--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/planemo/lib/python3.7/site-packages/pip/_internal/download.py	Fri Jul 31 00:32:28 2020 -0400
@@ -0,0 +1,1177 @@
+from __future__ import absolute_import
+
+import cgi
+import email.utils
+import json
+import logging
+import mimetypes
+import os
+import platform
+import re
+import shutil
+import sys
+
+from pip._vendor import requests, urllib3
+from pip._vendor.cachecontrol import CacheControlAdapter
+from pip._vendor.cachecontrol.caches import FileCache
+from pip._vendor.lockfile import LockError
+from pip._vendor.requests.adapters import BaseAdapter, HTTPAdapter
+from pip._vendor.requests.auth import AuthBase, HTTPBasicAuth
+from pip._vendor.requests.models import CONTENT_CHUNK_SIZE, Response
+from pip._vendor.requests.structures import CaseInsensitiveDict
+from pip._vendor.requests.utils import get_netrc_auth
+# NOTE: XMLRPC Client is not annotated in typeshed as on 2017-07-17, which is
+#       why we ignore the type on this import
+from pip._vendor.six.moves import xmlrpc_client  # type: ignore
+from pip._vendor.six.moves.urllib import parse as urllib_parse
+from pip._vendor.six.moves.urllib import request as urllib_request
+
+import pip
+from pip._internal.exceptions import HashMismatch, InstallationError
+from pip._internal.models.index import PyPI
+# Import ssl from compat so the initial import occurs in only one place.
+from pip._internal.utils.compat import HAS_TLS, ssl
+from pip._internal.utils.encoding import auto_decode
+from pip._internal.utils.filesystem import check_path_owner
+from pip._internal.utils.glibc import libc_ver
+from pip._internal.utils.marker_files import write_delete_marker_file
+from pip._internal.utils.misc import (
+    ARCHIVE_EXTENSIONS, ask, ask_input, ask_password, ask_path_exists,
+    backup_dir, consume, display_path, format_size, get_installed_version,
+    path_to_url, remove_auth_from_url, rmtree, split_auth_netloc_from_url,
+    splitext, unpack_file,
+)
+from pip._internal.utils.temp_dir import TempDirectory
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+from pip._internal.utils.ui import DownloadProgressProvider
+from pip._internal.vcs import vcs
+
+if MYPY_CHECK_RUNNING:
+    from typing import (
+        Optional, Tuple, Dict, IO, Text, Union
+    )
+    from optparse import Values
+    from pip._internal.models.link import Link
+    from pip._internal.utils.hashes import Hashes
+    from pip._internal.vcs.versioncontrol import AuthInfo, VersionControl
+
+    Credentials = Tuple[str, str, str]
+
+
+__all__ = ['get_file_content',
+           'is_url', 'url_to_path', 'path_to_url',
+           'is_archive_file', 'unpack_vcs_link',
+           'unpack_file_url', 'is_vcs_url', 'is_file_url',
+           'unpack_http_url', 'unpack_url',
+           'parse_content_disposition', 'sanitize_content_filename']
+
+
+logger = logging.getLogger(__name__)
+
+
+try:
+    import keyring  # noqa
+except ImportError:
+    keyring = None
+except Exception as exc:
+    logger.warning("Keyring is skipped due to an exception: %s",
+                   str(exc))
+    keyring = None
+
+# These are environment variables present when running under various
+# CI systems.  For each variable, some CI systems that use the variable
+# are indicated.  The collection was chosen so that for each of a number
+# of popular systems, at least one of the environment variables is used.
+# This list is used to provide some indication of and lower bound for
+# CI traffic to PyPI.  Thus, it is okay if the list is not comprehensive.
+# For more background, see: https://github.com/pypa/pip/issues/5499
+CI_ENVIRONMENT_VARIABLES = (
+    # Azure Pipelines
+    'BUILD_BUILDID',
+    # Jenkins
+    'BUILD_ID',
+    # AppVeyor, CircleCI, Codeship, Gitlab CI, Shippable, Travis CI
+    'CI',
+    # Explicit environment variable.
+    'PIP_IS_CI',
+)
+
+
+def looks_like_ci():
+    # type: () -> bool
+    """
+    Return whether it looks like pip is running under CI.
+    """
+    # We don't use the method of checking for a tty (e.g. using isatty())
+    # because some CI systems mimic a tty (e.g. Travis CI).  Thus that
+    # method doesn't provide definitive information in either direction.
+    return any(name in os.environ for name in CI_ENVIRONMENT_VARIABLES)
+
+
+def user_agent():
+    """
+    Return a string representing the user agent.
