pip/src/pip/_internal/index.py

"""Routines related to PyPI, indexes"""

# The following comment should be removed at some point in the future.
# mypy: strict-optional=False

from __future__ import absolute_import

import cgi
import itertools
import logging
import mimetypes
import os
import re

from pip._vendor import html5lib, requests
from pip._vendor.distlib.compat import unescape
from pip._vendor.packaging import specifiers
from pip._vendor.packaging.utils import canonicalize_name
from pip._vendor.packaging.version import parse as parse_version
from pip._vendor.requests.exceptions import HTTPError, RetryError, SSLError
from pip._vendor.six.moves.urllib import parse as urllib_parse
from pip._vendor.six.moves.urllib import request as urllib_request

from pip._internal.download import is_url, url_to_path
from pip._internal.exceptions import (
    BestVersionAlreadyInstalled,
    DistributionNotFound,
    InvalidWheelFilename,
    UnsupportedWheel,
)
from pip._internal.models.candidate import InstallationCandidate
from pip._internal.models.format_control import FormatControl
from pip._internal.models.link import Link
from pip._internal.models.selection_prefs import SelectionPreferences
from pip._internal.models.target_python import TargetPython
from pip._internal.utils.logging import indent_log
from pip._internal.utils.misc import (
    ARCHIVE_EXTENSIONS,
    SUPPORTED_EXTENSIONS,
    WHEEL_EXTENSION,
    build_netloc,
    path_to_url,
    redact_auth_from_url,
)
from pip._internal.utils.packaging import check_requires_python
from pip._internal.utils.typing import MYPY_CHECK_RUNNING
from pip._internal.wheel import Wheel

if MYPY_CHECK_RUNNING:
    from typing import (
        Any, Callable, FrozenSet, Iterable, List, MutableMapping, Optional,
        Sequence, Set, Text, Tuple, Union,
    )
    import xml.etree.ElementTree
    from pip._vendor.packaging.version import _BaseVersion
    from pip._vendor.requests import Response
    from pip._internal.models.search_scope import SearchScope
    from pip._internal.req import InstallRequirement
    from pip._internal.download import PipSession
    from pip._internal.pep425tags import Pep425Tag
    from pip._internal.utils.hashes import Hashes

    BuildTag = Tuple[Any, ...]  # either empty tuple or Tuple[int, str]
    CandidateSortingKey = (
        Tuple[int, int, int, _BaseVersion, BuildTag, Optional[int]]
    )
    HTMLElement = xml.etree.ElementTree.Element


__all__ = ['FormatControl', 'BestCandidateResult', 'PackageFinder']


logger = logging.getLogger(__name__)


def _match_vcs_scheme(url):
    # type: (str) -> Optional[str]
    """Look for VCS schemes in the URL.

    Returns the matched VCS scheme, or None if there's no match.
    """
    from pip._internal.vcs import vcs
    for scheme in vcs.schemes:
        if url.lower().startswith(scheme) and url[len(scheme)] in '+:':
            return scheme
    return None


def _is_url_like_archive(url):
    # type: (str) -> bool
    """Return whether the URL looks like an archive.
    """
    filename = Link(url).filename
    for bad_ext in ARCHIVE_EXTENSIONS:
        if filename.endswith(bad_ext):
            return True
    return False


class _NotHTML(Exception):
    def __init__(self, content_type, request_desc):
        # type: (str, str) -> None
        super(_NotHTML, self).__init__(content_type, request_desc)
        self.content_type = content_type
        self.request_desc = request_desc


def _ensure_html_header(response):
    # type: (Response) -> None
    """Check the Content-Type header to ensure the response contains HTML.

    Raises `_NotHTML` if the content type is not text/html.
    """
    content_type = response.headers.get("Content-Type", "")
    if not content_type.lower().startswith("text/html"):
        raise _NotHTML(content_type, response.request.method)


class _NotHTTP(Exception):
    pass


def _ensure_html_response(url, session):
    # type: (str, PipSession) -> None
    """Send a HEAD request to the URL, and ensure the response contains HTML.

    Raises `_NotHTTP` if the URL is not available for a HEAD request, or
    `_NotHTML` if the content type is not text/html.
    """
    scheme, netloc, path, query, fragment = urllib_parse.urlsplit(url)
    if scheme not in {'http', 'https'}:
        raise _NotHTTP()

    resp = session.head(url, allow_redirects=True)
    resp.raise_for_status()

    _ensure_html_header(resp)


def _get_html_response(url, session):
    # type: (str, PipSession) -> Response
    """Access an HTML page with GET, and return the response.

    This consists of three parts:

    1. If the URL looks suspiciously like an archive, send a HEAD first to
       check the Content-Type is HTML, to avoid downloading a large file.
       Raise `_NotHTTP` if the content type cannot be determined, or
       `_NotHTML` if it is not HTML.
    2. Actually perform the request. Raise HTTP exceptions on network failures.
    3. Check the Content-Type header to make sure we got HTML, and raise
       `_NotHTML` otherwise.
    """
    if _is_url_like_archive(url):
        _ensure_html_response(url, session=session)

    logger.debug('Getting page %s', redact_auth_from_url(url))

    resp = session.get(
        url,
        headers={
            "Accept": "text/html",
            # We don't want to blindly returned cached data for
            # /simple/, because authors generally expecting that
            # twine upload && pip install will function, but if
            # they've done a pip install in the last ~10 minutes
            # it won't. Thus by setting this to zero we will not
            # blindly use any cached data, however the benefit of
            # using max-age=0 instead of no-cache, is that we will
            # still support conditional requests, so we will still
            # minimize traffic sent in cases where the page hasn't
            # changed at all, we will just always incur the round
            # trip for the conditional GET now instead of only
            # once per 10 minutes.
            # For more information, please see pypa/pip#5670.
            "Cache-Control": "max-age=0",
        },
    )
    resp.raise_for_status()

