Create virtualenv if it doesn't exist in project API (#3499)

## Summary

This doesn't yet respect `--python` or the `requires-python` in the
project itself.

Closes #3449.

crates/uv-interpreter/src/environment.rs

@@ -18,12 +18,24 @@ pub struct PythonEnvironment {
 }
 
 impl PythonEnvironment {
-    /// Create a [`PythonEnvironment`] for an existing virtual environment.
+    /// Create a [`PythonEnvironment`] for an existing virtual environment, detected from the
+    /// environment variables and filesystem.
     pub fn from_virtualenv(cache: &Cache) -> Result<Self, Error> {
         let Some(venv) = detect_virtualenv()? else {
             return Err(Error::VenvNotFound);
         };
-        let venv = fs_err::canonicalize(venv)?;
+        Self::from_root(&venv, cache)
+    }
+
+    /// Create a [`PythonEnvironment`] from the virtual environment at the given root.
+    pub fn from_root(root: &Path, cache: &Cache) -> Result<Self, Error> {
+        let venv = match fs_err::canonicalize(root) {
+            Ok(venv) => venv,
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+                return Err(Error::VenvDoesNotExist(root.to_path_buf()));
+            }
+            Err(err) => return Err(err.into()),
+        };
         let executable = virtualenv_python_executable(&venv);
         let interpreter = Interpreter::query(&executable, cache)?;
 

crates/uv-interpreter/src/find_python.rs

@@ -736,7 +736,7 @@ mod windows {
                     ));
             assert_snapshot!(
                 format_err(result),
-                @"Failed to locate Python interpreter at `C:\\does\\not\\exists\\python3.12`"
+                @"Failed to locate Python interpreter at: `C:\\does\\not\\exists\\python3.12`"
             );
         }
     }
@@ -793,6 +793,6 @@ mod tests {
                 "/does/not/exists/python3.12".to_string(),
             ));
         assert_snapshot!(
-            format_err(result), @"Failed to locate Python interpreter at `/does/not/exists/python3.12`");
+            format_err(result), @"Failed to locate Python interpreter at: `/does/not/exists/python3.12`");
     }
 }

crates/uv-interpreter/src/lib.rs

@@ -14,6 +14,8 @@ use std::process::ExitStatus;
 
 use thiserror::Error;
 
+use uv_fs::Simplified;
+
 pub use crate::environment::PythonEnvironment;
 pub use crate::find_python::{find_best_python, find_default_python, find_requested_python};
 pub use crate::interpreter::Interpreter;
@@ -35,13 +37,15 @@ mod virtualenv;
 
 #[derive(Debug, Error)]
 pub enum Error {
-    #[error("Expected `{0}` to be a virtualenv, but `pyvenv.cfg` is missing")]
+    #[error("Expected `{}` to be a virtualenv, but `pyvenv.cfg` is missing", _0.user_display())]
     MissingPyVenvCfg(PathBuf),
     #[error("No versions of Python could be found. Is Python installed?")]
     PythonNotFound,
     #[error("Failed to locate a virtualenv or Conda environment (checked: `VIRTUAL_ENV`, `CONDA_PREFIX`, and `.venv`). Run `uv venv` to create a virtualenv.")]
     VenvNotFound,
-    #[error("Failed to locate Python interpreter at `{0}`")]
+    #[error("Virtualenv does not exist at: `{}`", _0.user_display())]
+    VenvDoesNotExist(PathBuf),
+    #[error("Failed to locate Python interpreter at: `{0}`")]
     RequestedPythonNotFound(String),
     #[error(transparent)]
     Io(#[from] io::Error),

crates/uv-interpreter/src/py_launcher.rs

@@ -120,7 +120,7 @@ mod tests {
                 ));
         assert_snapshot!(
             format_err(result),
-            @"Failed to locate Python interpreter at `C:\\does\\not\\exists\\python3.12`"
+            @"Failed to locate Python interpreter at: `C:\\does\\not\\exists\\python3.12`"
         );
     }
 }

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

@@ -7,7 +7,6 @@ use uv_cache::Cache;
 use uv_client::{BaseClientBuilder, RegistryClientBuilder};
 use uv_configuration::{ConfigSettings, NoBinary, NoBuild, PreviewMode, SetupPyStrategy};
 use uv_dispatch::BuildDispatch;
-use uv_interpreter::PythonEnvironment;
 use uv_requirements::{ExtrasSpecification, RequirementsSpecification};
 use uv_resolver::{FlatIndex, InMemoryIndex, OptionsBuilder};
 use uv_types::{BuildIsolation, HashStrategy, InFlight};
@@ -29,9 +28,6 @@ pub(crate) async fn lock(
         warn_user!("`uv lock` is experimental and may change without warning.");
     }
 
-    // TODO(charlie): If the environment doesn't exist, create it.
-    let venv = PythonEnvironment::from_virtualenv(cache)?;
-
     // Find the project requirements.
     let Some(project) = Project::find(std::env::current_dir()?)? else {
         return Err(anyhow::anyhow!(
@@ -39,6 +35,9 @@ pub(crate) async fn lock(
         ));
     };
 
+    // Discover or create the virtual environment.
+    let venv = project::init(&project, cache, printer)?;
+
     // TODO(zanieb): Support client configuration
     let client_builder = BaseClientBuilder::default();
 

