__init__.py

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import roslib; roslib.load_manifest('resource_retriever')
import subprocess
import rospkg
try:
    from urllib.request import urlopen
    from urllib.error import URLError
except ImportError:
    from urllib2 import urlopen
    from urllib2 import URLError

PACKAGE_PREFIX = 'package://'
r = rospkg.RosPack()

def get_filename(url, use_protocol=True):
    mod_url = url
    if url.find(PACKAGE_PREFIX) == 0:
        mod_url = url[len(PACKAGE_PREFIX):]
        pos = mod_url.find('/')
        if pos == -1:
            raise Exception("Could not parse package:// format into file:// format for " + url)

        package = mod_url[0:pos]
        mod_url = mod_url[pos:]
        package_path = r.get_path(package)

        if use_protocol:
            protocol = "file://"
        else:
            protocol = ""
        mod_url = protocol + package_path + mod_url;
    return mod_url

def get(url):
    filename = get_filename(url)
    try:
        return urlopen(filename).read()
    except URLError:
        raise Exception("Invalid URL: {}".format(filename))