    # The check for archives above only works if the url ends with
    # something that looks like an archive. However that is not a
    # requirement of an url. Unless we issue a HEAD request on every
    # url we cannot know ahead of time for sure if something is HTML
    # or not. However we can check after we've downloaded it.
    _ensure_html_header(resp)

    return resp


def _handle_get_page_fail(
    link,  # type: Link
    reason,  # type: Union[str, Exception]
    meth=None  # type: Optional[Callable[..., None]]
):
    # type: (...) -> None
    if meth is None:
        meth = logger.debug
    meth("Could not fetch URL %s: %s - skipping", link, reason)


def _get_html_page(link, session=None):
    # type: (Link, Optional[PipSession]) -> Optional[HTMLPage]
    if session is None:
        raise TypeError(
            "_get_html_page() missing 1 required keyword argument: 'session'"
        )

    url = link.url.split('#', 1)[0]

    # Check for VCS schemes that do not support lookup as web pages.
    vcs_scheme = _match_vcs_scheme(url)
    if vcs_scheme:
        logger.debug('Cannot look at %s URL %s', vcs_scheme, link)
        return None

    # Tack index.html onto file:// URLs that point to directories
    scheme, _, path, _, _, _ = urllib_parse.urlparse(url)
    if (scheme == 'file' and os.path.isdir(urllib_request.url2pathname(path))):
        # add trailing slash if not present so urljoin doesn't trim
        # final segment
        if not url.endswith('/'):
            url += '/'
        url = urllib_parse.urljoin(url, 'index.html')
        logger.debug(' file: URL is directory, getting %s', url)

    try:
        resp = _get_html_response(url, session=session)
    except _NotHTTP:
        logger.debug(
            'Skipping page %s because it looks like an archive, and cannot '
            'be checked by HEAD.', link,
        )
    except _NotHTML as exc:
        logger.debug(
            'Skipping page %s because the %s request got Content-Type: %s',
            link, exc.request_desc, exc.content_type,
        )
    except HTTPError as exc:
        _handle_get_page_fail(link, exc)
    except RetryError as exc:
        _handle_get_page_fail(link, exc)
    except SSLError as exc:
        reason = "There was a problem confirming the ssl certificate: "
        reason += str(exc)
        _handle_get_page_fail(link, reason, meth=logger.info)
    except requests.ConnectionError as exc:
        _handle_get_page_fail(link, "connection error: %s" % exc)
    except requests.Timeout:
        _handle_get_page_fail(link, "timed out")
    else:
        return HTMLPage(resp.content, resp.url, resp.headers)
    return None


def group_locations(locations, expand_dir=False):
    # type: (Sequence[str], bool) -> Tuple[List[str], List[str]]
    """
    Divide a list of locations into two groups: "files" (archives) and "urls."

    :return: A pair of lists (files, urls).
    """
    files = []
    urls = []

    # puts the url for the given file path into the appropriate list
    def sort_path(path):
        url = path_to_url(path)
        if mimetypes.guess_type(url, strict=False)[0] == 'text/html':
            urls.append(url)
        else:
            files.append(url)

    for url in locations:

        is_local_path = os.path.exists(url)
        is_file_url = url.startswith('file:')

        if is_local_path or is_file_url:
            if is_local_path:
                path = url
            else:
                path = url_to_path(url)
            if os.path.isdir(path):
                if expand_dir:
                    path = os.path.realpath(path)
                    for item in os.listdir(path):
                        sort_path(os.path.join(path, item))
                elif is_file_url:
                    urls.append(url)
                else:
                    logger.warning(
                        "Path '{0}' is ignored: "
                        "it is a directory.".format(path),
                    )
            elif os.path.isfile(path):
                sort_path(path)
            else:
                logger.warning(
                    "Url '%s' is ignored: it is neither a file "
                    "nor a directory.", url,
                )
        elif is_url(url):
            # Only add url with clear scheme
            urls.append(url)
        else:
            logger.warning(
                "Url '%s' is ignored. It is either a non-existing "
                "path or lacks a specific scheme.", url,
            )

    return files, urls


def _check_link_requires_python(
    link,  # type: Link
    version_info,  # type: Tuple[int, int, int]
    ignore_requires_python=False,  # type: bool
):
    # type: (...) -> bool
    """
    Return whether the given Python version is compatible with a link's
    "Requires-Python" value.

