Support resolving for an alternate Python distribution (#364)

## Summary

Low-priority but fun thing to end the day. You can now pass
`--target-version py37`, and we'll generate a resolution for Python 3.7.

See: https://github.com/astral-sh/puffin/issues/183.

crates/puffin-cli/src/commands/pip_compile.rs

@@ -1,13 +1,14 @@
+use std::borrow::Cow;
 use std::fmt::Write;
 use std::io::{stdout, BufWriter};
 use std::path::Path;
+use std::str::FromStr;
 use std::{env, fs};
 
 use anyhow::{anyhow, Result};
 use colored::Colorize;
 use fs_err::File;
 use itertools::Itertools;
-
 use tracing::debug;
 
 use pep508_rs::Requirement;
@@ -18,12 +19,12 @@ use puffin_dispatch::BuildDispatch;
 use puffin_interpreter::Virtualenv;
 use puffin_normalize::ExtraName;
 use puffin_resolver::{Manifest, PreReleaseMode, ResolutionMode};
-use std::str::FromStr;
 
 use crate::commands::reporters::ResolverReporter;
 use crate::commands::{elapsed, ExitStatus};
 use crate::index_urls::IndexUrls;
 use crate::printer::Printer;
+use crate::python_version::PythonVersion;
 use crate::requirements::{ExtrasSpecification, RequirementsSource, RequirementsSpecification};
 
 const VERSION: &str = env!("CARGO_PKG_VERSION");
@@ -40,6 +41,7 @@ pub(crate) async fn pip_compile(
     upgrade_mode: UpgradeMode,
     index_urls: Option<IndexUrls>,
     no_build: bool,
+    python_version: Option<PythonVersion>,
     cache: &Path,
     mut printer: Printer,
 ) -> Result<ExitStatus> {
@@ -118,6 +120,12 @@ pub(crate) async fn pip_compile(
         venv.interpreter_info().simple_version(),
     )?;
 
+    // Determine the markers to use for resolution.
+    let markers = python_version.map_or_else(
+        || Cow::Borrowed(venv.interpreter_info().markers()),
+        |python_version| Cow::Owned(python_version.markers(venv.interpreter_info().markers())),
+    );
+
     // Instantiate a client.
     let client = {
         let mut builder = RegistryClientBuilder::default();
@@ -142,14 +150,9 @@ pub(crate) async fn pip_compile(
     );
 
     // Resolve the dependencies.
-    let resolver = puffin_resolver::Resolver::new(
-        manifest,
-        venv.interpreter_info().markers(),
-        &tags,
-        &client,
-        &build_dispatch,
-    )
-    .with_reporter(ResolverReporter::from(printer));
+    let resolver =
+        puffin_resolver::Resolver::new(manifest, &markers, &tags, &client, &build_dispatch)
+            .with_reporter(ResolverReporter::from(printer));
     let resolution = match resolver.resolve().await {
         Err(puffin_resolver::ResolveError::PubGrub(err)) => {
             #[allow(clippy::print_stderr)]

crates/puffin-cli/src/main.rs

@@ -15,12 +15,14 @@ use requirements::ExtrasSpecification;
 
 use crate::commands::{extra_name_with_clap_error, ExitStatus};
 use crate::index_urls::IndexUrls;
+use crate::python_version::PythonVersion;
 use crate::requirements::RequirementsSource;
 
 mod commands;
 mod index_urls;
 mod logging;
 mod printer;
+mod python_version;
 mod requirements;
 
 #[derive(Parser)]
@@ -116,6 +118,10 @@ struct PipCompileArgs {
     /// cached wheels of already built source distributions will be reused.
     #[clap(long)]
     no_build: bool,
+
+    /// The minimum Python version that should be supported.
+    #[arg(long, short, value_enum)]
+    python_version: Option<PythonVersion>,
 }
 
 #[derive(Args)]
@@ -249,6 +255,7 @@ async fn inner() -> Result<ExitStatus> {
                 args.upgrade.into(),
                 index_urls,
                 args.no_build,
+                args.python_version,
                 &cache_dir,
                 printer,
             )