+    """
+    data = {
+        "installer": {"name": "pip", "version": pip.__version__},
+        "python": platform.python_version(),
+        "implementation": {
+            "name": platform.python_implementation(),
+        },
+    }
+
+    if data["implementation"]["name"] == 'CPython':
+        data["implementation"]["version"] = platform.python_version()
+    elif data["implementation"]["name"] == 'PyPy':
+        if sys.pypy_version_info.releaselevel == 'final':
+            pypy_version_info = sys.pypy_version_info[:3]
+        else:
+            pypy_version_info = sys.pypy_version_info
+        data["implementation"]["version"] = ".".join(
+            [str(x) for x in pypy_version_info]
+        )
+    elif data["implementation"]["name"] == 'Jython':
+        # Complete Guess
+        data["implementation"]["version"] = platform.python_version()
+    elif data["implementation"]["name"] == 'IronPython':
+        # Complete Guess
+        data["implementation"]["version"] = platform.python_version()
+
+    if sys.platform.startswith("linux"):
+        from pip._vendor import distro
+        distro_infos = dict(filter(
+            lambda x: x[1],
+            zip(["name", "version", "id"], distro.linux_distribution()),
+        ))
+        libc = dict(filter(
+            lambda x: x[1],
+            zip(["lib", "version"], libc_ver()),
+        ))
+        if libc:
+            distro_infos["libc"] = libc
+        if distro_infos:
+            data["distro"] = distro_infos
+
+    if sys.platform.startswith("darwin") and platform.mac_ver()[0]:
+        data["distro"] = {"name": "macOS", "version": platform.mac_ver()[0]}
+
+    if platform.system():
+        data.setdefault("system", {})["name"] = platform.system()
+
+    if platform.release():
+        data.setdefault("system", {})["release"] = platform.release()
+
+    if platform.machine():
+        data["cpu"] = platform.machine()
+
+    if HAS_TLS:
+        data["openssl_version"] = ssl.OPENSSL_VERSION
+
+    setuptools_version = get_installed_version("setuptools")
+    if setuptools_version is not None:
+        data["setuptools_version"] = setuptools_version
+
+    # Use None rather than False so as not to give the impression that
+    # pip knows it is not being run under CI.  Rather, it is a null or
+    # inconclusive result.  Also, we include some value rather than no
+    # value to make it easier to know that the check has been run.
+    data["ci"] = True if looks_like_ci() else None
+
+    user_data = os.environ.get("PIP_USER_AGENT_USER_DATA")
+    if user_data is not None:
+        data["user_data"] = user_data
+
+    return "{data[installer][name]}/{data[installer][version]} {json}".format(
+        data=data,
+        json=json.dumps(data, separators=(",", ":"), sort_keys=True),
+    )
+
+
+def _get_keyring_auth(url, username):
+    """Return the tuple auth for a given url from keyring."""
+    if not url or not keyring:
+        return None
+
+    try:
+        try:
+            get_credential = keyring.get_credential
+        except AttributeError:
+            pass
+        else:
+            logger.debug("Getting credentials from keyring for %s", url)
+            cred = get_credential(url, username)
+            if cred is not None:
+                return cred.username, cred.password
+            return None
+
+        if username:
+            logger.debug("Getting password from keyring for %s", url)
+            password = keyring.get_password(url, username)
+            if password:
+                return username, password
+
+    except Exception as exc:
+        logger.warning("Keyring is skipped due to an exception: %s",
+                       str(exc))
+
+
+class MultiDomainBasicAuth(AuthBase):
+
+    def __init__(self, prompting=True, index_urls=None):
+        # type: (bool, Optional[Values]) -> None
+        self.prompting = prompting
+        self.index_urls = index_urls
+        self.passwords = {}  # type: Dict[str, AuthInfo]
+        # When the user is prompted to enter credentials and keyring is
+        # available, we will offer to save them. If the user accepts,
+        # this value is set to the credentials they entered. After the
+        # request authenticates, the caller should call
+        # ``save_credentials`` to save these.
+        self._credentials_to_save = None  # type: Optional[Credentials]
+
+    def _get_index_url(self, url):
+        """Return the original index URL matching the requested URL.
+
+        Cached or dynamically generated credentials may work against
+        the original index URL rather than just the netloc.
+
+        The provided url should have had its username and password
+        removed already. If the original index url had credentials then
+        they will be included in the return value.
+
+        Returns None if no matching index was found, or if --no-index
+        was specified by the user.
+        """
+        if not url or not self.index_urls:
+            return None
+
+        for u in self.index_urls:
+            prefix = remove_auth_from_url(u).rstrip("/") + "/"
+            if url.startswith(prefix):
+                return u
+
+    def _get_new_credentials(self, original_url, allow_netrc=True,
+                             allow_keyring=True):
+        """Find and return credentials for the specified URL."""
+        # Split the credentials and netloc from the url.
+        url, netloc, url_user_password = split_auth_netloc_from_url(
+            original_url)
+
+        # Start with the credentials embedded in the url
+        username, password = url_user_password
+        if username is not None and password is not None:
+            logger.debug("Found credentials in url for %s", netloc)
+            return url_user_password
+
+        # Find a matching index url for this request
+        index_url = self._get_index_url(url)
+        if index_url:
+            # Split the credentials from the url.
