Allow use of older variants on newer systems

crates/uv-python/fetch-download-metadata.py

@@ -86,10 +86,12 @@ def __str__(self) -> str:
         return (self.family + "_" + self.variant) if self.variant else self.family
 
     def __gt__(self, other) -> bool:
-        return (self.family, self.variant or "") > (other.family, other.variant or "")
+        # Note this is inverted to prefer newer variants
+        return (self.family, self.variant or "") < (other.family, other.variant or "")
 
     def __lt__(self, other) -> bool:
-        return (self.family, self.variant or "") < (other.family, other.variant or "")
+        # Note this is inverted to prefer newer variants
+        return (self.family, self.variant or "") > (other.family, other.variant or "")
 
 
 type PlatformTripleKey = tuple[str, str, str]

crates/uv-python/src/downloads.rs

@@ -251,26 +251,30 @@ impl PythonDownloadRequest {
 
     /// Whether this request is satisfied by the key of an existing installation.
     pub fn satisfied_by_key(&self, key: &PythonInstallationKey) -> bool {
-        if let Some(arch) = &self.arch {
-            if key.arch != *arch {
+        if let Some(os) = &self.os {
+            if key.os != *os {
                 return false;
             }
         }
-        if let Some(os) = &self.os {
-            if key.os != *os {
+
+        if let Some(arch) = &self.arch {
+            if !arch.is_compatible(key.arch) {
                 return false;
             }
         }
+
         if let Some(libc) = &self.libc {
             if key.libc != *libc {
                 return false;
             }
         }
+
         if let Some(implementation) = &self.implementation {
             if key.implementation != LenientImplementationName::from(*implementation) {
                 return false;
             }
         }
+
         // If we don't allow pre-releases, don't match a key with a pre-release tag
         if !self.allows_prereleases() && key.prerelease.is_some() {
             return false;

crates/uv-python/src/managed.rs

@@ -237,16 +237,15 @@ impl ManagedPythonInstallations {
     pub fn find_matching_current_platform(
         &self,
     ) -> Result<impl DoubleEndedIterator<Item = ManagedPythonInstallation>, Error> {
-        let platform_key = platform_key_from_env()?;
+        let os = Os::from_env();
+        let arch = Arch::from_env();
+        let libc = Libc::from_env()?;
 
         let iter = ManagedPythonInstallations::from_settings(None)?
             .find_all()?
             .filter(move |installation| {
-                installation
-                    .path
-                    .file_name()
-                    .map(OsStr::to_string_lossy)
-                    .is_some_and(|filename| filename.ends_with(&platform_key))
+                let key = installation.key();
+                key.os == os && arch.is_compatible(key.arch) && key.libc == libc
             });
 
         Ok(iter)
@@ -585,14 +584,6 @@ impl ManagedPythonInstallation {
     }
 }
 
-/// Generate a platform portion of a key from the environment.
-fn platform_key_from_env() -> Result<String, Error> {
-    let os = Os::from_env();
-    let arch = Arch::from_env();
-    let libc = Libc::from_env()?;
-    Ok(format!("{os}-{arch}-{libc}").to_lowercase())
-}
-
 impl fmt::Display for ManagedPythonInstallation {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(

crates/uv-python/src/platform.rs

@@ -106,10 +106,30 @@ impl Arch {
             variant: ArchVariant::from_env(),
         }
     }
+
+    /// Is the current architecture compatible with `other`?
+    ///
+    /// `other` can be a compatible microarchitecture with worse performance characteristics.
+    pub(crate) fn is_compatible(self, other: Self) -> bool {
+        self.family == other.family
+            && ((self.variant.is_none() && other.variant.is_none())
+                || self
+                    .variant
+                    .is_some_and(|variant| variant.is_compatible(other.variant)))
+    }
+
+    #[must_use]
+    pub fn without_variant(self) -> Self {
+        Self {
+            family: self.family,
+            variant: None,
+        }
+    }
 }
 
 impl ArchVariant {
-    #[cfg(target_arch = "x86_64")]
+    /// Only Linux `x86_64` variants are published upstream at this time.
+    #[cfg(all(unix, target_arch = "x86_64"))]
     pub fn from_env() -> Option<Self> {
         if is_x86_feature_detected!("avx512f")
             && is_x86_feature_detected!("avx512bw")
@@ -141,10 +161,27 @@ impl ArchVariant {
         }
     }
 
-    #[cfg(not(target_arch = "x86_64"))]
+    #[cfg(not(all(unix, target_arch = "x86_64")))]
     pub fn from_env() -> Option<Self> {
         None
     }
+
+    /// Is the current variant compatible with `other`?
+    ///
+    /// `other` can be a compatible variant with lower priority, however, it must be a subset of
+    /// `self`.
+    pub(crate) fn is_compatible(self, other: Option<Self>) -> bool {
+        let Some(other) = other else {
+            // `None` can be used on any architecture
+            return true;
+        };
+        match self {
+            Self::V2 => matches!(other, Self::V2),
+            Self::V3 => matches!(other, Self::V2 | Self::V3),
+            // If on V4, any variant can be used.
+            Self::V4 => true,
+        }
+    }
 }
 
 impl Display for Libc {

crates/uv/tests/it/python_find.rs

@@ -95,7 +95,7 @@ fn python_find() {
 
     // Request CPython 3.12 for the current platform
     let os = Os::from_env();
-    let arch = Arch::from_env();
+    let arch = Arch::from_env().without_variant();
 
     uv_snapshot!(context.filters(), context.python_find()
     .arg(format!("cpython-3.12-{os}-{arch}"))

crates/uv/tests/it/python_pin.rs

@@ -562,7 +562,7 @@ fn python_pin_resolve() {
 
     // Request CPython 3.13 for the current platform
     let os = Os::from_env();
-    let arch = Arch::from_env();
+    let arch = Arch::from_env().without_variant();
 
     uv_snapshot!(context.filters(), context.python_pin().arg("--resolved")
     .arg(format!("cpython-3.13-{os}-{arch}"))