Surface the EXTERNALLY-MANAGED message to users (#2032)

## Summary

Per the
[spec](https://packaging.python.org/en/latest/specifications/externally-managed-environments/),
this message should be surfaced to users:

![Screenshot 2024-02-27 at 10 42
52 PM](https://github.com/astral-sh/uv/assets/1309177/dac3bd6b-dd05-4146-8faa-f046492e8a26)

crates/uv-interpreter/Cargo.toml

@@ -22,6 +22,7 @@ uv-cache = { path = "../uv-cache" }
 uv-fs = { path = "../uv-fs" }
 install-wheel-rs = { path = "../install-wheel-rs" }
 
+configparser = { workspace = true }
 fs-err = { workspace = true, features = ["tokio"] }
 once_cell = { workspace = true }
 regex = { workspace = true }

crates/uv-interpreter/src/interpreter.rs

@@ -3,6 +3,7 @@ use std::io::Write;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+use configparser::ini::Ini;
 use fs_err as fs;
 use once_cell::sync::OnceCell;
 use serde::{Deserialize, Serialize};
@@ -267,20 +268,41 @@ impl Interpreter {
         self.prefix != self.base_prefix
     }
 
-    /// Returns `true` if the environment is externally managed.
+    /// Returns `Some` if the environment is externally managed, optionally including an error
+    /// message from the `EXTERNALLY-MANAGED` file.
     ///
     /// See: <https://packaging.python.org/en/latest/specifications/externally-managed-environments/>
-    pub fn externally_managed(&self) -> bool {
+    pub fn is_externally_managed(&self) -> Option<ExternallyManaged> {
         // Per the spec, a virtual environment is never externally managed.
         if self.is_virtualenv() {
-            return false;
+            return None;
         }
 
-        // Per the spec, the existence of the file is the only requirement.
-        self.sysconfig_paths
-            .stdlib
-            .join("EXTERNALLY-MANAGED")
-            .is_file()
+        let Ok(contents) =
+            fs::read_to_string(self.sysconfig_paths.stdlib.join("EXTERNALLY-MANAGED"))
+        else {
+            return None;
+        };
+
+        let Ok(mut ini) = Ini::new_cs().read(contents) else {
+            // If a file exists but is not a valid INI file, we assume the environment is
+            // externally managed.
+            return Some(ExternallyManaged::default());
+        };
+
+        let Some(section) = ini.get_mut("externally-managed") else {
+            // If the file exists but does not contain an "externally-managed" section, we assume
+            // the environment is externally managed.
+            return Some(ExternallyManaged::default());
+        };
+
+        let Some(error) = section.remove("Error") else {
+            // If the file exists but does not contain an "Error" key, we assume the environment is
+            // externally managed.
+            return Some(ExternallyManaged::default());
+        };
+
+        Some(ExternallyManaged { error })
     }
 
     /// Returns the Python version.
@@ -403,6 +425,21 @@ impl Interpreter {
     }
 }
 
+/// The `EXTERNALLY-MANAGED` file in a Python installation.
+///
+/// See: <https://packaging.python.org/en/latest/specifications/externally-managed-environments/>
+#[derive(Debug, Default, Clone)]
+pub struct ExternallyManaged {
+    error: Option<String>,
+}
+
+impl ExternallyManaged {
+    /// Return the `EXTERNALLY-MANAGED` error message, if any.
+    pub fn into_error(self) -> Option<String> {
+        self.error
+    }
+}
+
 /// The installation paths returned by `sysconfig.get_paths()`.
 ///
 /// See: <https://docs.python.org/3.12/library/sysconfig.html#installation-paths>

crates/uv-interpreter/src/python_query.rs

@@ -121,9 +121,7 @@ fn find_python(
             }
             Err(Error::PyList(error)) => {
                 if error.kind() == std::io::ErrorKind::NotFound {
-                    tracing::debug!(
-                        "`py` is not installed. Falling back to searching Python on the path"
-                    );
+                    debug!("`py` is not installed. Falling back to searching Python on the path");
                     // Continue searching for python installations on the path.
                 }
             }
@@ -156,7 +154,7 @@ fn find_python(
                                 return Err(Error::Python2OrOlder);
                             }
                             // Skip over Python 2 or older installation when querying for a recent python installation.
-                            tracing::debug!("Found a Python 2 installation that isn't supported by uv, skipping.");
+                            debug!("Found a Python 2 installation that isn't supported by uv, skipping.");
                             continue;
                         }
                         Err(error) => return Err(error),

crates/uv/src/commands/pip_install.rs

@@ -118,12 +118,19 @@ pub(crate) async fn pip_install(
     );
 
     // If the environment is externally managed, abort.
-    // TODO(charlie): Surface the error from the `EXTERNALLY-MANAGED` file.
-    if venv.interpreter().externally_managed() {
-        return Err(anyhow::anyhow!(
-            "The environment at {} is externally managed",
-            venv.root().normalized_display()
-        ));
+    if let Some(externally_managed) = venv.interpreter().is_externally_managed() {
+        return if let Some(error) = externally_managed.into_error() {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed, and indicates the following:\n\n{}\n\nConsider creating a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan(),
+                textwrap::indent(&error, "  ").green(),
+            ))
+        } else {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed. Instead, create a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan()
+            ))
+        };
     }
 
     let _lock = venv.lock()?;

crates/uv/src/commands/pip_sync.rs

@@ -86,12 +86,19 @@ pub(crate) async fn pip_sync(
     );
 
     // If the environment is externally managed, abort.
-    // TODO(charlie): Surface the error from the `EXTERNALLY-MANAGED` file.
-    if venv.interpreter().externally_managed() {
-        return Err(anyhow::anyhow!(
-            "The environment at {} is externally managed",
-            venv.root().normalized_display()
-        ));
+    if let Some(externally_managed) = venv.interpreter().is_externally_managed() {
+        return if let Some(error) = externally_managed.into_error() {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed, and indicates the following:\n\n{}\n\nConsider creating a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan(),
+                textwrap::indent(&error, "  ").green(),
+            ))
+        } else {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed. Instead, create a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan()
+            ))
+        };
     }
 
     let _lock = venv.lock()?;

crates/uv/src/commands/pip_uninstall.rs

@@ -52,12 +52,19 @@ pub(crate) async fn pip_uninstall(
     );
 
     // If the environment is externally managed, abort.
-    // TODO(charlie): Surface the error from the `EXTERNALLY-MANAGED` file.
-    if venv.interpreter().externally_managed() {
-        return Err(anyhow::anyhow!(
-            "The environment at {} is externally managed",
-            venv.root().normalized_display()
-        ));
+    if let Some(externally_managed) = venv.interpreter().is_externally_managed() {
+        return if let Some(error) = externally_managed.into_error() {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed, and indicates the following:\n\n{}\n\nConsider creating a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan(),
+                textwrap::indent(&error, "  ").green(),
+            ))
+        } else {
+            Err(anyhow::anyhow!(
+                "The interpreter at {} is externally managed. Instead, create a virtual environment with `uv venv`.",
+                venv.root().normalized_display().cyan()
+            ))
+        };
     }
 
     let _lock = venv.lock()?;