Is there a library for retrieving a file from a re

The goal is just to retrieve a specific file without downloading the entire contents, using the HTTP range method as described: http://www.codeproject.com/KB/cs/remotezip.aspx

回答1:

You can solve this a bit more generally with less code. Essentially, create enough of a file-like object for ZipFile to use. So you wind up with z = ZipFile(HttpFile(url)) and it dynamically downloads just the portion needed. The advantage with this is you write less code, and it applies to more than just zip files. (In fact, I wonder if there is something like this already... I'm not finding it though.)

Using the same idea, you could also create a caching wrapper for HttpFile to avoid repeated downloads.

And here's the code: (note the lack of error-handling)

#!/usr/bin/python
import urllib2

class HttpFile(object):
    def __init__(self, url):
        self.url = url
        self.offset = 0
        self._size = -1

    def size(self):
        if self._size < 0:
            f = urllib2.urlopen(self.url)
            self._size = int(f.headers["Content-length"])
        return self._size

    def read(self, count=-1):
        req = urllib2.Request(self.url)
        if count < 0:
            end = self.size() - 1
        else:
            end = self.offset + count - 1
        req.headers['Range'] = "bytes=%s-%s" % (self.offset, end)
        f = urllib2.urlopen(req)
        data = f.read()
        # FIXME: should check that we got the range expected, etc.
        chunk = len(data)
        if count >= 0:
            assert chunk == count
        self.offset += chunk
        return data

    def seek(self, offset, whence=0):
        if whence == 0:
            self.offset = offset
        elif whence == 1:
            self.offset += offset
        elif whence == 2:
            self.offset = self.size() + offset
        else:
            raise Exception("Invalid whence")

    def tell(self):
        return self.offset

回答2:

Since there was no such library I have written a small module myself, most code and logic is is from zipfile with the seek/reads translated to HTTP range requests.

Feel free to review and suggest improvements:

The code:

"""
Read remote ZIP files using HTTP range requests
"""
import struct
import urllib2
import zlib
import cStringIO
from zipfile import ZipInfo, ZipExtFile, ZipInfo
from os.path import join, basename

# The code is mostly adatpted from the zipfile module
# NOTE: ZIP64 is not supported

# The "end of central directory" structure, magic number, size, and indices
# (section V.I in the format document)
structEndArchive = "<4s4H2LH"
stringEndArchive = "PK\005\006"
sizeEndCentDir = struct.calcsize(structEndArchive)

_ECD_SIGNATURE = 0
_ECD_DISK_NUMBER = 1
_ECD_DISK_START = 2
_ECD_ENTRIES_THIS_DISK = 3
_ECD_ENTRIES_TOTAL = 4
_ECD_SIZE = 5
_ECD_OFFSET = 6
_ECD_COMMENT_SIZE = 7
# These last two indices are not part of the structure as defined in the
# spec, but they are used internally by this module as a convenience
_ECD_COMMENT = 8
_ECD_LOCATION = 9

# The "central directory" structure, magic number, size, and indices
# of entries in the structure (section V.F in the format document)
structCentralDir = "<4s4B4HL2L5H2L"
stringCentralDir = "PK\001\002"
sizeCentralDir = struct.calcsize(structCentralDir)

# indexes of entries in the central directory structure
_CD_SIGNATURE = 0
_CD_CREATE_VERSION = 1
_CD_CREATE_SYSTEM = 2
_CD_EXTRACT_VERSION = 3
_CD_EXTRACT_SYSTEM = 4
_CD_FLAG_BITS = 5
_CD_COMPRESS_TYPE = 6
_CD_TIME = 7
_CD_DATE = 8
_CD_CRC = 9
_CD_COMPRESSED_SIZE = 10
_CD_UNCOMPRESSED_SIZE = 11
_CD_FILENAME_LENGTH = 12
_CD_EXTRA_FIELD_LENGTH = 13
_CD_COMMENT_LENGTH = 14
_CD_DISK_NUMBER_START = 15
_CD_INTERNAL_FILE_ATTRIBUTES = 16
_CD_EXTERNAL_FILE_ATTRIBUTES = 17
_CD_LOCAL_HEADER_OFFSET = 18

# The "local file header" structure, magic number, size, and indices
# (section V.A in the format document)
structFileHeader = "<4s2B4HL2L2H"
stringFileHeader = "PK\003\004"
sizeFileHeader = struct.calcsize(structFileHeader)

