3.3.10 使用difflib.unified_diff函数

可以使用本函数来生成统一的差异结果文本，只要拿到第一个文本，再通过这个差异文本进行操作，就可以得到第二个文本，从而减少储存两份文件，缩小储存空间。
例子：
#python3.4.3

import difflib

diff = difflib.unified_diff('one\ntwo\nthree\n'.splitlines(keepends=True),
'ore\ntwo\nthree\nemu\n'.splitlines(keepends=True))

for line in diff:
print(line)
结果输出如下：
--- 
 
+++ 
 
@@ -1,3 +1,4 @@
 
-one
 
+ore
 
 two
 
 three
 
+emu
通过上面的结果来看，@这行是说明两个文件差异的多少，以及总行数，后面前面有加号的表示相对第一个文件要添加，前面有减号的表示相对第一个文件要删除，如果什么都没有表示不用改变。有这个差异化文件，可以生成第一个文件，也可以生成第二个文件，因为生成第一个文件时，只要碰到减号就不要变化，空白的行直接保留，加号的行去掉；第二个文件时刚好相反。从而达到保存一个文件，就可以生成两个文件的目的。
 
 

3.3.11创建像diff的工具

有了文本比较库的功能，来实现一个文件差异比较工具就是很容易的事情。
例子：""" Command line interface to difflib.py providing diffs in four formats:

* ndiff: lists every line and highlights interline changes.
* context: highlights clusters of changes in a before/after format.
* unified: highlights clusters of changes in an inline format.
* html: generates side by side comparison with change highlights.

"""

import sys, os, time, difflib, optparse

def main():
# Configure the option parser
usage = "usage: %prog [options] fromfile tofile"
parser = optparse.OptionParser(usage)
parser.add_option("-c", action="store_true", default=False,
help='Produce a context format diff (default)')
parser.add_option("-u", action="store_true", default=False,
help='Produce a unified format diff')
hlp = 'Produce HTML side by side diff (can use -c and -l in conjunction)'
parser.add_option("-m", action="store_true", default=False, help=hlp)
parser.add_option("-n", action="store_true", default=False,
help='Produce a ndiff format diff')
parser.add_option("-l", "--lines", type="int", default=3,
help='Set number of context lines (default 3)')
(options, args) = parser.parse_args()

if len(args) == 0:
parser.print_help()
sys.exit(1)
if len(args) != 2:
parser.error("need to specify both a fromfile and tofile")

n = options.lines
fromfile, tofile = args # as specified in the usage string

# we're passing these as arguments to the diff function
fromdate = time.ctime(os.stat(fromfile).st_mtime)
todate = time.ctime(os.stat(tofile).st_mtime)
with open(fromfile) as fromf, open(tofile) as tof:
fromlines, tolines = list(fromf), list(tof)

if options.u:
diff = difflib.unified_diff(fromlines, tolines, fromfile, tofile,
fromdate, todate, n=n)
elif options.n:
diff = difflib.ndiff(fromlines, tolines)
elif options.m:
diff = difflib.HtmlDiff().make_file(fromlines, tolines, fromfile,
tofile, context=options.c,
numlines=n)
else:
diff = difflib.context_diff(fromlines, tolines, fromfile, tofile,
fromdate, todate, n=n)

# we're using writelines because diff is a generator
sys.stdout.writelines(diff)

if __name__ == '__main__':
main()
在这个例子里，可以使用不同的参数来选择产生比较后文本的结果方式，比如简单文本方式，也可以HTML方式，还可以HTML表格方式。

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