+            index_info = split_auth_netloc_from_url(index_url)
+            if index_info:
+                index_url, _, index_url_user_password = index_info
+                logger.debug("Found index url %s", index_url)
+
+        # If an index URL was found, try its embedded credentials
+        if index_url and index_url_user_password[0] is not None:
+            username, password = index_url_user_password
+            if username is not None and password is not None:
+                logger.debug("Found credentials in index url for %s", netloc)
+                return index_url_user_password
+
+        # Get creds from netrc if we still don't have them
+        if allow_netrc:
+            netrc_auth = get_netrc_auth(original_url)
+            if netrc_auth:
+                logger.debug("Found credentials in netrc for %s", netloc)
+                return netrc_auth
+
+        # If we don't have a password and keyring is available, use it.
+        if allow_keyring:
+            # The index url is more specific than the netloc, so try it first
+            kr_auth = (_get_keyring_auth(index_url, username) or
+                       _get_keyring_auth(netloc, username))
+            if kr_auth:
+                logger.debug("Found credentials in keyring for %s", netloc)
+                return kr_auth
+
+        return username, password
+
+    def _get_url_and_credentials(self, original_url):
+        """Return the credentials to use for the provided URL.
+
+        If allowed, netrc and keyring may be used to obtain the
+        correct credentials.
+
+        Returns (url_without_credentials, username, password). Note
+        that even if the original URL contains credentials, this
+        function may return a different username and password.
+        """
+        url, netloc, _ = split_auth_netloc_from_url(original_url)
+
+        # Use any stored credentials that we have for this netloc
+        username, password = self.passwords.get(netloc, (None, None))
+
+        if username is None and password is None:
+            # No stored credentials. Acquire new credentials without prompting
+            # the user. (e.g. from netrc, keyring, or the URL itself)
+            username, password = self._get_new_credentials(original_url)
+
+        if username is not None or password is not None:
+            # Convert the username and password if they're None, so that
+            # this netloc will show up as "cached" in the conditional above.
+            # Further, HTTPBasicAuth doesn't accept None, so it makes sense to
+            # cache the value that is going to be used.
+            username = username or ""
+            password = password or ""
+
+            # Store any acquired credentials.
+            self.passwords[netloc] = (username, password)
+
+        assert (
+            # Credentials were found
+            (username is not None and password is not None) or
+            # Credentials were not found
+            (username is None and password is None)
+        ), "Could not load credentials from url: {}".format(original_url)
+
+        return url, username, password
+
+    def __call__(self, req):
+        # Get credentials for this request
+        url, username, password = self._get_url_and_credentials(req.url)
+
+        # Set the url of the request to the url without any credentials
+        req.url = url
+
+        if username is not None and password is not None:
+            # Send the basic auth with this request
+            req = HTTPBasicAuth(username, password)(req)
+
+        # Attach a hook to handle 401 responses
+        req.register_hook("response", self.handle_401)
+
+        return req
+
+    # Factored out to allow for easy patching in tests
+    def _prompt_for_password(self, netloc):
+        username = ask_input("User for %s: " % netloc)
+        if not username:
+            return None, None
+        auth = _get_keyring_auth(netloc, username)
+        if auth:
+            return auth[0], auth[1], False
+        password = ask_password("Password: ")
+        return username, password, True
+
+    # Factored out to allow for easy patching in tests
+    def _should_save_password_to_keyring(self):
+        if not keyring:
+            return False
+        return ask("Save credentials to keyring [y/N]: ", ["y", "n"]) == "y"
+
+    def handle_401(self, resp, **kwargs):
+        # We only care about 401 responses, anything else we want to just
+        #   pass through the actual response
+        if resp.status_code != 401:
+            return resp
+
+        # We are not able to prompt the user so simply return the response
+        if not self.prompting:
+            return resp
+
+        parsed = urllib_parse.urlparse(resp.url)
+
+        # Prompt the user for a new username and password
+        username, password, save = self._prompt_for_password(parsed.netloc)
+
+        # Store the new username and password to use for future requests
+        self._credentials_to_save = None
+        if username is not None and password is not None:
+            self.passwords[parsed.netloc] = (username, password)
+
+            # Prompt to save the password to keyring
+            if save and self._should_save_password_to_keyring():
+                self._credentials_to_save = (parsed.netloc, username, password)
+
+        # Consume content and release the original connection to allow our new
+        #   request to reuse the same one.
+        resp.content
+        resp.raw.release_conn()
+
+        # Add our new username and password to the request
+        req = HTTPBasicAuth(username or "", password or "")(resp.request)
+        req.register_hook("response", self.warn_on_401)
+
+        # On successful request, save the credentials that were used to
+        # keyring. (Note that if the user responded "no" above, this member
+        # is not set and nothing will be saved.)
+        if self._credentials_to_save:
+            req.register_hook("response", self.save_credentials)
+
+        # Send our new request
+        new_resp = resp.connection.send(req, **kwargs)
+        new_resp.history.append(resp)
+
+        return new_resp
+
+    def warn_on_401(self, resp, **kwargs):
+        """Response callback to warn about incorrect credentials."""