_FH_SIGNATURE = 0
_FH_EXTRACT_VERSION = 1
_FH_EXTRACT_SYSTEM = 2
_FH_GENERAL_PURPOSE_FLAG_BITS = 3
_FH_COMPRESSION_METHOD = 4
_FH_LAST_MOD_TIME = 5
_FH_LAST_MOD_DATE = 6
_FH_CRC = 7
_FH_COMPRESSED_SIZE = 8
_FH_UNCOMPRESSED_SIZE = 9
_FH_FILENAME_LENGTH = 10
_FH_EXTRA_FIELD_LENGTH = 11


def _http_get_partial_data(url, start_range, end_range=None):
    req = urllib2.Request(url)
    range_header = "bytes=%s" % start_range
    if end_range is not None:
        range_header += "-%s" % end_range
    req.headers['Range'] = range_header
    f = urllib2.urlopen(req)    
    return f


def _EndRecData(url):
    """Return data from the "End of Central Directory" record, or None.

    The data is a list of the nine items in the ZIP "End of central dir"
    record followed by a tenth item, the file seek offset of this record."""
    ECD = _http_get_partial_data(url, -sizeEndCentDir)
    content_range =  ECD.headers.get('Content-Range')
    filesize = int(content_range.split('/')[1]) if content_range and '/' in content_range else 0
    data = ECD.read()
    ECD.close() 
    if data[0:4] == stringEndArchive and data[-2:] == "\000\000":
        # the signature is correct and there's no comment, unpack structure
        endrec = struct.unpack(structEndArchive, data)
        endrec = list(endrec)

        # Append a blank comment and record start offset
        endrec.append("")
        endrec.append(filesize - sizeEndCentDir)
        return endrec
    # Either this is not a ZIP file, or it is a ZIP file with an archive
    # comment.  Search the end of the file for the "end of central directory"
    # record signature. The comment is the last item in the ZIP file and may be
    # up to 64K long.  It is assumed that the "end of central directory" magic
    # number does not appear in the comment.

    # Search by retrieving chunks of 256, 1k and 64k
    try_ranges = (1 << 8, 1 << 10, 1 << 16)
    for check_range in try_ranges:
        ECD = _http_get_partial_data(url, -(check_range + sizeEndCentDir))      
        data = ECD.read()       
        content_range =  ECD.headers.get('Content-Range')       
        ECD.close()
        download_start = content_range.split('-')[0]
        start = data.rfind(stringEndArchive)        
        if start >= 0:          
            # found the magic number; attempt to unpack and interpret
            recData = data[start:start+sizeEndCentDir]
            endrec = list(struct.unpack(structEndArchive, recData))
            commentSize = endrec[_ECD_COMMENT_SIZE] #as claimed by the zip file
            comment = data[start+sizeEndCentDir:start+sizeEndCentDir+commentSize]
            endrec.append(comment)
            endrec.append(download_start + start)           
            return endrec

    raise IOError


class HTTPZipFile:
    def __init__(self, url):
        self.url = url
        self.NameToInfo = {}    # Find file info given name
        self.filelist = []      # List of ZipInfo instances for archive
        self.pwd = None
        self.comment = ''
        self.debug = 0
        self._RealGetContents()     

    def _RealGetContents(self):
        """Read in the table of contents for the ZIP file."""
        try:
            endrec = _EndRecData(self.url)
        except IOError:
            raise BadZipfile("File is not a zip file")
        if not endrec:
            raise BadZipfile, "File is not a zip file"
        if self.debug > 1:
            print endrec
        size_cd = endrec[_ECD_SIZE]             # bytes in central directory
        offset_cd = endrec[_ECD_OFFSET]         # offset of central directory
        self.comment = endrec[_ECD_COMMENT]     # archive comment

        # "concat" is zero, unless zip was concatenated to another file
        concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
        #if endrec[_ECD_SIGNATURE] == stringEndArchive64:
        #   # If Zip64 extension structures are present, account for them
        #   concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)