crates/puffin-cli/src/python_version.rs

@@ -0,0 +1,70 @@
+use std::str::FromStr;
+
+use pep508_rs::{MarkerEnvironment, StringVersion};
+
+#[derive(Debug, Clone, Copy, clap::ValueEnum)]
+pub(crate) enum PythonVersion {
+    Py37,
+    Py38,
+    Py39,
+    Py310,
+    Py311,
+    Py312,
+}
+
+impl PythonVersion {
+    /// Return the `python_version` marker for a [`PythonVersion`].
+    fn python_version(self) -> &'static str {
+        match self {
+            Self::Py37 => "3.7",
+            Self::Py38 => "3.8",
+            Self::Py39 => "3.9",
+            Self::Py310 => "3.10",
+            Self::Py311 => "3.11",
+            Self::Py312 => "3.12",
+        }
+    }
+
+    /// Return the `python_full_version` marker for a [`PythonVersion`].
+    fn python_full_version(self) -> &'static str {
+        match self {
+            Self::Py37 => "3.7.0",
+            Self::Py38 => "3.8.0",
+            Self::Py39 => "3.9.0",
+            Self::Py310 => "3.10.0",
+            Self::Py311 => "3.11.0",
+            Self::Py312 => "3.12.0",
+        }
+    }
+
+    /// Return the `implementation_version` marker for a [`PythonVersion`].
+    fn implementation_version(self) -> &'static str {
+        match self {
+            Self::Py37 => "3.7.0",
+            Self::Py38 => "3.8.0",
+            Self::Py39 => "3.9.0",
+            Self::Py310 => "3.10.0",
+            Self::Py311 => "3.11.0",
+            Self::Py312 => "3.12.0",
+        }
+    }
+
+    /// Return a [`MarkerEnvironment`] compatible with the given [`PythonVersion`], based on
+    /// a base [`MarkerEnvironment`].
+    ///
+    /// The returned [`MarkerEnvironment`] will preserve the base environment's platform markers,
+    /// but override its Python version markers.
+    pub(crate) fn markers(self, base: &MarkerEnvironment) -> MarkerEnvironment {
+        let mut markers = base.clone();
+        // Ex) `python_version == "3.12"`
+        markers.python_version = StringVersion::from_str(self.python_version()).unwrap();
+        // Ex) `python_full_version == "3.12.0"`
+        markers.python_full_version = StringVersion::from_str(self.python_full_version()).unwrap();
+        // Ex) `implementation_version == "3.12.0"`
+        if markers.implementation_name == "cpython" {
+            markers.implementation_version =
+                StringVersion::from_str(self.implementation_version()).unwrap();
+        }
+        markers
+    }
+}

crates/puffin-cli/tests/pip_compile.rs

@@ -554,6 +554,82 @@ optional-dependencies.foo = [
     Ok(())
 }
 
+/// Resolve a specific version of Black at Python 3.12.
+#[test]
+fn compile_python_312() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let venv = temp_dir.child(".venv");
+
+    Command::new(get_cargo_bin(BIN_NAME))
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+    venv.assert(predicates::path::is_dir());
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.touch()?;
+    requirements_in.write_str("black==23.10.1")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("py312")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir));
+    });
+
+    Ok(())
+}
+
+/// Resolve a specific version of Black at Python 3.7.
+#[test]
+fn compile_python_37() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let venv = temp_dir.child(".venv");
+
+    Command::new(get_cargo_bin(BIN_NAME))
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+    venv.assert(predicates::path::is_dir());
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.touch()?;
+    requirements_in.write_str("black==23.10.1")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("py37")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir));
+    });
+
+    Ok(())
+}
+
 /// Resolve a specific Flask wheel via a URL dependency.
 #[test]
 fn compile_wheel_url_dependency() -> Result<()> {

crates/puffin-cli/tests/snapshots/pip_compile__compile_python_312.snap

@@ -0,0 +1,34 @@
+---
+source: crates/puffin-cli/tests/pip_compile.rs
+info:
+  program: puffin
+  args:
+    - pip-compile
+    - requirements.in
+    - "--python-version"
+    - py312
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpbKzceW
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpZkKRNz/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+# This file was autogenerated by Puffin v0.0.1 via the following command:
+#    [BIN_PATH] pip-compile requirements.in --python-version py312 --cache-dir [CACHE_DIR]
+black==23.10.1
+click==8.1.7
+    # via black
+mypy-extensions==1.0.0
+    # via black
+packaging==23.2
+    # via black
+pathspec==0.11.2
+    # via black
+platformdirs==3.11.0
+    # via black
+
+----- stderr -----
+Resolved 6 packages in [TIME]
+

crates/puffin-cli/tests/snapshots/pip_compile__compile_python_37.snap

@@ -0,0 +1,45 @@
+---
+source: crates/puffin-cli/tests/pip_compile.rs
+info:
+  program: puffin
+  args:
+    - pip-compile
+    - requirements.in
+    - "--python-version"
+    - py37
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpQwHoBA
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmp1TmUIW/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+# This file was autogenerated by Puffin v0.0.1 via the following command:
+#    [BIN_PATH] pip-compile requirements.in --python-version py37 --cache-dir [CACHE_DIR]
+black==23.10.1
+click==8.1.7
+    # via black
+importlib-metadata==6.8.0
+    # via click
+mypy-extensions==1.0.0
+    # via black
+packaging==23.2
+    # via black
+pathspec==0.11.2
+    # via black
+platformdirs==3.11.0
+    # via black
+tomli==2.0.1
+    # via black
+typing-extensions==4.8.0
+    # via
+    #   black
+    #   importlib-metadata
+    #   platformdirs
+zipp==3.17.0
+    # via importlib-metadata
+
+----- stderr -----
+Resolved 10 packages in [TIME]
+