+        if resp.status_code == 401:
+            logger.warning('401 Error, Credentials not correct for %s',
+                           resp.request.url)
+
+    def save_credentials(self, resp, **kwargs):
+        """Response callback to save credentials on success."""
+        assert keyring is not None, "should never reach here without keyring"
+        if not keyring:
+            return
+
+        creds = self._credentials_to_save
+        self._credentials_to_save = None
+        if creds and resp.status_code < 400:
+            try:
+                logger.info('Saving credentials to keyring')
+                keyring.set_password(*creds)
+            except Exception:
+                logger.exception('Failed to save credentials')
+
+
+class LocalFSAdapter(BaseAdapter):
+
+    def send(self, request, stream=None, timeout=None, verify=None, cert=None,
+             proxies=None):
+        pathname = url_to_path(request.url)
+
+        resp = Response()
+        resp.status_code = 200
+        resp.url = request.url
+
+        try:
+            stats = os.stat(pathname)
+        except OSError as exc:
+            resp.status_code = 404
+            resp.raw = exc
+        else:
+            modified = email.utils.formatdate(stats.st_mtime, usegmt=True)
+            content_type = mimetypes.guess_type(pathname)[0] or "text/plain"
+            resp.headers = CaseInsensitiveDict({
+                "Content-Type": content_type,
+                "Content-Length": stats.st_size,
+                "Last-Modified": modified,
+            })
+
+            resp.raw = open(pathname, "rb")
+            resp.close = resp.raw.close
+
+        return resp
+
+    def close(self):
+        pass
+
+
+class SafeFileCache(FileCache):
+    """
+    A file based cache which is safe to use even when the target directory may
+    not be accessible or writable.
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(SafeFileCache, self).__init__(*args, **kwargs)
+
+        # Check to ensure that the directory containing our cache directory
+        # is owned by the user current executing pip. If it does not exist
+        # we will check the parent directory until we find one that does exist.
+        # If it is not owned by the user executing pip then we will disable
+        # the cache and log a warning.
+        if not check_path_owner(self.directory):
+            logger.warning(
+                "The directory '%s' or its parent directory is not owned by "
+                "the current user and the cache has been disabled. Please "
+                "check the permissions and owner of that directory. If "
+                "executing pip with sudo, you may want sudo's -H flag.",
+                self.directory,
+            )
+
+            # Set our directory to None to disable the Cache
+            self.directory = None
+
+    def get(self, *args, **kwargs):
+        # If we don't have a directory, then the cache should be a no-op.
+        if self.directory is None:
+            return
+
+        try:
+            return super(SafeFileCache, self).get(*args, **kwargs)
+        except (LockError, OSError, IOError):
+            # We intentionally silence this error, if we can't access the cache
+            # then we can just skip caching and process the request as if
+            # caching wasn't enabled.
+            pass
+
+    def set(self, *args, **kwargs):
+        # If we don't have a directory, then the cache should be a no-op.
+        if self.directory is None:
+            return
+
+        try:
+            return super(SafeFileCache, self).set(*args, **kwargs)
+        except (LockError, OSError, IOError):
+            # We intentionally silence this error, if we can't access the cache
+            # then we can just skip caching and process the request as if
+            # caching wasn't enabled.
+            pass
+
+    def delete(self, *args, **kwargs):
+        # If we don't have a directory, then the cache should be a no-op.
+        if self.directory is None:
+            return
+
+        try:
+            return super(SafeFileCache, self).delete(*args, **kwargs)
+        except (LockError, OSError, IOError):
+            # We intentionally silence this error, if we can't access the cache
+            # then we can just skip caching and process the request as if
+            # caching wasn't enabled.
+            pass
+
+
+class InsecureHTTPAdapter(HTTPAdapter):
+
+    def cert_verify(self, conn, url, verify, cert):
+        conn.cert_reqs = 'CERT_NONE'
+        conn.ca_certs = None
+
+
+class PipSession(requests.Session):
+
+    timeout = None  # type: Optional[int]
+
+    def __init__(self, *args, **kwargs):
+        retries = kwargs.pop("retries", 0)
+        cache = kwargs.pop("cache", None)
+        insecure_hosts = kwargs.pop("insecure_hosts", [])
+        index_urls = kwargs.pop("index_urls", None)
+
+        super(PipSession, self).__init__(*args, **kwargs)
+
+        # Attach our User Agent to the request
+        self.headers["User-Agent"] = user_agent()
+
+        # Attach our Authentication handler to the session
+        self.auth = MultiDomainBasicAuth(index_urls=index_urls)
+
+        # Create our urllib3.Retry instance which will allow us to customize
+        # how we handle retries.
+        retries = urllib3.Retry(
+            # Set the total number of retries that a particular request can
+            # have.
+            total=retries,
+
+            # A 503 error from PyPI typically means that the Fastly -> Origin
+            # connection got interrupted in some way. A 503 error in general
+            # is typically considered a transient error so we'll go ahead and
+            # retry it.