        if self.debug > 2:
            inferred = concat + offset_cd
            print "given, inferred, offset", offset_cd, inferred, concat
        # self.start_dir:  Position of start of central directory
        self.start_dir = offset_cd + concat
        ECD = _http_get_partial_data(self.url, self.start_dir, self.start_dir+size_cd-1)
        data = ECD.read()
        ECD.close()
        fp = cStringIO.StringIO(data)               
        total = 0
        while total < size_cd:
            centdir = fp.read(sizeCentralDir)
            if centdir[0:4] != stringCentralDir:
                raise BadZipfile, "Bad magic number for central directory"
            centdir = struct.unpack(structCentralDir, centdir)
            if self.debug > 2:
                print centdir
            filename = fp.read(centdir[_CD_FILENAME_LENGTH])
            # Create ZipInfo instance to store file information
            x = ZipInfo(filename)
            x.extra = fp.read(centdir[_CD_EXTRA_FIELD_LENGTH])
            x.comment = fp.read(centdir[_CD_COMMENT_LENGTH])
            x.header_offset = centdir[_CD_LOCAL_HEADER_OFFSET]
            (x.create_version, x.create_system, x.extract_version, x.reserved,
                x.flag_bits, x.compress_type, t, d,
                x.CRC, x.compress_size, x.file_size) = centdir[1:12]
            x.volume, x.internal_attr, x.external_attr = centdir[15:18]
            # Convert date/time code to (year, month, day, hour, min, sec)
            x._raw_time = t
            x.date_time = ( (d>>9)+1980, (d>>5)&0xF, d&0x1F,
                                     t>>11, (t>>5)&0x3F, (t&0x1F) * 2 )

            x._decodeExtra()
            x.header_offset = x.header_offset + concat
            x.filename = x._decodeFilename()
            self.filelist.append(x)
            self.NameToInfo[x.filename] = x

            # update total bytes read from central directory
            total = (total + sizeCentralDir + centdir[_CD_FILENAME_LENGTH]
                     + centdir[_CD_EXTRA_FIELD_LENGTH]
                     + centdir[_CD_COMMENT_LENGTH])

        if self.debug > 2:
            print "total", total

    def namelist(self):
        """Return a list of file names in the archive."""
        l = []
        for data in self.filelist:
            l.append(data.filename)
        return l

    def infolist(self):
        """Return a list of class ZipInfo instances for files in the
        archive."""
        return self.filelist

    def printdir(self):
        """Print a table of contents for the zip file."""
        print "%-46s %19s %12s" % ("File Name", "Modified    ", "Size")
        for zinfo in self.filelist:
            date = "%d-%02d-%02d %02d:%02d:%02d" % zinfo.date_time[:6]
            print "%-46s %s %12d" % (zinfo.filename, date, zinfo.file_size)

    def getinfo(self, name):
        """Return the instance of ZipInfo given 'name'."""
        info = self.NameToInfo.get(name)
        if info is None:
            raise KeyError(
                'There is no item named %r in the archive' % name)

        return info         

    def open(self, name, pwd=None):
        """Return file-like object for 'name'."""
        if not self.url:
            raise RuntimeError, \
                  "Attempt to read ZIP archive that was already closed"
        zinfo = self.getinfo(name)
        offset = zinfo.header_offset
        f = _http_get_partial_data(self.url, offset, offset+sizeFileHeader-1)
        fheader = f.read()
        f.close()

        fheader = struct.unpack(structFileHeader, fheader)
        offset += sizeFileHeader
        f = _http_get_partial_data(self.url, offset, offset+fheader[_FH_FILENAME_LENGTH]-1)
        fname = f.read()
        f.close()

        if fname != zinfo.orig_filename:
            raise BadZipfile, \
                      'File name in directory "%s" and header "%s" differ.' % (
                          zinfo.orig_filename, fname)

        is_encrypted = zinfo.flag_bits & 0x1
        if is_encrypted:
            raise RuntimeError, "File %s is encrypted, " \
                  "not supported." % name

        offset += fheader[_FH_FILENAME_LENGTH]+fheader[_FH_EXTRA_FIELD_LENGTH]
        f = _http_get_partial_data(self.url, offset, offset+fheader[_FH_COMPRESSED_SIZE]-1)
        data = f.read()
        return ZipExtFile(cStringIO.StringIO(data), 'r', zinfo)


if __name__ == "__main__":
    # Some tests
    link="http://dfn.dl.sourceforge.net/project/filezilla/FileZilla_Client/3.5.1/FileZilla_3.5.1_win32.zip"
    hzfile = HTTPZipFile(link)
    hzfile.printdir()
    for fname in ('GPL.html', 'resources/blukis/48x48/filter.png', 'resources/finished.wav'):
        source_name = join('FileZilla-3.5.1', fname)
        dest_fname = join('/tmp', basename(fname))
        print "Extracing %s to %s" % (source_name, dest_fname)
        with hzfile.open(source_name) as f:
            data = f.read()
            new_file = open(dest_fname, 'w')
            new_file.write(data)
            new_file.close()