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

@@ -13,8 +13,9 @@ use uv_cache::Cache;
 use uv_client::RegistryClient;
 use uv_configuration::{Constraints, NoBinary, Overrides, Reinstall};
 use uv_dispatch::BuildDispatch;
+use uv_fs::Simplified;
 use uv_installer::{Downloader, Plan, Planner, SitePackages};
-use uv_interpreter::{Interpreter, PythonEnvironment};
+use uv_interpreter::{find_default_python, Interpreter, PythonEnvironment};
 use uv_requirements::{
     ExtrasSpecification, LookaheadResolver, NamedRequirementsResolver, RequirementsSpecification,
     SourceTreeResolver,
@@ -25,6 +26,7 @@ use uv_resolver::{
 };
 use uv_types::{EmptyInstalledPackages, HashStrategy, InFlight};
 
+use crate::commands::project::discovery::Project;
 use crate::commands::reporters::{DownloadReporter, InstallReporter, ResolverReporter};
 use crate::commands::{elapsed, ChangeEvent, ChangeEventKind};
 use crate::printer::Printer;
@@ -45,6 +47,12 @@ pub(crate) enum Error {
     #[error(transparent)]
     Platform(#[from] platform_tags::PlatformError),
 
+    #[error(transparent)]
+    Interpreter(#[from] uv_interpreter::Error),
+
+    #[error(transparent)]
+    Virtualenv(#[from] uv_virtualenv::Error),
+
     #[error(transparent)]
     Hash(#[from] uv_types::HashStrategyError),
 
@@ -61,6 +69,47 @@ pub(crate) enum Error {
     Anyhow(#[from] anyhow::Error),
 }
 
+/// Initialize a virtual environment for the current project.
+pub(crate) fn init(
+    project: &Project,
+    cache: &Cache,
+    printer: Printer,
+) -> Result<PythonEnvironment, Error> {
+    let venv = project.root().join(".venv");
+
+    // Discover or create the virtual environment.
+    // TODO(charlie): If the environment isn't compatible with `--python`, recreate it.
+    match PythonEnvironment::from_root(&venv, cache) {
+        Ok(venv) => Ok(venv),
+        Err(uv_interpreter::Error::VenvDoesNotExist(_)) => {
+            // TODO(charlie): Respect `--python`; if unset, respect `Requires-Python`.
+            let interpreter = find_default_python(cache)?;
+
+            writeln!(
+                printer.stderr(),
+                "Using Python {} interpreter at: {}",
+                interpreter.python_version(),
+                interpreter.sys_executable().user_display().cyan()
+            )?;
+
+            writeln!(
+                printer.stderr(),
+                "Creating virtualenv at: {}",
+                venv.user_display().cyan()
+            )?;
+
+            Ok(uv_virtualenv::create_venv(
+                &venv,
+                interpreter,
+                uv_virtualenv::Prompt::None,
+                false,
+                false,
+            )?)
+        }
+        Err(e) => Err(e.into()),
+    }
+}
+
 /// Resolve a set of requirements, similar to running `pip compile`.
 #[allow(clippy::too_many_arguments)]
 pub(crate) async fn resolve(

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

@@ -68,7 +68,7 @@ pub(crate) async fn run(
             ));
         };
 
-        let venv = PythonEnvironment::from_virtualenv(cache)?;
+        let venv = project::init(&project, cache, printer)?;
 
         // Install the project requirements.
         Some(update_environment(venv, &project.requirements(), preview, cache, printer).await?)

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

@@ -7,7 +7,6 @@ use uv_client::RegistryClientBuilder;
 use uv_configuration::{ConfigSettings, NoBinary, NoBuild, PreviewMode, SetupPyStrategy};
 use uv_dispatch::BuildDispatch;
 use uv_installer::SitePackages;
-use uv_interpreter::PythonEnvironment;
 use uv_resolver::{FlatIndex, InMemoryIndex, Lock};
 use uv_types::{BuildIsolation, HashStrategy, InFlight};
 use uv_warnings::warn_user;
@@ -27,11 +26,6 @@ pub(crate) async fn sync(
         warn_user!("`uv sync` is experimental and may change without warning.");
     }
 
-    // TODO(charlie): If the environment doesn't exist, create it.
-    let venv = PythonEnvironment::from_virtualenv(cache)?;
-    let markers = venv.interpreter().markers();
-    let tags = venv.interpreter().tags()?;
-
     // Find the project requirements.
     let Some(project) = Project::find(std::env::current_dir()?)? else {
         return Err(anyhow::anyhow!(
@@ -39,6 +33,11 @@ pub(crate) async fn sync(
         ));
     };
 
+    // Discover or create the virtual environment.
+    let venv = project::init(&project, cache, printer)?;
+    let markers = venv.interpreter().markers();
+    let tags = venv.interpreter().tags()?;
+
     // Read the lockfile.
     let resolution = {
         let encoded = fs_err::tokio::read_to_string(project.root().join("uv.lock")).await?;

crates/uv/tests/pip_sync.rs

@@ -108,8 +108,7 @@ fn missing_venv() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    error: failed to canonicalize path `[VENV]/`
+    error: Virtualenv does not exist at: `[VENV]/`
-      Caused by: No such file or directory (os error 2)
     "###);
 
     assert!(predicates::path::missing().eval(&context.venv));