Error when workspace contains conflicting Python requirements (#10841)

## Summary

If members define disjoint Python requirements, we should error. Right
now, it seems that it maps to unbounded and leads to weird behavior.

Closes https://github.com/astral-sh/uv/issues/10835.

crates/uv-resolver/src/requires_python.rs

@@ -75,6 +75,11 @@ impl RequiresPython {
                 }
             })?;
 
+        // If the intersection is empty, return `None`.
+        if range.is_empty() {
+            return None;
+        }
+
         // Convert back to PEP 440 specifiers.
         let specifiers = VersionSpecifiers::from_release_only_bounds(range.iter());
 

crates/uv-workspace/src/workspace.rs

@@ -9,6 +9,7 @@ use tracing::{debug, trace, warn};
 use uv_distribution_types::Index;
 use uv_fs::{Simplified, CWD};
 use uv_normalize::{GroupName, PackageName, DEV_DEPENDENCIES};
+use uv_pep440::VersionSpecifiers;
 use uv_pep508::{MarkerTree, VerbatimUrl};
 use uv_pypi_types::{
     Conflicts, Requirement, RequirementSource, SupportedEnvironments, VerbatimParsedUrl,
@@ -383,6 +384,18 @@ impl Workspace {
         conflicting
     }
 
+    /// Returns an iterator over the `requires-python` values for each member of the workspace.
+    pub fn requires_python(&self) -> impl Iterator<Item = (&PackageName, &VersionSpecifiers)> {
+        self.packages().iter().filter_map(|(name, member)| {
+            member
+                .pyproject_toml()
+                .project
+                .as_ref()
+                .and_then(|project| project.requires_python.as_ref())
+                .map(|requires_python| (name, requires_python))
+        })
+    }
+
     /// Returns any requirements that are exclusive to the workspace root, i.e., not included in
     /// any of the workspace members.
     ///

crates/uv/src/commands/build_frontend.rs

@@ -11,7 +11,7 @@ use thiserror::Error;
 use tracing::instrument;
 
 use crate::commands::pip::operations;
-use crate::commands::project::find_requires_python;
+use crate::commands::project::{find_requires_python, ProjectError};
 use crate::commands::reporters::PythonDownloadReporter;
 use crate::commands::ExitStatus;
 use crate::printer::Printer;
@@ -69,6 +69,8 @@ enum Error {
     BuildDispatch(AnyErrorBuild),
     #[error(transparent)]
     BuildFrontend(#[from] uv_build_frontend::Error),
+    #[error(transparent)]
+    Project(#[from] ProjectError),
     #[error("Failed to write message")]
     Fmt(#[from] fmt::Error),
     #[error("Can't use `--force-pep517` with `--list`")]
@@ -464,7 +466,7 @@ async fn build_package(
     // (3) `Requires-Python` in `pyproject.toml`
     if interpreter_request.is_none() {
         if let Ok(workspace) = workspace {
-            interpreter_request = find_requires_python(workspace)
+            interpreter_request = find_requires_python(workspace)?
                 .as_ref()
                 .map(RequiresPython::specifiers)
                 .map(|specifiers| {

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

@@ -499,7 +499,12 @@ async fn init_project(
         };
 
         (requires_python, python_request)
-    } else if let Some(requires_python) = workspace.as_ref().and_then(find_requires_python) {
+    } else if let Some(requires_python) = workspace
+        .as_ref()
+        .map(find_requires_python)
+        .transpose()?
+        .flatten()
+    {
         // (3) `requires-python` from the workspace
         debug!("Using Python version from project workspace");
         let python_request = PythonRequest::Version(VersionRequest::Range(

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

@@ -418,7 +418,7 @@ async fn do_lock(
 
     // Determine the supported Python range. If no range is defined, and warn and default to the
     // current minor version.
-    let requires_python = target.requires_python();
+    let requires_python = target.requires_python()?;
 
     let requires_python = if let Some(requires_python) = requires_python {
         if requires_python.is_unbounded() {

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

@@ -188,14 +188,15 @@ impl<'lock> LockTarget<'lock> {
     }
 
     /// Return the `Requires-Python` bound for the [`LockTarget`].
-    pub(crate) fn requires_python(self) -> Option<RequiresPython> {
+    #[allow(clippy::result_large_err)]
+    pub(crate) fn requires_python(self) -> Result<Option<RequiresPython>, ProjectError> {
         match self {
             Self::Workspace(workspace) => find_requires_python(workspace),
-            Self::Script(script) => script
+            Self::Script(script) => Ok(script
                 .metadata
                 .requires_python
                 .as_ref()
-                .map(RequiresPython::from_specifiers),
+                .map(RequiresPython::from_specifiers)),
         }
     }
 