    :param version_info: A 3-tuple of ints representing the Python
        major-minor-micro version to check.
    :param ignore_requires_python: Whether to ignore the "Requires-Python"
        value if the given Python version isn't compatible.
    """
    try:
        is_compatible = check_requires_python(
            link.requires_python, version_info=version_info,
        )
    except specifiers.InvalidSpecifier:
        logger.debug(
            "Ignoring invalid Requires-Python (%r) for link: %s",
            link.requires_python, link,
        )
    else:
        if not is_compatible:
            version = '.'.join(map(str, version_info))
            if not ignore_requires_python:
                logger.debug(
                    'Link requires a different Python (%s not in: %r): %s',
                    version, link.requires_python, link,
                )
                return False

            logger.debug(
                'Ignoring failed Requires-Python check (%s not in: %r) '
                'for link: %s',
                version, link.requires_python, link,
            )

    return True


class LinkEvaluator(object):

    """
    Responsible for evaluating links for a particular project.
    """

    _py_version_re = re.compile(r'-py([123]\.?[0-9]?)$')

    # Don't include an allow_yanked default value to make sure each call
    # site considers whether yanked releases are allowed. This also causes
    # that decision to be made explicit in the calling code, which helps
    # people when reading the code.
    def __init__(
        self,
        project_name,    # type: str
        canonical_name,  # type: str
        formats,         # type: FrozenSet
        target_python,   # type: TargetPython
        allow_yanked,    # type: bool
        ignore_requires_python=None,  # type: Optional[bool]
    ):
        # type: (...) -> None
        """
        :param project_name: The user supplied package name.
        :param canonical_name: The canonical package name.
        :param formats: The formats allowed for this package. Should be a set
            with 'binary' or 'source' or both in it.
        :param target_python: The target Python interpreter to use when
            evaluating link compatibility. This is used, for example, to
            check wheel compatibility, as well as when checking the Python
            version, e.g. the Python version embedded in a link filename
            (or egg fragment) and against an HTML link's optional PEP 503
            "data-requires-python" attribute.
        :param allow_yanked: Whether files marked as yanked (in the sense
            of PEP 592) are permitted to be candidates for install.
        :param ignore_requires_python: Whether to ignore incompatible
            PEP 503 "data-requires-python" values in HTML links. Defaults
            to False.
        """
        if ignore_requires_python is None:
            ignore_requires_python = False

        self._allow_yanked = allow_yanked
        self._canonical_name = canonical_name
        self._ignore_requires_python = ignore_requires_python
        self._formats = formats
        self._target_python = target_python

        self.project_name = project_name

    def evaluate_link(self, link):
        # type: (Link) -> Tuple[bool, Optional[Text]]
        """
        Determine whether a link is a candidate for installation.

        :return: A tuple (is_candidate, result), where `result` is (1) a
            version string if `is_candidate` is True, and (2) if
            `is_candidate` is False, an optional string to log the reason
            the link fails to qualify.
        """
        version = None
        if link.is_yanked and not self._allow_yanked:
            reason = link.yanked_reason or '<none given>'
            # Mark this as a unicode string to prevent "UnicodeEncodeError:
            # 'ascii' codec can't encode character" in Python 2 when
            # the reason contains non-ascii characters.
            return (False, u'yanked for reason: {}'.format(reason))

        if link.egg_fragment:
            egg_info = link.egg_fragment
            ext = link.ext
        else:
            egg_info, ext = link.splitext()
            if not ext:
                return (False, 'not a file')
            if ext not in SUPPORTED_EXTENSIONS:
                return (False, 'unsupported archive format: %s' % ext)
            if "binary" not in self._formats and ext == WHEEL_EXTENSION:
                reason = 'No binaries permitted for %s' % self.project_name
                return (False, reason)
            if "macosx10" in link.path and ext == '.zip':
                return (False, 'macosx10 one')
            if ext == WHEEL_EXTENSION:
                try:
                    wheel = Wheel(link.filename)
                except InvalidWheelFilename:
                    return (False, 'invalid wheel filename')
                if canonicalize_name(wheel.name) != self._canonical_name:
                    reason = 'wrong project name (not %s)' % self.project_name
                    return (False, reason)

                supported_tags = self._target_python.get_tags()
                if not wheel.supported(supported_tags):
                    # Include the wheel's tags in the reason string to
                    # simplify troubleshooting compatibility issues.
                    file_tags = wheel.get_formatted_file_tags()
                    reason = (
                        "none of the wheel's tags match: {}".format(
                            ', '.join(file_tags)
                        )
                    )
                    return (False, reason)

                version = wheel.version

        # This should be up by the self.ok_binary check, but see issue 2700.
        if "source" not in self._formats and ext != WHEEL_EXTENSION:
            return (False, 'No sources permitted for %s' % self.project_name)

        if not version:
            version = _extract_version_from_fragment(
                egg_info, self._canonical_name,
            )
        if not version:
            return (
                False, 'Missing project version for %s' % self.project_name,
            )

        match = self._py_version_re.search(version)
        if match:
            version = version[:match.start()]
            py_version = match.group(1)
            if py_version != self._target_python.py_version:
                return (False, 'Python version is incorrect')

        supports_python = _check_link_requires_python(
            link, version_info=self._target_python.py_version_info,
            ignore_requires_python=self._ignore_requires_python,
        )
        if not supports_python:
            # Return None for the reason text to suppress calling
            # _log_skipped_link().
            return (False, None)

        logger.debug('Found link %s, version: %s', link, version)

        return (True, version)


def filter_unallowed_hashes(
    candidates,    # type: List[InstallationCandidate]
    hashes,        # type: Hashes
    project_name,  # type: str
):
    # type: (...) -> List[InstallationCandidate]
    """
    Filter out candidates whose hashes aren't allowed, and return a new
    list of candidates.

    If at least one candidate has an allowed hash, then all candidates with
    either an allowed hash or no hash specified are returned.  Otherwise,
    the given candidates are returned.

