在python中的文件的开头和结尾添加行

Question

Good, I have the following code:

numList = [6, 7, 8, 10, 15, 18, 31, 35, 51, 54]

with open('/home/user/test.nft') as f:
    lines = f.readlines()

for i, l in enumerate(lines): 
    for num in numList:
        if l.startswith('add rule ip filter vlan_%d' %num):
            if "oifname bond1.%d" %num in l:
                f = open('inet-filter-chain-vlan%d_in.nft' %num, 'a')
            else:
                f = open('inet-filter-chain-vlan%d_out.nft' %num, 'a')

            f.write(l)
            f.close()
            break

I want to add a line at the beginning and end of the generated files in the if: inet-filter-chain-vlan% d_in.nft and inet-filter-chain-vlan% d_out.nft.

For example, the contents of the inet-filter-chain-vlan20_in.nft file should be:

Custom line for file 20

......content...........

Custom line for file 20

Answer 1

You need to cache which files you are writing to.

This can be done in a simple manner with a set,

Why not use with open(...) as f: again too?

fileset = set()
for i, l in enumerate(lines): 
    for num in numList:
        if l.startswith('add rule ip filter vlan_%d' %num):
            in_out = 'in' if "oifname bond1.%d" %num in l else 'out'
            filename = 'inet-filter-chain-vlan%d_%s.nft' % (num, in_out)
            with open(filename, 'a') as f:
                # the first time we open a file, output the custom line
                if filename not in fileset:
                    custom_string="Custom line for %s\n" % filename
                    f.write(custom_string)
                    fileset.add(filename)
                f.write(l)
            break
# now we need to write the final line in each file
for filename in fileset:
    with open(filename, 'a') as f:
        custom_string="Custom line for %s\n" % filename
        f.write(custom_string)

There are other ways to do this, but this is simple and doesn't leave the files open, which has an overhead in opening and closing the files often (for each line) but doesn't potentially open many files.

If you want to keep the files open for write performace, I suggest using bare opens, use a dict (keyed by filename) to store the file references, then close them after writing the final line. It should look something like:

filedict = {}
for i, l in enumerate(lines): 
    for num in numList:
        if l.startswith('add rule ip filter vlan_%d' %num):
            in_out = 'in' if "oifname bond1.%d" %num in l else 'out'
            filename = 'inet-filter-chain-vlan%d_%s.nft' % (num, in_out)
                # the first time we open a file, output the custom line
                f = filedict.get(filename)
                if f is None:
                    custom_string="Custom line for %s\n" % filename
                    f = open(filename, 'a')
                    f.write(custom_string)
                    filedict[filename] = f
                f.write(l)
            break
# now we need to write the final line in each file
for filename, f in filedict:
    custom_string="Custom line for %s\n" % filename
    f.write(custom_string)
    f.close()