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

@@ -1,4 +1,4 @@
-use std::collections::BTreeSet;
+use std::collections::{BTreeMap, BTreeSet};
 use std::fmt::Write;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
@@ -155,6 +155,9 @@ pub(crate) enum ProjectError {
     #[error("Environment markers `{0}` don't overlap with Python requirement `{1}`")]
     DisjointEnvironment(MarkerTreeContents, VersionSpecifiers),
 
+    #[error("The workspace contains conflicting Python requirements:\n{}", _0.iter().map(|(name, specifiers)| format!("- `{name}`: `{specifiers}`")).join("\n"))]
+    DisjointRequiresPython(BTreeMap<PackageName, VersionSpecifiers>),
+
     #[error("Environment marker is empty")]
     EmptyEnvironment,
 
@@ -317,14 +320,27 @@ impl std::error::Error for ConflictError {}
 ///
 /// For a [`Workspace`] with multiple packages, the `Requires-Python` bound is the union of the
 /// `Requires-Python` bounds of all the packages.
-pub(crate) fn find_requires_python(workspace: &Workspace) -> Option<RequiresPython> {
-    RequiresPython::intersection(workspace.packages().values().filter_map(|member| {
-        member
-            .pyproject_toml()
-            .project
-            .as_ref()
-            .and_then(|project| project.requires_python.as_ref())
-    }))
+#[allow(clippy::result_large_err)]
+pub(crate) fn find_requires_python(
+    workspace: &Workspace,
+) -> Result<Option<RequiresPython>, ProjectError> {
+    // If there are no `Requires-Python` specifiers in the workspace, return `None`.
+    if workspace.requires_python().next().is_none() {
+        return Ok(None);
+    }
+    match RequiresPython::intersection(
+        workspace
+            .requires_python()
+            .map(|(.., specifiers)| specifiers),
+    ) {
+        Some(requires_python) => Ok(Some(requires_python)),
+        None => Err(ProjectError::DisjointRequiresPython(
+            workspace
+                .requires_python()
+                .map(|(name, specifiers)| (name.clone(), specifiers.clone()))
+                .collect(),
+        )),
+    }
 }
 
 /// Returns an error if the [`Interpreter`] does not satisfy the [`Workspace`] `requires-python`.
@@ -732,7 +748,7 @@ impl WorkspacePython {
         project_dir: &Path,
         no_config: bool,
     ) -> Result<Self, ProjectError> {
-        let requires_python = workspace.and_then(find_requires_python);
+        let requires_python = workspace.map(find_requires_python).transpose()?.flatten();
 
         let workspace_root = workspace.map(Workspace::install_path);
 

crates/uv/src/commands/python/pin.rs

@@ -256,7 +256,7 @@ fn assert_pin_compatible_with_project(pin: &Pin, virtual_project: &VirtualProjec
                 project_workspace.project_name(),
                 project_workspace.workspace().install_path().display()
             );
-            let requires_python = find_requires_python(project_workspace.workspace());
+            let requires_python = find_requires_python(project_workspace.workspace())?;
             (requires_python, "project")
         }
         VirtualProject::NonProject(workspace) => {
@@ -264,7 +264,7 @@ fn assert_pin_compatible_with_project(pin: &Pin, virtual_project: &VirtualProjec
                 "Discovered virtual workspace at: {}",
                 workspace.install_path().display()
             );
-            let requires_python = find_requires_python(workspace);
+            let requires_python = find_requires_python(workspace)?;
             (requires_python, "workspace")
         }
     };

crates/uv/tests/it/lock.rs

@@ -5242,6 +5242,58 @@ fn lock_requires_python_unbounded() -> Result<()> {
     Ok(())
 }
 
+/// Error if `Requires-Python` is disjoint across the workspace.
+#[test]
+fn lock_requires_python_disjoint() -> Result<()> {
+    let context = TestContext::new("3.11");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+
+        [tool.uv.workspace]
+        members = ["child"]
+        "#,
+    )?;
+
+    let pyproject_toml = context.temp_dir.child("child").child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "child"
+        version = "0.1.0"
+        requires-python = "==3.10"
+        dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: The workspace contains conflicting Python requirements:
+    - `child`: `==3.10`
+    - `project`: `>=3.12`
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn lock_requires_python_maximum_version() -> Result<()> {
     let context = TestContext::new("3.11");