Question

我试图在Python中解析来自Common Crawl的.warc文件。

由于文件很大，我想从头几条记录的样本/子集开始。

如何将文件截断为仅包含前X行，同时保留现有的换行符/回车符？

这是我尝试过的：

head -n 250 oldfile > newfile这将删除解析文件所需的一些返回。如果我尝试在我的Hadoop作业中使用此文件（使用warc包读取它），我会得到错误：

  Traceback (most recent call last):
      File "test.py", line 46, in <module>
        TagGrabber.run()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/job.py", line 461, in run
        mr_job.execute()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/job.py", line 479, in execute
        super(MRJob, self).execute()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/launch.py", line 151, in execute
        self.run_job()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/launch.py", line 214, in run_job
        runner.run()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/runner.py", line 464, in run
        self._run()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/sim.py", line 173, in _run
        self._invoke_step(step_num, 'mapper')
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/sim.py", line 264, in _invoke_step
        self.per_step_runner_finish(step_num)
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/local.py", line 152, in per_step_runner_finish
        self._wait_for_process(proc_dict, step_num)
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/mrjob/local.py", line 268, in _wait_for_process
        (proc_dict['args'], returncode, ''.join(tb_lines)))
    Exception: Command ['sh', '-ex', 'setup-wrapper.sh', '/var/cc-mrjob/venv/bin/python', 'test.py', '--step-num=0', '--mapper', '/tmp/test.root.20150520.071726.549519/input_part-00000'] returned non-zero exit status 1:
    Traceback (most recent call last):
      File "test.py", line 46, in <module>
        TagGrabber.run()
      File "/tmp/test.root.20150520.071726.549519/job_local_dir/0/mapper/0/mrjob.tar.gz/mrjob/job.py", line 461, in run
        mr_job.execute()
      File "/tmp/test.root.20150520.071726.549519/job_local_dir/0/mapper/0/mrjob.tar.gz/mrjob/job.py", line 470, in execute
        self.run_mapper(self.options.step_num)
      File "/tmp/test.root.20150520.071726.549519/job_local_dir/0/mapper/0/mrjob.tar.gz/mrjob/job.py", line 535, in run_mapper
        for out_key, out_value in mapper(key, value) or ():
      File "/var/cc-mrjob/mrcc.py", line 33, in mapper
        for i, record in enumerate(f):
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/warc/warc.py", line 390, in __iter__
        record = self.read_record()
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/warc/warc.py", line 373, in read_record
        header = self.read_header(fileobj)
      File "/var/cc-mrjob/venv/local/lib/python2.7/site-packages/warc/warc.py", line 331, in read_header
        raise IOError("Bad version line: %r" % version_line)
    IOError: Bad version line: 'WARC/1.0\n'

与＃1相同，但使用tail命令
与＃1相同，但之后使用tr或sed来替换任何丢失的换行符或^M（回车）字符。这导致warc包仍然抱怨预期的回车或换行没有到位。
unix2dos oldfile

Answer 1

正确处理换行很困难，因为.warc文件也可能包含二进制数据。截断也可能会产生损坏的.warc文件，因为例如python库信任Content-Length标头是有效的。

warc python lib一次只从.warc文件中读取一条记录（避免一次读取整个文件到内存），因此只能使用python处理子集。例如：

import warc
from itertools import islice

N = 10
warc_file = warc.open('/path/to/file.warc')
for record in islice(warc_file, N):
    do_stuff_with(record)

如何从warc文件中读取记录的子集

1 个答案: