distribution-types: flip the ordering of index preference

Previously, we would prioritize `--index-url` over all
`--extra-index-url` values. But now, we prioritize all
`--extra-index-url` values over `--index-url`. That is, `--index-url`
has gone from the "primary" index to the "fallback" index. In most
setups, `--index-url` is left as its default value, which is PyPI.

The ordering of `--extra-index-url` with respect to one another remains
the same. That is, in `--extra-index-url foo --extra-index-url bar`,
`foo` will be tried before `bar`.

Finally, note that this specifically does not match `pip`'s behavior.
`pip` will attempt to look at versions of a package from all indexes in
which in occurs. `uv` will stop looking for versions of a package once
it finds it in an index. That is, for any given package, `uv` will only
utilize versions of it from a single index.

Ref #171, Fixes #1377, Fixes #1451, Fixes #1600

crates/distribution-types/src/index_url.rs

@@ -288,7 +288,7 @@ impl Default for IndexUrls {
 }
 
 impl<'a> IndexUrls {
-    /// Return the primary [`IndexUrl`] entry.
+    /// Return the fallback [`IndexUrl`] entry.
     ///
     /// If `--no-index` is set, return `None`.
     ///
@@ -318,7 +318,7 @@ impl<'a> IndexUrls {
     /// If `no_index` was enabled, then this always returns an empty
     /// iterator.
     pub fn indexes(&'a self) -> impl Iterator<Item = &'a IndexUrl> + 'a {
-        self.index().into_iter().chain(self.extra_index())
+        self.extra_index().chain(self.index())
     }
 }
 

crates/uv/src/main.rs

@@ -284,10 +284,25 @@ struct PipCompileArgs {
     refresh_package: Vec<PackageName>,
 
     /// The URL of the Python package index (by default: <https://pypi.org/simple>).
+    ///
+    /// The index given by this flag is given lower priority than all other
+    /// indexes specified via the `--extra-index-url` flag.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, short, env = "UV_INDEX_URL")]
     index_url: Option<IndexUrl>,
 
     /// Extra URLs of package indexes to use, in addition to `--index-url`.
+    ///
+    /// All indexes given via this flag take priority over the index
+    /// in `--index-url` (which defaults to PyPI). And when multiple
+    /// `--extra-index-url` flags are given, earlier values take priority.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, env = "UV_EXTRA_INDEX_URL")]
     extra_index_url: Vec<IndexUrl>,
 
@@ -421,10 +436,25 @@ struct PipSyncArgs {
     link_mode: install_wheel_rs::linker::LinkMode,
 
     /// The URL of the Python package index (by default: <https://pypi.org/simple>).
+    ///
+    /// The index given by this flag is given lower priority than all other
+    /// indexes specified via the `--extra-index-url` flag.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, short, env = "UV_INDEX_URL")]
     index_url: Option<IndexUrl>,
 
     /// Extra URLs of package indexes to use, in addition to `--index-url`.
+    ///
+    /// All indexes given via this flag take priority over the index
+    /// in `--index-url` (which defaults to PyPI). And when multiple
+    /// `--extra-index-url` flags are given, earlier values take priority.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, env = "UV_EXTRA_INDEX_URL")]
     extra_index_url: Vec<IndexUrl>,
 
@@ -619,10 +649,25 @@ struct PipInstallArgs {
     output_file: Option<PathBuf>,
 
     /// The URL of the Python package index (by default: <https://pypi.org/simple>).
+    ///
+    /// The index given by this flag is given lower priority than all other
+    /// indexes specified via the `--extra-index-url` flag.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, short, env = "UV_INDEX_URL")]
     index_url: Option<IndexUrl>,
 
     /// Extra URLs of package indexes to use, in addition to `--index-url`.
+    ///
+    /// All indexes given via this flag take priority over the index
+    /// in `--index-url` (which defaults to PyPI). And when multiple
+    /// `--extra-index-url` flags are given, earlier values take priority.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, env = "UV_EXTRA_INDEX_URL")]
     extra_index_url: Vec<IndexUrl>,
 
@@ -892,10 +937,25 @@ struct VenvArgs {
     prompt: Option<String>,
 
     /// The URL of the Python package index (by default: <https://pypi.org/simple>).
+    ///
+    /// The index given by this flag is given lower priority than all other
+    /// indexes specified via the `--extra-index-url` flag.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, short, env = "UV_INDEX_URL")]
     index_url: Option<IndexUrl>,
 
     /// Extra URLs of package indexes to use, in addition to `--index-url`.
+    ///
+    /// All indexes given via this flag take priority over the index
+    /// in `--index-url` (which defaults to PyPI). And when multiple
+    /// `--extra-index-url` flags are given, earlier values take priority.
+    ///
+    /// Unlike `pip`, `uv` will stop looking for versions of a package as soon
+    /// as it finds it in an index. That is, it isn't possible for `uv` to
+    /// consider versions of the same package across multiple indexes.
     #[clap(long, env = "UV_EXTRA_INDEX_URL")]
     extra_index_url: Vec<IndexUrl>,
 

crates/uv/tests/pip_install.rs

@@ -36,14 +36,24 @@ fn decode_token(content: &[&str]) -> String {
 
 /// Create a `pip install` command with options shared across scenarios.
 fn command(context: &TestContext) -> Command {
+    let mut command = command_without_exclude_newer(context);
+    command.arg("--exclude-newer").arg(EXCLUDE_NEWER);
+    command
+}
+
+/// Create a `pip install` command with no `--exclude-newer` option.
+///
+/// One should avoid using this in tests to the extent possible because
+/// it can result in tests failing when the index state changes. Therefore,
+/// if you use this, there should be some other kind of mitigation in place.
+/// For example, pinning package versions.
+fn command_without_exclude_newer(context: &TestContext) -> Command {
     let mut command = Command::new(get_bin());
     command
         .arg("pip")
         .arg("install")
         .arg("--cache-dir")
         .arg(context.cache_dir.path())
-        .arg("--exclude-newer")
-        .arg(EXCLUDE_NEWER)
         .env("VIRTUAL_ENV", context.venv.as_os_str())
         .current_dir(&context.temp_dir);
     command
@@ -802,6 +812,56 @@ fn install_no_index_version() {
     context.assert_command("import flask").failure();
 }
 
+/// Install a package via --extra-index-url.
+///
+/// This is a regression test where previously `uv` would consult test.pypi.org
+/// first, and if the package was found there, `uv` would not look at any other
+/// indexes. We fixed this by flipping the priority order of indexes so that
+/// test.pypi.org becomes the fallback (in this example) and the extra indexes
+/// (regular PyPI) are checked first.
+///
+/// (Neither approach matches `pip`'s behavior, which considers versions of
+/// each package from all indexes. `uv` stops at the first index it finds a
+/// package in.)
+///
+/// Ref: <https://github.com/astral-sh/uv/issues/1600>
+#[test]
+fn install_extra_index_url_has_priority() {
+    let context = TestContext::new("3.12");
+
+    uv_snapshot!(command_without_exclude_newer(&context)
+        .arg("--index-url")
+        .arg("https://test.pypi.org/simple")
+        .arg("--extra-index-url")
+        .arg("https://pypi.org/simple")
+        // This tests what we want because BOTH of the following
+        // are true: `black` is on pypi.org and test.pypi.org, AND
+        // `black==24.2.0` is on pypi.org and NOT test.pypi.org. So
+        // this would previously check for `black` on test.pypi.org,
+        // find it, but then not find a compatible version. After
+        // the fix, `uv` will check pypi.org first since it is given
+        // priority via --extra-index-url.
+        .arg("black==24.2.0"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 6 packages in [TIME]
+    Downloaded 6 packages in [TIME]
+    Installed 6 packages in [TIME]
+     + black==24.2.0
+     + click==8.1.7
+     + mypy-extensions==1.0.0
+     + packaging==23.2
+     + pathspec==0.12.1
+     + platformdirs==4.2.0
+    "###
+    );
+
+    context.assert_command("import flask").failure();
+}
+
 /// Install a package from a public GitHub repository
 #[test]
 #[cfg(feature = "git")]