    Including the candidates with no hash specified when there is a match
    allows a warning to be logged if there is a more preferred candidate
    with no hash specified.  Returning all candidates in the case of no
    matches lets pip report the hash of the candidate that would otherwise
    have been installed (e.g. permitting the user to more easily update
    their requirements file with the desired hash).
    """
    if not hashes:
        logger.debug(
            'Given no hashes to check %s links for project %r: '
            'discarding no candidates',
            len(candidates),
            project_name,
        )
        # Make sure we're not returning back the given value.
        return list(candidates)

    matches_or_no_digest = []
    # Collect the non-matches for logging purposes.
    non_matches = []
    match_count = 0
    for candidate in candidates:
        link = candidate.link
        if not link.has_hash:
            pass
        elif link.is_hash_allowed(hashes=hashes):
            match_count += 1
        else:
            non_matches.append(candidate)
            continue

        matches_or_no_digest.append(candidate)

    if match_count:
        filtered = matches_or_no_digest
    else:
        # Make sure we're not returning back the given value.
        filtered = list(candidates)

    if len(filtered) == len(candidates):
        discard_message = 'discarding no candidates'
    else:
        discard_message = 'discarding {} non-matches:\n  {}'.format(
            len(non_matches),
            '\n  '.join(str(candidate.link) for candidate in non_matches)
        )

    logger.debug(
        'Checked %s links for project %r against %s hashes '
        '(%s matches, %s no digest): %s',
        len(candidates),
        project_name,
        hashes.digest_count,
        match_count,
        len(matches_or_no_digest) - match_count,
        discard_message
    )

    return filtered


class CandidatePreferences(object):

    """
    Encapsulates some of the preferences for filtering and sorting
    InstallationCandidate objects.
    """

    def __init__(
        self,
        prefer_binary=False,  # type: bool
        allow_all_prereleases=False,  # type: bool
    ):
        # type: (...) -> None
        """
        :param allow_all_prereleases: Whether to allow all pre-releases.
        """
        self.allow_all_prereleases = allow_all_prereleases
        self.prefer_binary = prefer_binary


class BestCandidateResult(object):
    """A collection of candidates, returned by `PackageFinder.find_best_candidate`.

    This class is only intended to be instantiated by CandidateEvaluator's
    `compute_best_candidate()` method.
    """

    def __init__(
        self,
        candidates,             # type: List[InstallationCandidate]
        applicable_candidates,  # type: List[InstallationCandidate]
        best_candidate,         # type: Optional[InstallationCandidate]
    ):
        # type: (...) -> None
        """
        :param candidates: A sequence of all available candidates found.
        :param applicable_candidates: The applicable candidates.
        :param best_candidate: The most preferred candidate found, or None
            if no applicable candidates were found.
        """
        assert set(applicable_candidates) <= set(candidates)

        if best_candidate is None:
            assert not applicable_candidates
        else:
            assert best_candidate in applicable_candidates

        self._applicable_candidates = applicable_candidates
        self._candidates = candidates

        self.best_candidate = best_candidate

    def iter_all(self):
        # type: () -> Iterable[InstallationCandidate]
        """Iterate through all candidates.
        """
        return iter(self._candidates)

    def iter_applicable(self):
        # type: () -> Iterable[InstallationCandidate]
        """Iterate through the applicable candidates.
        """
        return iter(self._applicable_candidates)


class CandidateEvaluator(object):

    """
    Responsible for filtering and sorting candidates for installation based
    on what tags are valid.
    """

    @classmethod
    def create(
        cls,
        project_name,         # type: str
        target_python=None,   # type: Optional[TargetPython]
        prefer_binary=False,  # type: bool
        allow_all_prereleases=False,  # type: bool
        specifier=None,       # type: Optional[specifiers.BaseSpecifier]
        hashes=None,          # type: Optional[Hashes]
    ):
        # type: (...) -> CandidateEvaluator
        """Create a CandidateEvaluator object.

        :param target_python: The target Python interpreter to use when
            checking compatibility. If None (the default), a TargetPython
            object will be constructed from the running Python.
        :param specifier: An optional object implementing `filter`
            (e.g. `packaging.specifiers.SpecifierSet`) to filter applicable
            versions.
        :param hashes: An optional collection of allowed hashes.
        """
        if target_python is None:
            target_python = TargetPython()
        if specifier is None:
            specifier = specifiers.SpecifierSet()

        supported_tags = target_python.get_tags()

        return cls(
            project_name=project_name,
            supported_tags=supported_tags,
            specifier=specifier,
            prefer_binary=prefer_binary,
            allow_all_prereleases=allow_all_prereleases,
            hashes=hashes,
        )

    def __init__(
        self,
        project_name,         # type: str
        supported_tags,       # type: List[Pep425Tag]
        specifier,            # type: specifiers.BaseSpecifier
        prefer_binary=False,  # type: bool
        allow_all_prereleases=False,  # type: bool
        hashes=None,                  # type: Optional[Hashes]
    ):
        # type: (...) -> None
        """
        :param supported_tags: The PEP 425 tags supported by the target
            Python in order of preference (most preferred first).
        """
        self._allow_all_prereleases = allow_all_prereleases
        self._hashes = hashes
        self._prefer_binary = prefer_binary
        self._project_name = project_name
        self._specifier = specifier
        self._supported_tags = supported_tags

    def get_applicable_candidates(
        self,
        candidates,  # type: List[InstallationCandidate]
    ):
        # type: (...) -> List[InstallationCandidate]
        """
        Return the applicable candidates from a list of candidates.
        """
        # Using None infers from the specifier instead.
        allow_prereleases = self._allow_all_prereleases or None
        specifier = self._specifier
        versions = {
            str(v) for v in specifier.filter(
                # We turn the version object into a str here because otherwise
                # when we're debundled but setuptools isn't, Python will see
                # packaging.version.Version and
                # pkg_resources._vendor.packaging.version.Version as different
                # types. This way we'll use a str as a common data interchange
                # format. If we stop using the pkg_resources provided specifier
                # and start using our own, we can drop the cast to str().
                (str(c.version) for c in candidates),
                prereleases=allow_prereleases,
            )
        }

