pypa/pip · error · PylockSelectError

Multiple packages with the name {package.name!r} are selecte

Error message

Multiple packages with the name {package.name!r} are selected at packages[{package_index}] and packages[{selected_packages_by_name[package.name][0]}]

What it means

Raised as PylockSelectError (pylock.py:845) by Pylock.select() when two distinct package entries share the same canonical name and both have markers that match the selection environment. The spec forbids ambiguous selections; the error names both conflicting indices.

Source

Thrown at src/pip/_vendor/packaging/pylock.py:845

            if package.marker and not package.marker.evaluate(env, context="lock_file"):
                continue

            # #. If :ref:`pylock-packages-requires-python` is specified, check if it is
            #    satisfied; an error MUST be raised if it isn't.
            if package.requires_python and not package.requires_python.contains(
                env_python_full_version,
            ):
                raise PylockSelectError(
                    f"python_full_version {env_python_full_version!r} "
                    f"in provided environment does not satisfy the Python version "
                    f"requirement {str(package.requires_python)!r} for package "
                    f"{package.name!r} at packages[{package_index}]"
                )

            # #. Check that no other conflicting instance of the package has been slated
            #    to be installed; an error about the ambiguity MUST be raised otherwise.
            if package.name in selected_packages_by_name:
                raise PylockSelectError(
                    f"Multiple packages with the name {package.name!r} are "
                    f"selected at packages[{package_index}] and "
                    f"packages[{selected_packages_by_name[package.name][0]}]"
                )

            # #. Check that the source of the package is specified appropriately (i.e.
            #    there are no conflicting sources in the package entry);
            #    an error MUST be raised if any issues are found.
            # Covered by lock.validate() which is a precondition for this method.

            # #. Add the package to the set of packages to install.
            selected_packages_by_name[package.name] = (package_index, package)

        # #. For each package to be installed:
        for package_index, package in selected_packages_by_name.values():
            # - If :ref:`pylock-packages-vcs` is set:
            if package.vcs is not None:
                yield package, package.vcs

View on GitHub (pinned to d7d0d0a394)

Solutions

1. Make the two packages' markers mutually exclusive for your environment (e.g. 'python_version < "3.10"' vs 'python_version >= "3.10"').
2. Remove the duplicate package entry that should not apply.
3. Provide an environment that disambiguates which entry should win.

Example fix

# before: both entries match on the same interpreter
[[packages]]
name = "foo"
marker = "python_version >= '3.8'"
[[packages]]
name = "foo"
marker = "python_version >= '3.10'"

# after: mutually exclusive
[[packages]]
name = "foo"
marker = "python_version < '3.10'"
[[packages]]
name = "foo"
marker = "python_version >= '3.10'"

from collections import Counter
from pip._vendor.packaging.markers import Marker
def has_duplicate_selected_packages(packages, env):
    selected = [p['name'] for p in packages
                if (not p.get('marker')) or Marker(p['marker']).evaluate(env)]
    return [n for n, c in Counter(selected).items() if c > 1]

null

from pip._vendor.packaging.pylock import PylockSelectError
try:
    list(pylock.select(environment=env))
except PylockSelectError as e:
    if 'Multiple packages' in str(e):
        make_markers_mutually_exclusive(e)

When it happens

Trigger: A lock contains [[packages]] name='foo' twice (e.g. one for Python <3.10 and one for >=3.10) but both entries' markers evaluate True for the current environment. Triggered in the package loop inside select() right after marker evaluation.

Common situations: Authoring mutually-exclusive markers incorrectly (both branches true). A lock merge that duplicated an entry. A marker that references a variable not present in the environment so it defaults to truthy in both copies.

Related errors

• Provided environment does not satisfy any of the environment
• python_full_version {env_python_full_version!r} in provided
• python_full_version {env_python_full_version!r} in provided
• No wheel found matching the provided tags for package {packa
• Platform and interpreter constraints using --python-version,

AI-assisted analysis of pypa/pip@d7d0d0a394 (2026-08-04). Data as JSON: /data/errors/3eb3b49cc52733cb.json. Report an issue: GitHub.