如何在远程（HTTP）文件上寻找特定位置，以便只下载该部分？
让我们说远程文件中的字节是：1234567890
我想寻求4并从那里下载3个字节，所以我会：456
此外，如何检查远程文件是否存在？ 我试过，os.path.isfile（）但是当我传递一个远程文件url时它返回False。
答案 0 :(得分：16)
如果您通过HTTP下载远程文件，则需要设置Range标题。
检查in this example如何完成。看起来像这样：
myUrlclass.addheader("Range","bytes=%s-" % (existSize))
编辑：I just found a better implementation。这个类使用起来非常简单，因为它可以在docstring中看到。
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')
更新：“更好的实施”已移至github: excid3/urlgrabber文件中的byterange.py。
答案 1 :(得分：5)
我强烈建议您使用requests库。它是我用过的最好的HTTP库。特别是，要完成你所描述的内容，你会做类似的事情：
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()
答案 2 :(得分：4)
AFAIK，这不可能使用fseek（）或类似的。您需要使用HTTP Range标头来实现此目的。服务器可能支持也可能不支持此标题，因此您的里程可能会有所不同。
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())
编辑：这当然是假设远程文件是指存储在HTTP服务器上的文件......
如果您想要的文件位于FTP服务器上，则FTP仅允许指定 start 偏移而不是范围。如果这是你想要的，那么下面的代码应该这样做（未经过测试！）
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()
答案 3 :(得分：1)
我没有找到任何现有的带有seek（）到HTTP URL的类文件接口的实现，所以我推出了自己的简单版本：https://github.com/valgur/pyhttpio。它取决于urllib.request，但如果需要，可以很容易地修改为使用requests。
完整代码：
import cgi
import time
import urllib.request
from io import IOBase
from sys import stderr
class SeekableHTTPFile(IOBase):
def __init__(self, url, name=None, repeat_time=-1, debug=False):
"""Allow a file accessible via HTTP to be used like a local file by utilities
that use `seek()` to read arbitrary parts of the file, such as `ZipFile`.
Seeking is done via the 'range: bytes=xx-yy' HTTP header.
Parameters
----------
url : str
A HTTP or HTTPS URL
name : str, optional
The filename of the file.
Will be filled from the Content-Disposition header if not provided.
repeat_time : int, optional
In case of HTTP errors wait `repeat_time` seconds before trying again.
Negative value or `None` disables retrying and simply passes on the exception (the default).
"""
super().__init__()
self.url = url
self.name = name
self.repeat_time = repeat_time
self.debug = debug
self._pos = 0
self._seekable = True
with self._urlopen() as f:
if self.debug:
print(f.getheaders())
self.content_length = int(f.getheader("Content-Length", -1))
if self.content_length < 0:
self._seekable = False
if f.getheader("Accept-Ranges", "none").lower() != "bytes":
self._seekable = False
if name is None:
header = f.getheader("Content-Disposition")
if header:
value, params = cgi.parse_header(header)
self.name = params["filename"]
def seek(self, offset, whence=0):
if not self.seekable():
raise OSError
if whence == 0:
self._pos = 0
elif whence == 1:
pass
elif whence == 2:
self._pos = self.content_length
self._pos += offset
return self._pos
def seekable(self, *args, **kwargs):
return self._seekable
def readable(self, *args, **kwargs):
return not self.closed
def writable(self, *args, **kwargs):
return False
def read(self, amt=-1):
if self._pos >= self.content_length:
return b""
if amt < 0:
end = self.content_length - 1
else:
end = min(self._pos + amt - 1, self.content_length - 1)
byte_range = (self._pos, end)
self._pos = end + 1
with self._urlopen(byte_range) as f:
return f.read()
def readall(self):
return self.read(-1)
def tell(self):
return self._pos
def __getattribute__(self, item):
attr = object.__getattribute__(self, item)
if not object.__getattribute__(self, "debug"):
return attr
if hasattr(attr, '__call__'):
def trace(*args, **kwargs):
a = ", ".join(map(str, args))
if kwargs:
a += ", ".join(["{}={}".format(k, v) for k, v in kwargs.items()])
print("Calling: {}({})".format(item, a))
return attr(*args, **kwargs)
return trace
else:
return attr
def _urlopen(self, byte_range=None):
header = {}
if byte_range:
header = {"range": "bytes={}-{}".format(*byte_range)}
while True:
try:
r = urllib.request.Request(self.url, headers=header)
return urllib.request.urlopen(r)
except urllib.error.HTTPError as e:
if self.repeat_time is None or self.repeat_time < 0:
raise
print("Server responded with " + str(e), file=stderr)
print("Sleeping for {} seconds before trying again".format(self.repeat_time), file=stderr)
time.sleep(self.repeat_time)
一个小用法示例：
url = "https://www.python.org/ftp/python/3.5.0/python-3.5.0-embed-amd64.zip"
f = SeekableHTTPFile(url, debug=True)
zf = ZipFile(f)
zf.printdir()
zf.extract("python.exe")
编辑：在这个答案中实际上有一个大致相同但稍微更简单的实现：https://stackoverflow.com/a/7852229/2997179