逐行比较两个文件

Question

I have this program that simply takes two files and compares them line by line. It works fine as long as both files have the same amount of lines. My problem would be what if for example file2 has more lines than file1? Or the other way around. I get the IndexError: list index out of range error when this happens. What can I do to take this into account?

#Compares two files
def compare(baseline, newestFile):



    baselineHolder = open(baseline)
    newestFileHolder = open(newestFile)



    lines1 = baselineHolder.readlines()
    a = returnName(baseline)
    b = returnName(newestFile)


    for i,lines2 in enumerate(newestFileHolder):
        if lines2 != lines1[i]:
            add1 = i + 1
            print ("line ", add1, " in newestFile is different \n")
            print("TAKE A LOOK HERE----------------------TAKE A LOOK HERE")
            print (lines2)
        else:
            addRow = 1 + i
            print ("line  " + str(addRow) + " is identical")

Answer 1

Instead of reinventing the wheel, why not use the built-in difflib ? Here is an example using difflib.unified_diff from the docs:

 >>> s1 = ['bacon\\n', 'eggs\\n', 'ham\\n', 'guido\\n'] >>> s2 = ['python\\n', 'eggy\\n', 'hamster\\n', 'guido\\n'] >>> for line in unified_diff(s1, s2, fromfile='before.py', tofile='after.py'): ... sys.stdout.write(line) --- before.py +++ after.py @@ -1,4 +1,4 @@ -bacon -eggs -ham +python +eggy +hamster guido 

Answer 2

Perhaps you can use itertools.izip_longest . If one sequence has been exhausted, it emits some fill value (by default, None ):

import itertools

for l, r in itertools.izip_longest(open('foo.txt'), open('bar.txt')):
    if l is None: # foo.txt has been exhausted
        ...
    elif r is None: # bar.txt has been exhausted
        ...
    else: # both still have lines - compare now the content of l and r
        ...

Edit As @danidee correctly notes, for Py3 it is zip_longest .

Answer 3

You should catch the IndexError and then stop your comparison

    for i,lines2 in enumerate(newestFileHolder):
        try:
            if lines2 != lines1[i]:
                add1 = i + 1
                print ("line ", add1, " in newestFile is different \n")
                print("TAKE A LOOK HERE----------------------TAKE A LOOK HERE")    
                print (lines2)
            else:
                addRow = 1 + i
                print ("line  " + str(addRow) + " is identical")
        except IndexError:
            print("Exit comparison")
            break