodamio

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You can find most of the text similarity methods and how they are calculated under this link: https://github.com/luozhouyang/python-string-similarity#python-string-similarity Here some examples;

• Normalized, metric, similarity and distance

• (Normalized) similarity and distance

• Metric distances

• Shingles (n-gram) based similarity and distance
• Levenshtein
• Normalized Levenshtein
• Weighted Levenshtein
• Damerau-Levenshtein
• Optimal String Alignment
• Jaro-Winkler
• Longest Common Subsequence
• Metric Longest Common Subsequence
• N-Gram
• Shingle(n-gram) based algorithms
• Q-Gram
• Cosine similarity
• Jaccard index
• Sorensen-Dice coefficient
• Overlap coefficient (i.e.,Szymkiewicz-Simpson)

answered Apr 9, 2020 at 14:38

MikeMike

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BLEUscore

BLEU, or the Bilingual Evaluation Understudy, is a score for comparing a candidate translation of text to one or more reference translations.

A perfect match results in a score of 1.0, whereas a perfect mismatch results in a score of 0.0.

Although developed for translation, it can be used to evaluate text generated for a suite of natural language processing tasks.

Code:

import nltk
from nltk.translate import bleu
from nltk.translate.bleu_score import SmoothingFunction
smoothie = SmoothingFunction().method4

C1='Text'
C2='Best'

print('BLEUscore:',bleu([C1], C2, smoothing_function=smoothie))

Examples: By updating C1 and C2.

C1='Test' C2='Test'

BLEUscore: 1.0

C1='Test' C2='Best'

BLEUscore: 0.2326589746035907

C1='Test' C2='Text'

BLEUscore: 0.2866227639866161

You can also compare sentence similarity:

C1='It is tough.' C2='It is rough.'

BLEUscore: 0.7348889200874658

C1='It is tough.' C2='It is tough.'

BLEUscore: 1.0

answered Feb 15, 2021 at 11:53

Reema Q KhanReema Q Khan

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There are many metrics to define similarity and distance between strings as mentioned above. I will give my 5 cents by showing an example of Jaccard similarity with Q-Grams and an example with edit distance.

The libraries

from nltk.metrics.distance import jaccard_distance
from nltk.util import ngrams
from nltk.metrics.distance  import edit_distance

Jaccard Similarity

1-jaccard_distance(set(ngrams('Apple', 2)), set(ngrams('Appel', 2)))

and we get:

0.33333333333333337

And for the Apple and Mango

1-jaccard_distance(set(ngrams('Apple', 2)), set(ngrams('Mango', 2)))

and we get:

0.0

Edit Distance

edit_distance('Apple', 'Appel')

and we get:

2

And finally,

edit_distance('Apple', 'Mango')

and we get:

5

Cosine Similarity on Q-Grams (q=2)

Another solution is to work with the textdistance library. I will provide an example of Cosine Similarity

import textdistance
1-textdistance.Cosine(qval=2).distance('Apple', 'Appel')

and we get:

0.5

answered Sep 10, 2020 at 22:48

George PipisGeorge Pipis

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Textdistance:

TextDistance – python library for comparing distance between two or more sequences by many algorithms. It has Textdistance

• 30+ algorithms
• Pure python implementation
• Simple usage
• More than two sequences comparing
• Some algorithms have more than one implementation in one class.
• Optional numpy usage for maximum speed.