        # Again, converting version to str to deal with debundling.
        applicable_candidates = [
            c for c in candidates if str(c.version) in versions
        ]

        return filter_unallowed_hashes(
            candidates=applicable_candidates,
            hashes=self._hashes,
            project_name=self._project_name,
        )

    def _sort_key(self, candidate):
        # type: (InstallationCandidate) -> CandidateSortingKey
        """
        Function to pass as the `key` argument to a call to sorted() to sort
        InstallationCandidates by preference.

        Returns a tuple such that tuples sorting as greater using Python's
        default comparison operator are more preferred.

        The preference is as follows:

        First and foremost, candidates with allowed (matching) hashes are
        always preferred over candidates without matching hashes. This is
        because e.g. if the only candidate with an allowed hash is yanked,
        we still want to use that candidate.

        Second, excepting hash considerations, candidates that have been
        yanked (in the sense of PEP 592) are always less preferred than
        candidates that haven't been yanked. Then:

        If not finding wheels, they are sorted by version only.
        If finding wheels, then the sort order is by version, then:
          1. existing installs
          2. wheels ordered via Wheel.support_index_min(self._supported_tags)
          3. source archives
        If prefer_binary was set, then all wheels are sorted above sources.

        Note: it was considered to embed this logic into the Link
              comparison operators, but then different sdist links
              with the same version, would have to be considered equal
        """
        valid_tags = self._supported_tags
        support_num = len(valid_tags)
        build_tag = tuple()  # type: BuildTag
        binary_preference = 0
        link = candidate.link
        if link.is_wheel:
            # can raise InvalidWheelFilename
            wheel = Wheel(link.filename)
            if not wheel.supported(valid_tags):
                raise UnsupportedWheel(
                    "%s is not a supported wheel for this platform. It "
                    "can't be sorted." % wheel.filename
                )
            if self._prefer_binary:
                binary_preference = 1
            pri = -(wheel.support_index_min(valid_tags))
            if wheel.build_tag is not None:
                match = re.match(r'^(\d+)(.*)$', wheel.build_tag)
                build_tag_groups = match.groups()
                build_tag = (int(build_tag_groups[0]), build_tag_groups[1])
        else:  # sdist
            pri = -(support_num)
        has_allowed_hash = int(link.is_hash_allowed(self._hashes))
        yank_value = -1 * int(link.is_yanked)  # -1 for yanked.
        return (
            has_allowed_hash, yank_value, binary_preference, candidate.version,
            build_tag, pri,
        )

    def sort_best_candidate(
        self,
        candidates,    # type: List[InstallationCandidate]
    ):
        # type: (...) -> Optional[InstallationCandidate]
        """
        Return the best candidate per the instance's sort order, or None if
        no candidate is acceptable.
        """
        if not candidates:
            return None

        best_candidate = max(candidates, key=self._sort_key)

        # Log a warning per PEP 592 if necessary before returning.
        link = best_candidate.link
        if link.is_yanked:
            reason = link.yanked_reason or '<none given>'
            msg = (
                # Mark this as a unicode string to prevent
                # "UnicodeEncodeError: 'ascii' codec can't encode character"
                # in Python 2 when the reason contains non-ascii characters.
                u'The candidate selected for download or install is a '
                'yanked version: {candidate}\n'
                'Reason for being yanked: {reason}'
            ).format(candidate=best_candidate, reason=reason)
            logger.warning(msg)

        return best_candidate

    def compute_best_candidate(
        self,
        candidates,      # type: List[InstallationCandidate]
    ):
        # type: (...) -> BestCandidateResult
        """
        Compute and return a `BestCandidateResult` instance.
        """
        applicable_candidates = self.get_applicable_candidates(candidates)

        best_candidate = self.sort_best_candidate(applicable_candidates)

        return BestCandidateResult(
            candidates,
            applicable_candidates=applicable_candidates,
            best_candidate=best_candidate,
        )


class PackageFinder(object):
    """This finds packages.

    This is meant to match easy_install's technique for looking for
    packages, by reading pages and looking for appropriate links.
    """

    def __init__(
        self,
        search_scope,         # type: SearchScope
        session,  # type: PipSession
        target_python,        # type: TargetPython
        allow_yanked,         # type: bool
        format_control=None,  # type: Optional[FormatControl]
        candidate_prefs=None,         # type: CandidatePreferences
        ignore_requires_python=None,  # type: Optional[bool]
    ):
        # type: (...) -> None
        """
        This constructor is primarily meant to be used by the create() class
        method and from tests.

