Respect .python-version in pip-compile

crates/uv/src/commands/pip/compile.rs

@@ -1,3 +1,4 @@
+use fs_err as fs;
 use std::collections::{BTreeMap, BTreeSet};
 use std::env;
 use std::ffi::OsStr;
@@ -156,6 +157,51 @@ pub(crate) async fn pip_compile(
         }
     }
 
+    // Respect `.python-version` file
+    if python.is_none() && python_version.is_none() {
+        let mut version_file: Option<PathBuf> = None;
+
+        if PathBuf::from(".python-version").exists() {
+            // If there is `.python-version` in the current directory, use that.
+            // This is for cases like `uv pip compile requirements.in`.
+            version_file = Some(PathBuf::from(".python-version"));
+        } else if !requirements.is_empty() {
+            // If there is no `.python-version` present, try to find a sibling
+            // to the first RequirementsSource, which contains a path.
+            let file_name = match &requirements[0] {
+                RequirementsSource::PylockToml(file_name) => Some(file_name),
+                RequirementsSource::RequirementsTxt(file_name) => Some(file_name),
+                RequirementsSource::PyprojectToml(file_name) => Some(file_name),
+                RequirementsSource::SetupPy(file_name) => Some(file_name),
+                RequirementsSource::SetupCfg(file_name) => Some(file_name),
+                RequirementsSource::EnvironmentYml(file_name) => Some(file_name),
+                _ => None,
+            };
+
+            if let Some(file_name) = file_name {
+                let version_file_path = file_name
+                    .as_path()
+                    .parent()
+                    .unwrap()
+                    .join(PathBuf::from(".python-version"));
+
+                if version_file_path.exists() {
+                    version_file = Some(version_file_path);
+                }
+            }
+        }
+
+        if let Some(version_file) = version_file {
+            if let Ok(request) = fs::read_to_string(version_file) {
+                if !request.is_empty() {
+                    if let Some(first_line) = request.lines().next() {
+                        python = Some(first_line.trim().to_string());
+                    }
+                }
+            }
+        }
+    }
+
     // If `--python` / `-p` is a simple Python version request, we treat it as `--python-version`
     // for backwards compatibility. `-p` was previously aliased to `--python-version` but changed to
     // `--python` for consistency with the rest of the CLI in v0.6.0. Since we assume metadata is

crates/uv/tests/it/pip_compile.rs

@@ -17603,3 +17603,80 @@ fn omit_python_patch_universal() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn compile_requirements_in_respect_python_version_default() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.12", "3.11"]).with_filtered_python_sources();
+
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("requests\nrich ; python_version >= '3.12'\n")?;
+
+    uv_snapshot!(context.filters(), context.pip_compile()
+        .arg("requirements.in"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in
+    certifi==2024.2.2
+        # via requests
+    charset-normalizer==3.3.2
+        # via requests
+    idna==3.6
+        # via requests
+    markdown-it-py==3.0.0
+        # via rich
+    mdurl==0.1.2
+        # via markdown-it-py
+    pygments==2.17.2
+        # via rich
+    requests==2.31.0
+        # via -r requirements.in
+    rich==13.7.1
+        # via -r requirements.in
+    urllib3==2.2.1
+        # via requests
+
+    ----- stderr -----
+    Resolved 9 packages in [TIME]
+    "
+    );
+
+    Ok(())
+}
+
+#[test]
+fn compile_requirements_in_respect_python_version() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.12", "3.11"]).with_filtered_python_sources();
+
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("requests\nrich ; python_version >= '3.12'\n")?;
+
+    let python_version = context.temp_dir.child(".python-version");
+    python_version.write_str("3.11")?;
+
+    uv_snapshot!(context.filters(), context.pip_compile()
+        .arg("requirements.in"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in
+    certifi==2024.2.2
+        # via requests
+    charset-normalizer==3.3.2
+        # via requests
+    idna==3.6
+        # via requests
+    requests==2.31.0
+        # via -r requirements.in
+    urllib3==2.2.1
+        # via requests
+
+    ----- stderr -----
+    Resolved 5 packages in [TIME]
+    "
+    );
+
+    Ok(())
+}