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# 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.

## Overview

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 https://pypi.org/simple/ ) 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 main PackageFinder class,

- the LinkCollector class,

- the LinkEvaluator class,

- the CandidateEvaluator class,

- the CandidatePreferences class, and

- the BestCandidateResult class.

## 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:

- pip download

- pip install

- pip list

- pip wheel

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 `self_outdated_check.py` 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_sources()` 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_sources()` 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 platform 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.