        :param session: The Session to use to make requests.
        :param format_control: A FormatControl object, used to control
            the selection of source packages / binary packages when consulting
            the index and links.
        :param candidate_prefs: Options to use when creating a
            CandidateEvaluator object.
        """
        if candidate_prefs is None:
            candidate_prefs = CandidatePreferences()

        format_control = format_control or FormatControl(set(), set())

        self._allow_yanked = allow_yanked
        self._candidate_prefs = candidate_prefs
        self._ignore_requires_python = ignore_requires_python
        self._target_python = target_python

        self.search_scope = search_scope
        self.session = session
        self.format_control = format_control

        # These are boring links that have already been logged somehow.
        self._logged_links = set()  # type: Set[Link]

    # Don't include an allow_yanked default value to make sure each call
    # site considers whether yanked releases are allowed. This also causes
    # that decision to be made explicit in the calling code, which helps
    # people when reading the code.
    @classmethod
    def create(
        cls,
        search_scope,  # type: SearchScope
        selection_prefs,     # type: SelectionPreferences
        session=None,        # type: Optional[PipSession]
        target_python=None,  # type: Optional[TargetPython]
    ):
        # type: (...) -> PackageFinder
        """Create a PackageFinder.

        :param selection_prefs: The candidate selection preferences, as a
            SelectionPreferences object.
        :param session: The Session to use to make requests.
        :param target_python: The target Python interpreter to use when
            checking compatibility. If None (the default), a TargetPython
            object will be constructed from the running Python.
        """
        if session is None:
            raise TypeError(
                "PackageFinder.create() missing 1 required keyword argument: "
                "'session'"
            )
        if target_python is None:
            target_python = TargetPython()

        candidate_prefs = CandidatePreferences(
            prefer_binary=selection_prefs.prefer_binary,
            allow_all_prereleases=selection_prefs.allow_all_prereleases,
        )

        return cls(
            candidate_prefs=candidate_prefs,
            search_scope=search_scope,
            session=session,
            target_python=target_python,
            allow_yanked=selection_prefs.allow_yanked,
            format_control=selection_prefs.format_control,
            ignore_requires_python=selection_prefs.ignore_requires_python,
        )

    @property
    def find_links(self):
        # type: () -> List[str]
        return self.search_scope.find_links

    @property
    def index_urls(self):
        # type: () -> List[str]
        return self.search_scope.index_urls

    @property
    def trusted_hosts(self):
        # type: () -> Iterable[str]
        for host_port in self.session.pip_trusted_origins:
            yield build_netloc(*host_port)

    @property
    def allow_all_prereleases(self):
        # type: () -> bool
        return self._candidate_prefs.allow_all_prereleases

    def set_allow_all_prereleases(self):
        # type: () -> None
        self._candidate_prefs.allow_all_prereleases = True

    def make_link_evaluator(self, project_name):
        # type: (str) -> LinkEvaluator
        canonical_name = canonicalize_name(project_name)
        formats = self.format_control.get_allowed_formats(canonical_name)

        return LinkEvaluator(
            project_name=project_name,
            canonical_name=canonical_name,
            formats=formats,
            target_python=self._target_python,
            allow_yanked=self._allow_yanked,
            ignore_requires_python=self._ignore_requires_python,
        )

    def _sort_links(self, links):
        # type: (Iterable[Link]) -> List[Link]
        """
        Returns elements of links in order, non-egg links first, egg links
        second, while eliminating duplicates
        """
        eggs, no_eggs = [], []
        seen = set()  # type: Set[Link]
        for link in links:
            if link not in seen:
                seen.add(link)
                if link.egg_fragment:
                    eggs.append(link)
                else:
                    no_eggs.append(link)
        return no_eggs + eggs

    def _log_skipped_link(self, link, reason):
        # type: (Link, Text) -> None
        if link not in self._logged_links:
            # Mark this as a unicode string to prevent "UnicodeEncodeError:
            # 'ascii' codec can't encode character" in Python 2 when
            # the reason contains non-ascii characters.
            #   Also, put the link at the end so the reason is more visible
            # and because the link string is usually very long.
            logger.debug(u'Skipping link: %s: %s', reason, link)
            self._logged_links.add(link)

    def get_install_candidate(self, link_evaluator, link):
        # type: (LinkEvaluator, Link) -> Optional[InstallationCandidate]
        """
        If the link is a candidate for install, convert it to an
        InstallationCandidate and return it. Otherwise, return None.
        """
        is_candidate, result = link_evaluator.evaluate_link(link)
        if not is_candidate:
            if result:
                self._log_skipped_link(link, reason=result)
            return None

        return InstallationCandidate(
            project=link_evaluator.project_name,
            link=link,
            # Convert the Text result to str since InstallationCandidate
            # accepts str.
            version=str(result),
        )

    def evaluate_links(self, link_evaluator, links):
        # type: (LinkEvaluator, Iterable[Link]) -> List[InstallationCandidate]
        """
        Convert links that are candidates to InstallationCandidate objects.
        """
        candidates = []
        for link in self._sort_links(links):
            candidate = self.get_install_candidate(link_evaluator, link)
            if candidate is not None:
                candidates.append(candidate)

        return candidates

    def find_all_candidates(self, project_name):
        # type: (str) -> List[InstallationCandidate]
        """Find all available InstallationCandidate for project_name

        This checks index_urls and find_links.
        All versions found are returned as an InstallationCandidate list.

