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4.4.3 Differ Objects

Note that `Differ`-generated deltas make no claim to be minimal diffs. To the contrary, minimal diffs are often counter-intuitive, because they synch up anywhere possible, sometimes accidental matches 100 pages apart. Restricting synch points to contiguous matches preserves some notion of locality, at the occasional cost of producing a longer diff.

The `Differ` class has this constructor:

class `Differ`([linejunk[, charjunk]])
  Optional keyword parameters linejunk and charjunk are for filter functions (or `None`):

  linejunk: A function that should accept a single string argument, and return true if the string is junk. The default is module-level function `IS_LINE_JUNK()`, which filters out lines without visible characters, except for at most one pound character ("`#`").

  charjunk: A function that should accept a string of length 1. The default is module-level function `IS_CHARACTER_JUNK()`, which filters out whitespace characters (a blank or tab; note: bad idea to include newline in this!).

`Differ` objects are used (deltas generated) via a single method:

`compare`(a, b)
  Compare two sequences of lines, and generate the delta (a sequence of lines).

  Each sequence must contain individual single-line strings ending with newlines. Such sequences can be obtained from the `readlines()` method of file-like objects. The delta generated also consists of newline-terminated strings, ready to be printed as-is via the `writelines()` method of a file-like object.