Difflib.ndiff (Problem on identification of changes as Diff Style)

[vincenzogisondi]

BPO	27913

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-08-31.14:43:22.322>
created_at = <Date 2016-08-31.14:37:05.391>
labels = ['invalid', 'type-bug', 'library']
title = 'Difflib.ndiff (Problem on identification of changes as Diff Style)'
updated_at = <Date 2016-08-31.15:04:57.491>
user = 'https://bugs.python.org/vincenzogisondi'

bugs.python.org fields:

activity = <Date 2016-08-31.15:04:57.491>
actor = 'vincenzo gisondi'
assignee = 'none'
closed = True
closed_date = <Date 2016-08-31.14:43:22.322>
closer = 'SilentGhost'
components = ['Library (Lib)']
creation = <Date 2016-08-31.14:37:05.391>
creator = 'vincenzo gisondi'
dependencies = []
files = []
hgrepos = []
issue_num = 27913
keywords = []
message_count = 3.0
messages = ['274023', '274025', '274027']
nosy_count = 2.0
nosy_names = ['SilentGhost', 'vincenzo gisondi']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27913'
versions = ['Python 2.7']

[vincenzogisondi]

I found an anomaly on identification of changes as Diff Style (^).

There are the tests that i done:

1. First Test (only one character is different)

>>> a = "Hello Vincenzo\n".splitlines(1)
>>> b = "Hello Vincenza\n".splitlines(1)
>>> a
['Hello Vincenzo\n']
>>> b
['Hello Vincenza\n']
>>> diff = difflib.ndiff(a,b)
>>> print(''.join(diff))
- Hello Vincenzo
?              ^
+ Hello Vincenza
?              ^

2. Second Test (two characters are differents)

>>> a = "Hello Vincenzo\n".splitlines(1)
>>> b = "Hello Vincensa\n".splitlines(1)
>>> a
['Hello Vincenzo\n']
>>> b
['Hello Vincensa\n']
>>> diff = difflib.ndiff(a,b)
>>> print(''.join(diff))
- Hello Vincenzo
?             ^^
+ Hello Vincensa
?             ^^

3. Third Test (three characters are differents)

>>> a = "Hello Vincenzo\n".splitlines(1)
>>> b = "Helto Bincenza\n".splitlines(1)
>>> a
['Hello Vincenzo\n']
>>> b
['Helto Bincenza\n']
>>> diff = difflib.ndiff(a,b)
>>> print(''.join(diff))
- Hello Vincenzo
?    ^  ^      ^
+ Helto Bincenza
?    ^  ^      ^

4. Fourth test (four characters are differents -> Anomaly)

>>> a = "Hello Vincenzo\n".splitlines(1)
>>> b = "Halto Bincenza\n".splitlines(1)
>>> a
['Hello Vincenzo\n']
>>> b
['Halto Bincenza\n']
>>> diff = difflib.ndiff(a,b)
>>> print(''.join(diff))
- Hello Vincenzo
+ Halto Bincenza

In this last test I expected 4 "^" characters as in previous tests, like this:

• Hello Vincenzo
  ? ^ ^ ^ ^
  + Halto Bincenza
  ? ^ ^ ^ ^

but I have a response completly different. This is a Bug or I did not understand something :)

Thank you very much for your support.

[SilentGhost]

Since there are more than a quarter of all characters changed between two sequences, they are considered sufficiently different and no by-character comparison is shown.

[vincenzogisondi]

Ok now it is all clear.
Thank you very much, for your rapid and clear answer.