CODEWITHSUNDEEP
pythondiff
microo8
microo8

Reputation: 3774

python diff SequenceMatcher - patching a list

i patch a list to look like another:

a = [x for x in "qabxcd"]
b = [x for x in "abycdf"]
c = a[:]
s = SequenceMatcher(None, a, b)
for tag, i1, i2, j1, j2 in s.get_opcodes():
    print ("%7s a[%d:%d] (%s) b[%d:%d] (%s)" % 
    (tag, i1, i2, a[i1:i2], j1, j2, b[j1:j2]))
    if tag == "delete":
        del c[i1:i2]
    elif tag == "replace":
        c[i1:i2] = b[j1-1:j2-1]
    elif tag == "insert":
        c[i1:i2] = b[j1:j2]
print c
print b
print c == b
a == b

but the list is not equal:

 delete a[0:1] (['q']) b[0:0] ([])
  equal a[1:3] (['a', 'b']) b[0:2] (['a', 'b'])
replace a[3:4] (['x']) b[2:3] (['y'])
  equal a[4:6] (['c', 'd']) b[3:5] (['c', 'd'])
 insert a[6:6] ([]) b[5:6] (['f'])
['a', 'b', 'x', 'b', 'd', 'f']
['a', 'b', 'y', 'c', 'd', 'f']
False

what is the problem?

Upvotes: 4

Views: 744

Answers (3)

djvg
djvg

Reputation: 14255

The OP's example can be simplified slightly by initializing c to an empty string. No need to handle the "delete" case then:

a = "qabxcd"
b = "abycdf"
c = ""  # <-- initialize to empty string

s = SequenceMatcher(None, a, b)
for tag, i1, i2, j1, j2 in s.get_opcodes():
    ...
    if tag == "equal":
        c += a[i1:i2]
    elif tag in ["replace", "insert"]:
        c += b[j1:j2]

print(c)
print(b)
assert b == c

Upvotes: 0

microo8
microo8

Reputation: 3774

All the action shift the indexes. When i will to do it, i must count the changes:

a = [x for x in "abyffgh fg99"]
b = [x for x in "999aby99ff9h9"]
c = a[:]

s = SequenceMatcher(None, a, b)

i = 0
for tag, i1, i2, j1, j2 in s.get_opcodes():
    print ("%7s a[%d:%d] (%s) b[%d:%d] (%s) c[%d:%d] (%s)" % 
    (tag, i1, i2, a[i1:i2], j1, j2, b[j1:j2], i1, i2, c[i1 + i:i2 + i]))
    if tag == "delete":
        del c[i1 + i:i2 + i]
        i -= i2 - i1
    elif tag == "replace":
        c[i1 + i:i2 + i] = b[j1:j2]
        i -= i2 - i1 - j2 + j1
    elif tag == "insert":
        c[i1 + i:i2 + i] = b[j1:j2]
        i += j2 - j1
    print c
    print i
print c
print b
print c == b
a == b

output:

['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', ' ', 'f', 'g', '9', '9']
5
 delete a[7:10] ([' ', 'f', 'g']) b[12:12] ([]) c[7:10] ([' ', 'f', 'g'])
['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', '9', '9']
1
  equal a[10:11] (['9']) b[12:13] (['9']) c[10:11] (['h'])
['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', '9', '9']
1
 delete a[11:12] (['9']) b[13:13] ([]) c[11:12] (['9'])
['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', '9']
-1
['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', '9']
['9', '9', '9', 'a', 'b', 'y', '9', '9', 'f', 'f', '9', 'h', '9']
True

Upvotes: 2

Emmanuel
Emmanuel

Reputation: 14209

I think I can see why: the 5-tuples returned by s.get_opcodes() are valid on the initial state of your containers, i.e. they have to be adapted if your object changes: that is the case of the delete operation notably, that changes the indexes (that's why 'x' is not turned into 'y').

As far as I could see, the delete operation is the only one changing the indexes, so I would replace the deleted items by a marker (I used '#') and remove it at the end:

>>> c = a[:]
>>> for tag, i1, i2, j1, j2 in s.get_opcodes():
    print ("%7s a[%d:%d] (%s) b[%d:%d] (%s)" % 
    (tag, i1, i2, a[i1:i2], j1, j2, b[j1:j2]))
    if tag == "delete":
        c[i1:i2] = ['#' for i in range(i1, i2)]
    elif tag == "replace":
        c[i1:i2] = b[j1:j2]
    elif tag == "insert":
        c[i1:i1] = b[j1:j2]
    print c


 delete a[0:1] (['q']) b[0:0] ([])
['#', 'a', 'b', 'x', 'c', 'd']
  equal a[1:3] (['a', 'b']) b[0:2] (['a', 'b'])
['#', 'a', 'b', 'x', 'c', 'd']
replace a[3:4] (['x']) b[2:3] (['y'])
['#', 'a', 'b', 'y', 'c', 'd']
  equal a[4:6] (['c', 'd']) b[3:5] (['c', 'd'])
['#', 'a', 'b', 'y', 'c', 'd']
 insert a[6:6] ([]) b[5:6] (['f'])
['#', 'a', 'b', 'y', 'c', 'd', 'f']
>>> c = [i for i in c if i != '#']
>>> c
['a', 'b', 'y', 'c', 'd', 'f']
>>>

Upvotes: 1

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