Example1:

import textdistance
textdistance.hamming('test', 'text')

Output:

1

Example2:

import textdistance

textdistance.hamming.normalized_similarity('test', 'text')

Output:

0.75

Thanks and Cheers!!!

answered Oct 19, 2020 at 19:38

DRVDRV

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Adding the Spacy NLP library also to the mix;

@profile
def main():
    str1= "Mar 31 09:08:41  The world is beautiful"
    str2= "Mar 31 19:08:42  Beautiful is the world"
    print("NLP Similarity=",nlp(str1).similarity(nlp(str2)))
    print("Diff lib similarity",SequenceMatcher(None, str1, str2).ratio()) 
    print("Jellyfish lib similarity",jellyfish.jaro_distance(str1, str2))

if __name__ == '__main__':

    #python3 -m spacy download en_core_web_sm
    #nlp = spacy.load("en_core_web_sm")
    nlp = spacy.load("en_core_web_md")
    main()

Run with Robert Kern's line_profiler

kernprof -l -v ./python/loganalysis/testspacy.py

NLP Similarity= 0.9999999821467294
Diff lib similarity 0.5897435897435898
Jellyfish lib similarity 0.8561253561253562

However the time's are revealing

Function: main at line 32

Line #      Hits         Time  Per Hit   % Time  Line Contents
==============================================================
    32                                           @profile
    33                                           def main():
    34         1          1.0      1.0      0.0      str1= "Mar 31 09:08:41  The world is beautiful"
    35         1          0.0      0.0      0.0      str2= "Mar 31 19:08:42  Beautiful is the world"
    36         1      43248.0  43248.0     99.1      print("NLP Similarity=",nlp(str1).similarity(nlp(str2)))
    37         1        375.0    375.0      0.9      print("Diff lib similarity",SequenceMatcher(None, str1, str2).ratio()) 
    38         1         30.0     30.0      0.1      print("Jellyfish lib similarity",jellyfish.jaro_distance(str1, str2))

answered Apr 21 at 8:05

Alex PunnenAlex Punnen

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Here's what i thought of:

import string

def match(a,b):
    a,b = a.lower(), b.lower()
    error = 0
    for i in string.ascii_lowercase:
            error += abs(a.count(i) - b.count(i))
    total = len(a) + len(b)
    return (total-error)/total

if __name__ == "__main__":
    print(match("pple inc", "Apple Inc."))

answered Dec 1, 2020 at 21:22

• Python3.6+=

• No Libuary Imported

• Works Well in most scenarios

In stack overflow, when you tries to add a tag or post a question, it bring up all relevant stuff. This is so convenient and is exactly the algorithm that I am looking for. Therefore, I coded a query set similarity filter.

def compare(qs, ip):
    al = 2
    v = 0
    for ii, letter in enumerate(ip):
        if letter == qs[ii]:
            v += al
        else:
            ac = 0
            for jj in range(al):
                if ii - jj < 0 or ii + jj > len(qs) - 1: 
                    break
                elif letter == qs[ii - jj] or letter == qs[ii + jj]:
                    ac += jj
                    break
            v += ac
    return v


def getSimilarQuerySet(queryset, inp, length):
    return [k for tt, (k, v) in enumerate(reversed(sorted({it: compare(it, inp) for it in queryset}.items(), key=lambda item: item[1])))][:length]
        


if __name__ == "__main__":
    print(compare('apple', 'mongo'))
    # 0
    print(compare('apple', 'apple'))
    # 10
    print(compare('apple', 'appel'))
    # 7
    print(compare('dude', 'ud'))
    # 1
    print(compare('dude', 'du'))
    # 4
    print(compare('dude', 'dud'))
    # 6

    print(compare('apple', 'mongo'))
    # 2
    print(compare('apple', 'appel'))
    # 8

    print(getSimilarQuerySet(
        [
            "java",
            "jquery",
            "javascript",
            "jude",
            "aja",
        ], 
        "ja",
        2,
    ))
    # ['javascript', 'java']

Explanation

• compare takes two string and returns a positive integer.
• you can edit the al allowed variable in compare, it indicates how large the range we need to search through. It works like this: two strings are iterated, if same character is find at same index, then accumulator will be added to a largest value. Then, we search in the index range of allowed, if matched, add to the accumulator based on how far the letter is. (the further, the smaller)
• length indicate how many items you want as result, that is most similar to input string.

answered Oct 29, 2021 at 14:06

WeiloryWeilory

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Can you use == to compare strings in Python?

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