Add a dedicated error for members with mismatched Python requirements (…

…#5695)

## Summary

Gives you a nice error message if you attempt to sync with, e.g., `-p
3.8` when that version is supported by at least one workspace member,
but your project's minimum requirement is `>=3.12`

Closes #5662.

crates/uv-workspace/src/workspace.rs

@@ -1483,7 +1483,7 @@ mod tests {
                     "root": "[ROOT]/albatross-root-workspace/packages/bird-feeder",
                     "project": {
                       "name": "bird-feeder",
-                      "requires-python": ">=3.12",
+                      "requires-python": ">=3.8",
                       "optional-dependencies": null
                     },
                     "pyproject_toml": "[PYPROJECT_TOML]"

crates/uv/src/commands/project/mod.rs

@@ -1,11 +1,12 @@
 use std::fmt::Write;
+use std::path::PathBuf;
 
 use itertools::Itertools;
 use owo_colors::OwoColorize;
 use tracing::debug;
 
 use distribution_types::{Resolution, UnresolvedRequirementSpecification};
-use pep440_rs::Version;
+use pep440_rs::{Version, VersionSpecifiers};
 use pypi_types::Requirement;
 use uv_auth::store_credentials_from_url;
 use uv_cache::Cache;
@@ -17,6 +18,7 @@ use uv_dispatch::BuildDispatch;
 use uv_distribution::DistributionDatabase;
 use uv_fs::Simplified;
 use uv_installer::{SatisfiesResult, SitePackages};
+use uv_normalize::PackageName;
 use uv_python::{
     request_from_version_file, EnvironmentPreference, Interpreter, PythonEnvironment, PythonFetch,
     PythonInstallation, PythonPreference, PythonRequest, VersionRequest,
@@ -60,6 +62,15 @@ pub(crate) enum ProjectError {
     #[error("The requested Python interpreter ({0}) is incompatible with the project Python requirement: `{1}`")]
     RequestedPythonIncompatibility(Version, RequiresPython),
 
+    #[error("The requested Python interpreter ({0}) is incompatible with the project Python requirement: `{1}`. However, a workspace member (`{member}`) supports Python {3}. To install the workspace member on its own, navigate to `{path}`, then run `{venv}` followed by `{install}`.", member = _2.cyan(), venv = format!("uv venv --python {_0}").green(), install = "uv pip install -e .".green(), path = _4.user_display().cyan() )]
+    RequestedMemberPythonIncompatibility(
+        Version,
+        RequiresPython,
+        PackageName,
+        VersionSpecifiers,
+        PathBuf,
+    ),
+
     #[error(transparent)]
     Python(#[from] uv_python::Error),
 
@@ -239,6 +250,29 @@ impl FoundInterpreter {
 
         if let Some(requires_python) = requires_python.as_ref() {
             if !requires_python.contains(interpreter.python_version()) {
+                // If the Python version is compatible with one of the workspace _members_, raise
+                // a dedicated error. For example, if the workspace root requires Python >=3.12, but
+                // a library in the workspace is compatible with Python >=3.8, the user may attempt
+                // to sync on Python 3.8. This will fail, but we should provide a more helpful error
+                // message.
+                for (name, member) in workspace.packages() {
+                    let Some(project) = member.pyproject_toml().project.as_ref() else {
+                        continue;
+                    };
+                    let Some(specifiers) = project.requires_python.as_ref() else {
+                        continue;
+                    };
+                    if specifiers.contains(interpreter.python_version()) {
+                        return Err(ProjectError::RequestedMemberPythonIncompatibility(
+                            interpreter.python_version().clone(),
+                            requires_python.clone(),
+                            name.clone(),
+                            specifiers.clone(),
+                            member.root().clone(),
+                        ));
+                    }
+                }
+
                 return Err(ProjectError::RequestedPythonIncompatibility(
                     interpreter.python_version().clone(),
                     requires_python.clone(),

crates/uv/tests/sync.rs

@@ -355,7 +355,7 @@ fn mixed_requires_python() -> Result<()> {
     ----- stderr -----
     warning: `uv sync` is experimental and may change without warning
     Using Python 3.8.[X] interpreter at: [PYTHON-3.8]
-    error: The requested Python interpreter (3.8.[X]) is incompatible with the project Python requirement: `>=3.12`
+    error: The requested Python interpreter (3.8.[X]) is incompatible with the project Python requirement: `>=3.12`. However, a workspace member (`bird-feeder`) supports Python >=3.8. To install the workspace member on its own, navigate to `packages/bird-feeder`, then run `uv venv --python 3.8.[X]` followed by `uv pip install -e .`.
     "###);
 
     Ok(())

scripts/workspaces/albatross-root-workspace/packages/bird-feeder/pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "bird-feeder"
 version = "1.0.0"
-requires-python = ">=3.12"
+requires-python = ">=3.8"
 dependencies = ["anyio>=4.3.0,<5", "seeds"]
 
 [tool.uv.sources]