+            # A 500 may indicate transient error in Amazon S3
+            # A 520 or 527 - may indicate transient error in CloudFlare
+            status_forcelist=[500, 503, 520, 527],
+
+            # Add a small amount of back off between failed requests in
+            # order to prevent hammering the service.
+            backoff_factor=0.25,
+        )
+
+        # We want to _only_ cache responses on securely fetched origins. We do
+        # this because we can't validate the response of an insecurely fetched
+        # origin, and we don't want someone to be able to poison the cache and
+        # require manual eviction from the cache to fix it.
+        if cache:
+            secure_adapter = CacheControlAdapter(
+                cache=SafeFileCache(cache, use_dir_lock=True),
+                max_retries=retries,
+            )
+        else:
+            secure_adapter = HTTPAdapter(max_retries=retries)
+
+        # Our Insecure HTTPAdapter disables HTTPS validation. It does not
+        # support caching (see above) so we'll use it for all http:// URLs as
+        # well as any https:// host that we've marked as ignoring TLS errors
+        # for.
+        insecure_adapter = InsecureHTTPAdapter(max_retries=retries)
+        # Save this for later use in add_insecure_host().
+        self._insecure_adapter = insecure_adapter
+
+        self.mount("https://", secure_adapter)
+        self.mount("http://", insecure_adapter)
+
+        # Enable file:// urls
+        self.mount("file://", LocalFSAdapter())
+
+        # We want to use a non-validating adapter for any requests which are
+        # deemed insecure.
+        for host in insecure_hosts:
+            self.add_insecure_host(host)
+
+    def add_insecure_host(self, host):
+        # type: (str) -> None
+        self.mount('https://{}/'.format(host), self._insecure_adapter)
+
+    def request(self, method, url, *args, **kwargs):
+        # Allow setting a default timeout on a session
+        kwargs.setdefault("timeout", self.timeout)
+
+        # Dispatch the actual request
+        return super(PipSession, self).request(method, url, *args, **kwargs)
+
+
+def get_file_content(url, comes_from=None, session=None):
+    # type: (str, Optional[str], Optional[PipSession]) -> Tuple[str, Text]
+    """Gets the content of a file; it may be a filename, file: URL, or
+    http: URL.  Returns (location, content).  Content is unicode.
+
+    :param url:         File path or url.
+    :param comes_from:  Origin description of requirements.
+    :param session:     Instance of pip.download.PipSession.
+    """
+    if session is None:
+        raise TypeError(
+            "get_file_content() missing 1 required keyword argument: 'session'"
+        )
+
+    match = _scheme_re.search(url)
+    if match:
+        scheme = match.group(1).lower()
+        if (scheme == 'file' and comes_from and
+                comes_from.startswith('http')):
+            raise InstallationError(
+                'Requirements file %s references URL %s, which is local'
+                % (comes_from, url))
+        if scheme == 'file':
+            path = url.split(':', 1)[1]
+            path = path.replace('\\', '/')
+            match = _url_slash_drive_re.match(path)
+            if match:
+                path = match.group(1) + ':' + path.split('|', 1)[1]
+            path = urllib_parse.unquote(path)
+            if path.startswith('/'):
+                path = '/' + path.lstrip('/')
+            url = path
+        else:
+            # FIXME: catch some errors
+            resp = session.get(url)
+            resp.raise_for_status()
+            return resp.url, resp.text
+    try:
+        with open(url, 'rb') as f:
+            content = auto_decode(f.read())
+    except IOError as exc:
+        raise InstallationError(
+            'Could not open requirements file: %s' % str(exc)
+        )
+    return url, content
+
+
+_scheme_re = re.compile(r'^(http|https|file):', re.I)
+_url_slash_drive_re = re.compile(r'/*([a-z])\|', re.I)
+
+
+def is_url(name):
+    # type: (Union[str, Text]) -> bool
+    """Returns true if the name looks like a URL"""
+    if ':' not in name:
+        return False
+    scheme = name.split(':', 1)[0].lower()
+    return scheme in ['http', 'https', 'file', 'ftp'] + vcs.all_schemes
+
+
+def url_to_path(url):
+    # type: (str) -> str
+    """
+    Convert a file: URL to a path.
+    """
+    assert url.startswith('file:'), (
+        "You can only turn file: urls into filenames (not %r)" % url)
+
+    _, netloc, path, _, _ = urllib_parse.urlsplit(url)
+
+    if not netloc or netloc == 'localhost':
+        # According to RFC 8089, same as empty authority.
+        netloc = ''
+    elif sys.platform == 'win32':
+        # If we have a UNC path, prepend UNC share notation.
+        netloc = '\\\\' + netloc
+    else:
+        raise ValueError(
+            'non-local file URIs are not supported on this platform: %r'
+            % url
+        )
+
+    path = urllib_request.url2pathname(netloc + path)
+    return path
+
+
+def is_archive_file(name):
+    # type: (str) -> bool
+    """Return True if `name` is a considered as an archive file."""
