Finding and choosing files (index and PackageFinder)

The pip._internal.index sub-package in pip is responsible for deciding what file to download and from where, given a requirement for a project. The package’s functionality is largely exposed through and coordinated by the package’s PackageFinder class.


Here is a rough description of the process that pip uses to choose what file to download for a package, given a requirement:

  1. Collect together the various network and file system locations containing project package files. These locations are derived, for example, from pip’s --index-url (with default ) setting and any configured --extra-index-url locations. Each of the project page URL’s is an HTML page of anchor links, as defined in PEP 503, the “Simple Repository API.”

  2. For each project page URL, fetch the HTML and parse out the anchor links, creating a Link object from each one. The LinkCollector class is responsible for both the previous step and fetching the HTML over the network.

  3. Determine which of the links are minimally relevant, using the LinkEvaluator class. Create an InstallationCandidate object (aka candidate for install) for each of these relevant links.

  4. Further filter the collection of InstallationCandidate objects (using the CandidateEvaluator class) to a collection of “applicable” candidates.

  5. If there are applicable candidates, choose the best candidate by sorting them (again using the CandidateEvaluator class).

The remainder of this section is organized by documenting some of the classes inside the index package, in the following order:

The PackageFinder class

The PackageFinder class is the primary way through which code in pip interacts with index package. It is an umbrella class that encapsulates and groups together various package-finding functionality.

The PackageFinder class is responsible for searching the network and file system for what versions of a package pip can install, and also for deciding which version is most preferred, given the user’s preferences, target Python environment, etc.

The pip commands that use the PackageFinder class are:

The pip commands requiring use of the PackageFinder class generally instantiate PackageFinder only once for the whole pip invocation. In fact, pip creates this PackageFinder instance when command options are first parsed.

With the exception of pip list, each of the above commands is implemented as a Command class inheriting from RequirementCommand (for example pip download is implemented by DownloadCommand), and the PackageFinder instance is created by calling the RequirementCommand class’s _build_package_finder() method. pip list, on the other hand, constructs its PackageFinder instance by calling the ListCommand class’s _build_package_finder(). (This difference may simply be historical and may not actually be necessary.)

Each of these commands also uses the PackageFinder class for pip’s “self-check,” (i.e. to check whether a pip upgrade is available). In this case, the PackageFinder instance is created by the module’s pip_self_version_check() function.

The PackageFinder class is responsible for doing all of the things listed in the Overview section like fetching and parsing PEP 503 simple repository HTML pages, evaluating which links in the simple repository pages are relevant for each requirement, and further filtering and sorting by preference the candidates for install coming from the relevant links.

One of PackageFinder’s main top-level methods is find_best_candidate(). This method does the following two things:

  1. Calls its find_all_candidates() method, which gathers all possible package links by reading and parsing the index URL’s and locations provided by the user (the LinkCollector class’s collect_links() method), constructs a LinkEvaluator object to filter out some of those links, and then returns a list of InstallationCandidates (aka candidates for install). This corresponds to steps 1-3 of the Overview above.

  2. Constructs a CandidateEvaluator object and uses that to determine the best candidate. It does this by calling the CandidateEvaluator class’s compute_best_candidate() method on the return value of find_all_candidates(). This corresponds to steps 4-5 of the Overview.

PackageFinder also has a process_project_url() method (called by find_best_candidate()) to process a PEP 503 “simple repository” project page. This method fetches and parses the HTML from a PEP 503 project page URL, extracts the anchor elements and creates Link objects from them, and then evaluates those links.

The LinkCollector class

The LinkCollector class is the class responsible for collecting the raw list of “links” to package files (represented as Link objects) from file system locations, as well as the PEP 503 project page URL’s that PackageFinder should access.

The LinkCollector class takes into account the user’s --find-links, --extra-index-url, and related options when deciding which locations to collect links from. The class’s main method is the collect_links() method. The PackageFinder class invokes this method as the first step of its find_all_candidates() method.

LinkCollector also has a fetch_page() method to fetch the HTML from a project page URL. This method is “unintelligent” in that it doesn’t parse the HTML.

The LinkCollector class is the only class in the index sub-package that makes network requests and is the only class in the sub-package that depends directly on PipSession, which stores pip’s configuration options and state for making requests.

The LinkEvaluator class

The LinkEvaluator class contains the business logic for determining whether a link (e.g. in a simple repository page) satisfies minimal conditions to be a candidate for install (resulting in an InstallationCandidate object). When making this determination, the LinkEvaluator instance uses information like the target Python interpreter as well as user preferences like whether binary files are allowed or preferred, etc.

Specifically, the LinkEvaluator class has an evaluate_link() method that returns whether a link is a candidate for install.

Instances of this class are created by the PackageFinder class’s make_link_evaluator() on a per-requirement basis.

The CandidateEvaluator class

The CandidateEvaluator class contains the business logic for evaluating which InstallationCandidate objects should be preferred. This can be viewed as a determination that is finer-grained than that performed by the LinkEvaluator class.

In particular, the CandidateEvaluator class uses the whole set of InstallationCandidate objects when making its determinations, as opposed to evaluating each candidate in isolation, as LinkEvaluator does. For example, whether a pre-release is eligible for selection or whether a file whose hash doesn’t match is eligible depends on properties of the collection as a whole.

The CandidateEvaluator class uses information like the list of PEP 425 tags compatible with the target Python interpreter, hashes provided by the user, and other user preferences, etc.

Specifically, the class has a get_applicable_candidates() method. This accepts the InstallationCandidate objects resulting from the links accepted by the LinkEvaluator class’s evaluate_link() method, filters them to a list of “applicable” candidates and orders them by preference.

The CandidateEvaluator class also has a sort_best_candidate() method that returns the best (i.e. most preferred) candidate.

Finally, the class has a compute_best_candidate() method that calls get_applicable_candidates() followed by sort_best_candidate(), and then returning a BestCandidateResult object encapsulating both the intermediate and final results of the decision.

Instances of CandidateEvaluator are created by the PackageFinder class’s make_candidate_evaluator() method on a per-requirement basis.

The CandidatePreferences class

The CandidatePreferences class is a simple container class that groups together some of the user preferences that PackageFinder uses to construct CandidateEvaluator objects (via the PackageFinder class’s make_candidate_evaluator() method).

A PackageFinder instance has a _candidate_prefs attribute whose value is a CandidatePreferences instance. Since PackageFinder has a number of responsibilities and options that control its behavior, grouping the preferences specific to CandidateEvaluator helps maintainers know which attributes are needed only for CandidateEvaluator.

The BestCandidateResult class

The BestCandidateResult class is a convenience “container” class that encapsulates the result of finding the best candidate for a requirement. (By “container” we mean an object that simply contains data and has no business logic or state-changing methods of its own.) It stores not just the final result but also intermediate values used to determine the result.

The class is the return type of both the CandidateEvaluator class’s compute_best_candidate() method and the PackageFinder class’s find_best_candidate() method.


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