Python seek on remote file using HTTP

问题

How do I seek to a particular position on a remote (HTTP) file so I can download only that part?

Lets say the bytes on a remote file were: 1234567890

I wanna seek to 4 and download 3 bytes from there so I would have: 456

and also, how do I check if a remote file exists? I tried, os.path.isfile() but it returns False when I'm passing a remote file url.

回答1:

If you are downloading the remote file through HTTP, you need to set the Range header.

Check in this example how it can be done. Looks like this:

myUrlclass.addheader("Range","bytes=%s-" % (existSize))

EDIT: I just found a better implementation. This class is very simple to use, as it can be seen in the docstring.

class HTTPRangeHandler(urllib2.BaseHandler):
"""Handler that enables HTTP Range headers.

This was extremely simple. The Range header is a HTTP feature to
begin with so all this class does is tell urllib2 that the 
"206 Partial Content" reponse from the HTTP server is what we 
expected.

Example:
    import urllib2
    import byterange

    range_handler = range.HTTPRangeHandler()
    opener = urllib2.build_opener(range_handler)

    # install it
    urllib2.install_opener(opener)

    # create Request and set Range header
    req = urllib2.Request('http://www.python.org/')
    req.header['Range'] = 'bytes=30-50'
    f = urllib2.urlopen(req)
"""

def http_error_206(self, req, fp, code, msg, hdrs):
    # 206 Partial Content Response
    r = urllib.addinfourl(fp, hdrs, req.get_full_url())
    r.code = code
    r.msg = msg
    return r

def http_error_416(self, req, fp, code, msg, hdrs):
    # HTTP's Range Not Satisfiable error
    raise RangeError('Requested Range Not Satisfiable')

Update: The "better implementation" has moved to github: excid3/urlgrabber in the byterange.py file.

回答2:

I highly recommend using the requests library. It is easily the best HTTP library I have ever used. In particular, to accomplish what you have described, you would do something like:

import requests

url = "http://www.sffaudio.com/podcasts/ShellGameByPhilipK.Dick.pdf"

# Retrieve bytes between offsets 3 and 5 (inclusive).
r = requests.get(url, headers={"range": "bytes=3-5"})

# If a 4XX client error or a 5XX server error is encountered, we raise it.
r.raise_for_status()

回答3:

AFAIK, this is not possible using fseek() or similar. You need to use the HTTP Range header to achieve this. This header may or may not be supported by the server, so your mileage may vary.

import urllib2

myHeaders = {'Range':'bytes=0-9'}

req = urllib2.Request('http://www.promotionalpromos.com/mirrors/gnu/gnu/bash/bash-1.14.3-1.14.4.diff.gz',headers=myHeaders)

partialFile = urllib2.urlopen(req)

s2 = (partialFile.read())

EDIT: This is of course assuming that by remote file you mean a file stored on a HTTP server...

If the file you want is on an FTP server, FTP only allows to to specify a start offset and not a range. If this is what you want, then the following code should do it (not tested!)

import ftplib
fileToRetrieve = 'somefile.zip'
fromByte = 15
ftp = ftplib.FTP('ftp.someplace.net')
outFile = open('partialFile', 'wb')
ftp.retrbinary('RETR '+ fileToRetrieve, outFile.write, rest=str(fromByte))
outFile.close()

回答4:

You can use httpio to access remote HTTP files as if they were local:

pip install httpio

import zipfile
import httpio

url = "http://some/large/file.zip"
with httpio.open(url) as fp:
    zf = zipfile.ZipFile(fp)
    print(zf.namelist())

来源：https://stackoverflow.com/questions/1971240/python-seek-on-remote-file-using-http

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python

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seek