+    ext = splitext(name)[1].lower()
+    if ext in ARCHIVE_EXTENSIONS:
+        return True
+    return False
+
+
+def unpack_vcs_link(link, location):
+    vcs_backend = _get_used_vcs_backend(link)
+    vcs_backend.unpack(location, url=link.url)
+
+
+def _get_used_vcs_backend(link):
+    # type: (Link) -> Optional[VersionControl]
+    """
+    Return a VersionControl object or None.
+    """
+    for vcs_backend in vcs.backends:
+        if link.scheme in vcs_backend.schemes:
+            return vcs_backend
+    return None
+
+
+def is_vcs_url(link):
+    # type: (Link) -> bool
+    return bool(_get_used_vcs_backend(link))
+
+
+def is_file_url(link):
+    # type: (Link) -> bool
+    return link.url.lower().startswith('file:')
+
+
+def is_dir_url(link):
+    # type: (Link) -> bool
+    """Return whether a file:// Link points to a directory.
+
+    ``link`` must not have any other scheme but file://. Call is_file_url()
+    first.
+
+    """
+    link_path = url_to_path(link.url_without_fragment)
+    return os.path.isdir(link_path)
+
+
+def _progress_indicator(iterable, *args, **kwargs):
+    return iterable
+
+
+def _download_url(
+    resp,  # type: Response
+    link,  # type: Link
+    content_file,  # type: IO
+    hashes,  # type: Optional[Hashes]
+    progress_bar  # type: str
+):
+    # type: (...) -> None
+    try:
+        total_length = int(resp.headers['content-length'])
+    except (ValueError, KeyError, TypeError):
+        total_length = 0
+
+    cached_resp = getattr(resp, "from_cache", False)
+    if logger.getEffectiveLevel() > logging.INFO:
+        show_progress = False
+    elif cached_resp:
+        show_progress = False
+    elif total_length > (40 * 1000):
+        show_progress = True
+    elif not total_length:
+        show_progress = True
+    else:
+        show_progress = False
+
+    show_url = link.show_url
+
+    def resp_read(chunk_size):
+        try:
+            # Special case for urllib3.
+            for chunk in resp.raw.stream(
+                    chunk_size,
+                    # We use decode_content=False here because we don't
+                    # want urllib3 to mess with the raw bytes we get
+                    # from the server. If we decompress inside of
+                    # urllib3 then we cannot verify the checksum
+                    # because the checksum will be of the compressed
+                    # file. This breakage will only occur if the
+                    # server adds a Content-Encoding header, which
+                    # depends on how the server was configured:
+                    # - Some servers will notice that the file isn't a
+                    #   compressible file and will leave the file alone
+                    #   and with an empty Content-Encoding
+                    # - Some servers will notice that the file is
+                    #   already compressed and will leave the file
+                    #   alone and will add a Content-Encoding: gzip
+                    #   header
+                    # - Some servers won't notice anything at all and
+                    #   will take a file that's already been compressed
+                    #   and compress it again and set the
+                    #   Content-Encoding: gzip header
+                    #
+                    # By setting this not to decode automatically we
+                    # hope to eliminate problems with the second case.
+                    decode_content=False):
+                yield chunk
+        except AttributeError:
+            # Standard file-like object.
+            while True:
+                chunk = resp.raw.read(chunk_size)
+                if not chunk:
+                    break
+                yield chunk
+
+    def written_chunks(chunks):
+        for chunk in chunks:
+            content_file.write(chunk)
+            yield chunk
+
+    progress_indicator = _progress_indicator
+
+    if link.netloc == PyPI.netloc:
+        url = show_url
+    else:
+        url = link.url_without_fragment
+
+    if show_progress:  # We don't show progress on cached responses
+        progress_indicator = DownloadProgressProvider(progress_bar,
+                                                      max=total_length)
+        if total_length:
+            logger.info("Downloading %s (%s)", url, format_size(total_length))
+        else:
+            logger.info("Downloading %s", url)
+    elif cached_resp:
+        logger.info("Using cached %s", url)
+    else:
+        logger.info("Downloading %s", url)
+
+    logger.debug('Downloading from URL %s', link)
+
+    downloaded_chunks = written_chunks(
+        progress_indicator(
+            resp_read(CONTENT_CHUNK_SIZE),
+            CONTENT_CHUNK_SIZE
+        )
+    )
+    if hashes:
+        hashes.check_against_chunks(downloaded_chunks)
+    else:
+        consume(downloaded_chunks)
+
+
+def _copy_file(filename, location, link):
+    copy = True
+    download_location = os.path.join(location, link.filename)
+    if os.path.exists(download_location):
+        response = ask_path_exists(
+            'The file %s exists. (i)gnore, (w)ipe, (b)ackup, (a)abort' %
+            display_path(download_location), ('i', 'w', 'b', 'a'))
+        if response == 'i':
+            copy = False
+        elif response == 'w':
+            logger.warning('Deleting %s', display_path(download_location))
+            os.remove(download_location)
+        elif response == 'b':
+            dest_file = backup_dir(download_location)
+            logger.warning(
+                'Backing up %s to %s',
+                display_path(download_location),
+                display_path(dest_file),
+            )
+            shutil.move(download_location, dest_file)
+        elif response == 'a':
+            sys.exit(-1)
+    if copy:
+        shutil.copy(filename, download_location)
+        logger.info('Saved %s', display_path(download_location))
+
+
+def unpack_http_url(
+    link,  # type: Link
+    location,  # type: str
+    download_dir=None,  # type: Optional[str]
+    session=None,  # type: Optional[PipSession]
+    hashes=None,  # type: Optional[Hashes]
+    progress_bar="on"  # type: str
+):
+    # type: (...) -> None
+    if session is None:
+        raise TypeError(
+            "unpack_http_url() missing 1 required keyword argument: 'session'"
+        )
+
+    with TempDirectory(kind="unpack") as temp_dir:
+        # If a download dir is specified, is the file already downloaded there?
