Why pip Downloads Build Dependencies Before Installing Your Package

A local source archive can still require network access. Before pip can install it, pip may need a build backend and supporting packages to produce a wheel.

Locate the failure in this sequence: target archive retrieval, build-requirement installation, backend execution, then installation of the resulting package. Backend hooks can request additional build requirements; those are separate from the package's runtime dependencies.

Identify the artifact pip still needs

At “Installing build dependencies,” record the failed package and URL. Inspect the effective index, proxy route, account context and trust for that request, using the installed pip version's documented settings. An earlier successful download does not establish access to every host or the environment of every helper. Public build requirements may use an HTTP candidate from the HTTP proxy list when that route is permitted; keep the required index and trust policy. Historical setup.py download recipes can use different subprocesses.

If another machine installs successfully, compare the selected artifacts first. Its Python version or platform may have a compatible prebuilt wheel, while this machine selected a source distribution and therefore needs build tooling.

Supply the build environment deliberately

--no-build-isolation transfers responsibility for the necessary build dependencies to you. It does not repair proxy authentication or certificate trust. A locally installed backend can hide a missing or incompatible requirement.

For an intentionally offline build, prepare the target artifact, build requirements and required tooling for the project and platform through the approved process. A directory containing only the source archive is incomplete. Keep the failed requirement and stage in the report so either the route or the prepared artifact set can be corrected.