        See LinkEvaluator.evaluate_link() for details on which files
        are accepted.
        """
        search_scope = self.search_scope
        index_locations = search_scope.get_index_urls_locations(project_name)
        index_file_loc, index_url_loc = group_locations(index_locations)
        fl_file_loc, fl_url_loc = group_locations(
            self.find_links, expand_dir=True,
        )

        file_locations = (Link(url) for url in itertools.chain(
            index_file_loc, fl_file_loc,
        ))

        # We trust every url that the user has given us whether it was given
        #   via --index-url or --find-links.
        # We want to filter out any thing which does not have a secure origin.
        url_locations = [
            link for link in itertools.chain(
                (Link(url) for url in index_url_loc),
                (Link(url) for url in fl_url_loc),
            )
            if self.session.is_secure_origin(link)
        ]

        logger.debug('%d location(s) to search for versions of %s:',
                     len(url_locations), project_name)

        for location in url_locations:
            logger.debug('* %s', location)

        link_evaluator = self.make_link_evaluator(project_name)
        find_links_versions = self.evaluate_links(
            link_evaluator,
            # We trust every directly linked archive in find_links
            links=(Link(url, '-f') for url in self.find_links),
        )

        page_versions = []
        for page in self._get_pages(url_locations, project_name):
            logger.debug('Analyzing links from page %s', page.url)
            with indent_log():
                new_versions = self.evaluate_links(
                    link_evaluator,
                    links=page.iter_links(),
                )
                page_versions.extend(new_versions)

        file_versions = self.evaluate_links(
            link_evaluator,
            links=file_locations,
        )
        if file_versions:
            file_versions.sort(reverse=True)
            logger.debug(
                'Local files found: %s',
                ', '.join([
                    url_to_path(candidate.link.url)
                    for candidate in file_versions
                ])
            )

        # This is an intentional priority ordering
        return file_versions + find_links_versions + page_versions

    def make_candidate_evaluator(
        self,
        project_name,    # type: str
        specifier=None,  # type: Optional[specifiers.BaseSpecifier]
        hashes=None,     # type: Optional[Hashes]
    ):
        # type: (...) -> CandidateEvaluator
        """Create a CandidateEvaluator object to use.
        """
        candidate_prefs = self._candidate_prefs
        return CandidateEvaluator.create(
            project_name=project_name,
            target_python=self._target_python,
            prefer_binary=candidate_prefs.prefer_binary,
            allow_all_prereleases=candidate_prefs.allow_all_prereleases,
            specifier=specifier,
            hashes=hashes,
        )

    def find_best_candidate(
        self,
        project_name,       # type: str
        specifier=None,     # type: Optional[specifiers.BaseSpecifier]
        hashes=None,        # type: Optional[Hashes]
    ):
        # type: (...) -> BestCandidateResult
        """Find matches for the given project and specifier.

        :param specifier: An optional object implementing `filter`
            (e.g. `packaging.specifiers.SpecifierSet`) to filter applicable
            versions.

        :return: A `BestCandidateResult` instance.
        """
        candidates = self.find_all_candidates(project_name)
        candidate_evaluator = self.make_candidate_evaluator(
            project_name=project_name,
            specifier=specifier,
            hashes=hashes,
        )
        return candidate_evaluator.compute_best_candidate(candidates)

    def find_requirement(self, req, upgrade):
        # type: (InstallRequirement, bool) -> Optional[Link]
        """Try to find a Link matching req

        Expects req, an InstallRequirement and upgrade, a boolean
        Returns a Link if found,
        Raises DistributionNotFound or BestVersionAlreadyInstalled otherwise
        """
        hashes = req.hashes(trust_internet=False)
        best_candidate_result = self.find_best_candidate(
            req.name, specifier=req.specifier, hashes=hashes,
        )
        best_candidate = best_candidate_result.best_candidate

        installed_version = None    # type: Optional[_BaseVersion]
        if req.satisfied_by is not None:
            installed_version = parse_version(req.satisfied_by.version)

        def _format_versions(cand_iter):
            # This repeated parse_version and str() conversion is needed to
            # handle different vendoring sources from pip and pkg_resources.
            # If we stop using the pkg_resources provided specifier and start
            # using our own, we can drop the cast to str().
            return ", ".join(sorted(
                {str(c.version) for c in cand_iter},
                key=parse_version,
            )) or "none"

        if installed_version is None and best_candidate is None:
            logger.critical(
                'Could not find a version that satisfies the requirement %s '
                '(from versions: %s)',
                req,
                _format_versions(best_candidate_result.iter_all()),
            )

            raise DistributionNotFound(
                'No matching distribution found for %s' % req
            )

        best_installed = False
        if installed_version and (
                best_candidate is None or
                best_candidate.version <= installed_version):
            best_installed = True

        if not upgrade and installed_version is not None:
            if best_installed:
                logger.debug(
                    'Existing installed version (%s) is most up-to-date and '
                    'satisfies requirement',
                    installed_version,
                )
            else:
                logger.debug(
                    'Existing installed version (%s) satisfies requirement '
                    '(most up-to-date version is %s)',
                    installed_version,
                    best_candidate.version,
                )
            return None

        if best_installed:
            # We have an existing version, and its the best version
            logger.debug(
                'Installed version (%s) is most up-to-date (past versions: '
                '%s)',
                installed_version,
                _format_versions(best_candidate_result.iter_applicable()),
            )
            raise BestVersionAlreadyInstalled

        logger.debug(
            'Using version %s (newest of versions: %s)',
            best_candidate.version,
            _format_versions(best_candidate_result.iter_applicable()),
        )
        return best_candidate.link

    def _get_pages(self, locations, project_name):
        # type: (Iterable[Link], str) -> Iterable[HTMLPage]
        """
        Yields (page, page_url) from the given locations, skipping
        locations that have errors.
        """
        seen = set()  # type: Set[Link]
        for location in locations:
            if location in seen:
                continue
            seen.add(location)

            page = _get_html_page(location, session=self.session)
            if page is None:
                continue

            yield page


def _find_name_version_sep(fragment, canonical_name):
    # type: (str, str) -> int
    """Find the separator's index based on the package's canonical name.