+        already_downloaded_path = None
+        if download_dir:
+            already_downloaded_path = _check_download_dir(link,
+                                                          download_dir,
+                                                          hashes)
+
+        if already_downloaded_path:
+            from_path = already_downloaded_path
+            content_type = mimetypes.guess_type(from_path)[0]
+        else:
+            # let's download to a tmp dir
+            from_path, content_type = _download_http_url(link,
+                                                         session,
+                                                         temp_dir.path,
+                                                         hashes,
+                                                         progress_bar)
+
+        # unpack the archive to the build dir location. even when only
+        # downloading archives, they have to be unpacked to parse dependencies
+        unpack_file(from_path, location, content_type, link)
+
+        # a download dir is specified; let's copy the archive there
+        if download_dir and not already_downloaded_path:
+            _copy_file(from_path, download_dir, link)
+
+        if not already_downloaded_path:
+            os.unlink(from_path)
+
+
+def unpack_file_url(
+    link,  # type: Link
+    location,  # type: str
+    download_dir=None,  # type: Optional[str]
+    hashes=None  # type: Optional[Hashes]
+):
+    # type: (...) -> None
+    """Unpack link into location.
+
+    If download_dir is provided and link points to a file, make a copy
+    of the link file inside download_dir.
+    """
+    link_path = url_to_path(link.url_without_fragment)
+
+    # If it's a url to a local directory
+    if is_dir_url(link):
+        if os.path.isdir(location):
+            rmtree(location)
+        shutil.copytree(link_path, location, symlinks=True)
+        if download_dir:
+            logger.info('Link is a directory, ignoring download_dir')
+        return
+
+    # If --require-hashes is off, `hashes` is either empty, the
+    # link's embedded hash, or MissingHashes; it is required to
+    # match. If --require-hashes is on, we are satisfied by any
+    # hash in `hashes` matching: a URL-based or an option-based
+    # one; no internet-sourced hash will be in `hashes`.
+    if hashes:
+        hashes.check_against_path(link_path)
+
+    # If a download dir is specified, is the file already there and valid?
+    already_downloaded_path = None
+    if download_dir:
+        already_downloaded_path = _check_download_dir(link,
+                                                      download_dir,
+                                                      hashes)
+
+    if already_downloaded_path:
+        from_path = already_downloaded_path
+    else:
+        from_path = link_path
+
+    content_type = mimetypes.guess_type(from_path)[0]
+
+    # unpack the archive to the build dir location. even when only downloading
+    # archives, they have to be unpacked to parse dependencies
+    unpack_file(from_path, location, content_type, link)
+
+    # a download dir is specified and not already downloaded
+    if download_dir and not already_downloaded_path:
+        _copy_file(from_path, download_dir, link)
+
+
+class PipXmlrpcTransport(xmlrpc_client.Transport):
+    """Provide a `xmlrpclib.Transport` implementation via a `PipSession`
+    object.
+    """
+
+    def __init__(self, index_url, session, use_datetime=False):
+        xmlrpc_client.Transport.__init__(self, use_datetime)
+        index_parts = urllib_parse.urlparse(index_url)
+        self._scheme = index_parts.scheme
+        self._session = session
+
+    def request(self, host, handler, request_body, verbose=False):
+        parts = (self._scheme, host, handler, None, None, None)
+        url = urllib_parse.urlunparse(parts)
+        try:
+            headers = {'Content-Type': 'text/xml'}
+            response = self._session.post(url, data=request_body,
+                                          headers=headers, stream=True)
+            response.raise_for_status()
+            self.verbose = verbose
+            return self.parse_response(response.raw)
+        except requests.HTTPError as exc:
+            logger.critical(
+                "HTTP error %s while getting %s",
+                exc.response.status_code, url,
+            )
+            raise
+
+
+def unpack_url(
+    link,  # type: Link
+    location,  # type: str
+    download_dir=None,  # type: Optional[str]
+    only_download=False,  # type: bool
+    session=None,  # type: Optional[PipSession]
+    hashes=None,  # type: Optional[Hashes]
+    progress_bar="on"  # type: str
+):
+    # type: (...) -> None
+    """Unpack link.
