CODEWITHSUNDEEP
pythonpython-2.xdifflib
ealeon
ealeon

Reputation: 12452

Python difflib: not detecting changes

import difflib

test1 = ")\n )"
test2 = "#)\n #)"

d = difflib.Differ()
diff = d.compare(test1.splitlines(), test2.splitlines())
print "\n".join(diff)

OUTPUT:

- )
+ #)
-  )
+  #)
?  +

as you can see, it didnt detect the change for the first line ( no ? line )but it did in the second line

Anyone know why difflib think its a delete/add, and not change?

Upvotes: 1

Views: 396

Answers (1)

alexanderlukanin13
alexanderlukanin13

Reputation: 4715

One-character string is an edge case. For two or more characters, insertion of one character is always detected correctly. Here is a simple algorithm to demonstrate that:

import difflib

def show_diffs(limit):
    characters = 'abcdefghijklmnopqrstuvwxyz'
    differ = difflib.Differ()
    for length in range(1, limit + 1):
        for pos in range(0, length + 1):
            line_a = characters[:length]
            line_b = line_a[:pos] + 'A' + line_a[pos:]
            diff = list(differ.compare([line_a], [line_b]))
            if len(diff) == 2 and diff[0][0] == '-' and diff[1][0] == '+':
                marker = 'N'  # Insertion not detected
            elif len(diff) == 3 and diff[0][0] == '-' and diff[1][0] == '+' and diff[2][0] == '?':
                marker = 'Y'  # Insertion detected
            else:
                print('ERROR: unexpected diff for %r -> %r:\n%r' % (line_a, line_b, diff))
                return
            print('%s %r -> %r' % (marker, line_a, line_b))

show_diffs(limit=3)

It "fails" only for 1-character strings:

N 'a' -> 'Aa'
N 'a' -> 'aA'
Y 'ab' -> 'Aab'
Y 'ab' -> 'aAb'
Y 'ab' -> 'abA'
Y 'abc' -> 'Aabc'
Y 'abc' -> 'aAbc'
Y 'abc' -> 'abAc'
Y 'abc' -> 'abcA'

Upvotes: 2

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