    :param fragment: A <package>+<version> filename "fragment" (stem) or
        egg fragment.
    :param canonical_name: The package's canonical name.

    This function is needed since the canonicalized name does not necessarily
    have the same length as the egg info's name part. An example::

    >>> fragment = 'foo__bar-1.0'
    >>> canonical_name = 'foo-bar'
    >>> _find_name_version_sep(fragment, canonical_name)
    8
    """
    # Project name and version must be separated by one single dash. Find all
    # occurrences of dashes; if the string in front of it matches the canonical
    # name, this is the one separating the name and version parts.
    for i, c in enumerate(fragment):
        if c != "-":
            continue
        if canonicalize_name(fragment[:i]) == canonical_name:
            return i
    raise ValueError("{} does not match {}".format(fragment, canonical_name))


def _extract_version_from_fragment(fragment, canonical_name):
    # type: (str, str) -> Optional[str]
    """Parse the version string from a <package>+<version> filename
    "fragment" (stem) or egg fragment.

    :param fragment: The string to parse. E.g. foo-2.1
    :param canonical_name: The canonicalized name of the package this
        belongs to.
    """
    try:
        version_start = _find_name_version_sep(fragment, canonical_name) + 1
    except ValueError:
        return None
    version = fragment[version_start:]
    if not version:
        return None
    return version


def _determine_base_url(document, page_url):
    """Determine the HTML document's base URL.

    This looks for a ``<base>`` tag in the HTML document. If present, its href
    attribute denotes the base URL of anchor tags in the document. If there is
    no such tag (or if it does not have a valid href attribute), the HTML
    file's URL is used as the base URL.

    :param document: An HTML document representation. The current
        implementation expects the result of ``html5lib.parse()``.
    :param page_url: The URL of the HTML document.
    """
    for base in document.findall(".//base"):
        href = base.get("href")
        if href is not None:
            return href
    return page_url


def _get_encoding_from_headers(headers):
    """Determine if we have any encoding information in our headers.
    """
    if headers and "Content-Type" in headers:
        content_type, params = cgi.parse_header(headers["Content-Type"])
        if "charset" in params:
            return params['charset']
    return None


def _clean_link(url):
    # type: (str) -> str
    """Makes sure a link is fully encoded.  That is, if a ' ' shows up in
    the link, it will be rewritten to %20 (while not over-quoting
    % or other characters)."""
    # Split the URL into parts according to the general structure
    # `scheme://netloc/path;parameters?query#fragment`. Note that the
    # `netloc` can be empty and the URI will then refer to a local
    # filesystem path.
    result = urllib_parse.urlparse(url)
    # In both cases below we unquote prior to quoting to make sure
    # nothing is double quoted.
    if result.netloc == "":
        # On Windows the path part might contain a drive letter which
        # should not be quoted. On Linux where drive letters do not
        # exist, the colon should be quoted. We rely on urllib.request
        # to do the right thing here.
        path = urllib_request.pathname2url(
            urllib_request.url2pathname(result.path))
    else:
        # In addition to the `/` character we protect `@` so that
        # revision strings in VCS URLs are properly parsed.
        path = urllib_parse.quote(urllib_parse.unquote(result.path), safe="/@")
    return urllib_parse.urlunparse(result._replace(path=path))


def _create_link_from_element(
    anchor,    # type: HTMLElement
    page_url,  # type: str
    base_url,  # type: str
):
    # type: (...) -> Optional[Link]
    """
    Convert an anchor element in a simple repository page to a Link.
    """
    href = anchor.get("href")
    if not href:
        return None

    url = _clean_link(urllib_parse.urljoin(base_url, href))
    pyrequire = anchor.get('data-requires-python')
    pyrequire = unescape(pyrequire) if pyrequire else None

    yanked_reason = anchor.get('data-yanked')
    if yanked_reason:
        # This is a unicode string in Python 2 (and 3).
        yanked_reason = unescape(yanked_reason)

    link = Link(
        url,
        comes_from=page_url,
        requires_python=pyrequire,
        yanked_reason=yanked_reason,
    )

    return link


class HTMLPage(object):
    """Represents one page, along with its URL"""

    def __init__(self, content, url, headers=None):
        # type: (bytes, str, MutableMapping[str, str]) -> None
        self.content = content
        self.url = url
        self.headers = headers

    def __str__(self):
        return redact_auth_from_url(self.url)

    def iter_links(self):
        # type: () -> Iterable[Link]
        """Yields all links in the page"""
        document = html5lib.parse(
            self.content,
            transport_encoding=_get_encoding_from_headers(self.headers),
            namespaceHTMLElements=False,
        )
        base_url = _determine_base_url(document, self.url)
        for anchor in document.findall(".//a"):
            link = _create_link_from_element(
                anchor,
                page_url=self.url,
                base_url=base_url,
            )
            if link is None:
                continue
            yield link