+       If link is a VCS link:
+         if only_download, export into download_dir and ignore location
+          else unpack into location
+       for other types of link:
+         - unpack into location
+         - if download_dir, copy the file into download_dir
+         - if only_download, mark location for deletion
+
+    :param hashes: A Hashes object, one of whose embedded hashes must match,
+        or HashMismatch will be raised. If the Hashes is empty, no matches are
+        required, and unhashable types of requirements (like VCS ones, which
+        would ordinarily raise HashUnsupported) are allowed.
+    """
+    # non-editable vcs urls
+    if is_vcs_url(link):
+        unpack_vcs_link(link, location)
+
+    # file urls
+    elif is_file_url(link):
+        unpack_file_url(link, location, download_dir, hashes=hashes)
+
+    # http urls
+    else:
+        if session is None:
+            session = PipSession()
+
+        unpack_http_url(
+            link,
+            location,
+            download_dir,
+            session,
+            hashes=hashes,
+            progress_bar=progress_bar
+        )
+    if only_download:
+        write_delete_marker_file(location)
+
+
+def sanitize_content_filename(filename):
+    # type: (str) -> str
+    """
+    Sanitize the "filename" value from a Content-Disposition header.
+    """
+    return os.path.basename(filename)
+
+
+def parse_content_disposition(content_disposition, default_filename):
+    # type: (str, str) -> str
+    """
+    Parse the "filename" value from a Content-Disposition header, and
+    return the default filename if the result is empty.
+    """
+    _type, params = cgi.parse_header(content_disposition)
+    filename = params.get('filename')
+    if filename:
+        # We need to sanitize the filename to prevent directory traversal
+        # in case the filename contains ".." path parts.
+        filename = sanitize_content_filename(filename)
+    return filename or default_filename
+
+
+def _download_http_url(
+    link,  # type: Link
+    session,  # type: PipSession
+    temp_dir,  # type: str
+    hashes,  # type: Optional[Hashes]
+    progress_bar  # type: str
+):
+    # type: (...) -> Tuple[str, str]
+    """Download link url into temp_dir using provided session"""
+    target_url = link.url.split('#', 1)[0]
+    try:
+        resp = session.get(
+            target_url,
+            # We use Accept-Encoding: identity here because requests
+            # defaults to accepting compressed responses. This breaks in
+            # a variety of ways depending on how the server is configured.
+            # - Some servers will notice that the file isn't a compressible
+            #   file and will leave the file alone and with an empty
+            #   Content-Encoding
+            # - Some servers will notice that the file is already
+            #   compressed and will leave the file alone and will add a
+            #   Content-Encoding: gzip header
+            # - Some servers won't notice anything at all and will take
+            #   a file that's already been compressed and compress it again
+            #   and set the Content-Encoding: gzip header
+            # By setting this to request only the identity encoding We're
+            # hoping to eliminate the third case. Hopefully there does not
+            # exist a server which when given a file will notice it is
+            # already compressed and that you're not asking for a
+            # compressed file and will then decompress it before sending
+            # because if that's the case I don't think it'll ever be
+            # possible to make this work.
+            headers={"Accept-Encoding": "identity"},
+            stream=True,
+        )
+        resp.raise_for_status()
+    except requests.HTTPError as exc:
+        logger.critical(
+            "HTTP error %s while getting %s", exc.response.status_code, link,
+        )
+        raise
+
+    content_type = resp.headers.get('content-type', '')
+    filename = link.filename  # fallback
+    # Have a look at the Content-Disposition header for a better guess
+    content_disposition = resp.headers.get('content-disposition')
+    if content_disposition:
+        filename = parse_content_disposition(content_disposition, filename)
+    ext = splitext(filename)[1]  # type: Optional[str]
+    if not ext:
+        ext = mimetypes.guess_extension(content_type)
+        if ext:
+            filename += ext
+    if not ext and link.url != resp.url:
+        ext = os.path.splitext(resp.url)[1]
+        if ext:
+            filename += ext
+    file_path = os.path.join(temp_dir, filename)
+    with open(file_path, 'wb') as content_file:
+        _download_url(resp, link, content_file, hashes, progress_bar)
+    return file_path, content_type
+
+
+def _check_download_dir(link, download_dir, hashes):
+    # type: (Link, str, Optional[Hashes]) -> Optional[str]
+    """ Check download_dir for previously downloaded file with correct hash
+        If a correct file is found return its path else None
+    """
+    download_path = os.path.join(download_dir, link.filename)
+    if os.path.exists(download_path):
+        # If already downloaded, does its hash match?
+        logger.info('File was already downloaded %s', download_path)
+        if hashes:
+            try:
+                hashes.check_against_path(download_path)
+            except HashMismatch:
+                logger.warning(
+                    'Previously-downloaded file %s has bad hash. '
+                    'Re-downloading.',
+                    download_path
+                )
+                os.unlink(download_path)
+                return None
+        return download_path
+    return None