Add an empty group with uv add -r (#16490)

## Summary

`uv add -r requirements.txt --group foo` will now create `foo` even if
`requirements.txt` is empty.

Closes #16361.

Author: charliermarsh

crates/uv-workspace/src/pyproject_mut.rs

@@ -622,6 +622,41 @@ impl PyProjectTomlMut {
         Ok(added)
     }
 
+    /// Ensure that an optional dependency group exists, creating an empty group if it doesn't.
+    pub fn ensure_optional_dependency(&mut self, extra: &ExtraName) -> Result<(), Error> {
+        // Get or create `project.optional-dependencies`.
+        let optional_dependencies = self
+            .project()?
+            .entry("optional-dependencies")
+            .or_insert(Item::Table(Table::new()))
+            .as_table_like_mut()
+            .ok_or(Error::MalformedDependencies)?;
+
+        // Check if the extra already exists.
+        let extra_exists = optional_dependencies
+            .iter()
+            .any(|(key, _value)| ExtraName::from_str(key).is_ok_and(|e| e == *extra));
+
+        // If the extra doesn't exist, create it.
+        if !extra_exists {
+            optional_dependencies.insert(extra.as_ref(), Item::Value(Value::Array(Array::new())));
+        }
+
+        // If `project.optional-dependencies` is an inline table, reformat it.
+        //
+        // Reformatting can drop comments between keys, but you can't put comments
+        // between items in an inline table anyway.
+        if let Some(optional_dependencies) = self
+            .project()?
+            .get_mut("optional-dependencies")
+            .and_then(Item::as_inline_table_mut)
+        {
+            optional_dependencies.fmt();
+        }
+
+        Ok(())
+    }
+
     /// Adds a dependency to `dependency-groups`.
     ///
     /// Returns `true` if the dependency was added, `false` if it was updated.
@@ -693,6 +728,54 @@ impl PyProjectTomlMut {
         Ok(added)
     }
 
+    /// Ensure that a dependency group exists, creating an empty group if it doesn't.
+    pub fn ensure_dependency_group(&mut self, group: &GroupName) -> Result<(), Error> {
+        // Get or create `dependency-groups`.
+        let dependency_groups = self
+            .doc
+            .entry("dependency-groups")
+            .or_insert(Item::Table(Table::new()))
+            .as_table_like_mut()
+            .ok_or(Error::MalformedDependencies)?;
+
+        let was_sorted = dependency_groups
+            .get_values()
+            .iter()
+            .filter_map(|(dotted_ks, _)| dotted_ks.first())
+            .map(|k| k.get())
+            .is_sorted();
+
+        // Check if the group already exists.
+        let group_exists = dependency_groups
+            .iter()
+            .any(|(key, _value)| GroupName::from_str(key).is_ok_and(|g| g == *group));
+
+        // If the group doesn't exist, create it.
+        if !group_exists {
+            dependency_groups.insert(group.as_ref(), Item::Value(Value::Array(Array::new())));
+
+            // To avoid churn in pyproject.toml, we only sort new group keys if the
+            // existing keys were sorted.
+            if was_sorted {
+                dependency_groups.sort_values();
+            }
+        }
+
+        // If `dependency-groups` is an inline table, reformat it.
+        //
+        // Reformatting can drop comments between keys, but you can't put comments
+        // between items in an inline table anyway.
+        if let Some(dependency_groups) = self
+            .doc
+            .get_mut("dependency-groups")
+            .and_then(Item::as_inline_table_mut)
+        {
+            dependency_groups.fmt();
+        }
+
+        Ok(())
+    }
+
     /// Set the constraint for a requirement for an existing dependency.
     pub fn set_dependency_bound(
         &mut self,

crates/uv/src/commands/project/add.rs

@@ -645,6 +645,22 @@ pub(crate) async fn add(
         &mut toml,
     )?;
 
+    // If no requirements were added but a dependency group or optional dependency was specified,
+    // ensure the group/extra exists. This handles the case where `uv add -r requirements.txt
+    // --group <name>` or `uv add -r requirements.txt --optional <extra>` is called with an empty
+    // requirements file.
+    if edits.is_empty() {
+        match &dependency_type {
+            DependencyType::Group(group) => {
+                toml.ensure_dependency_group(group)?;
+            }
+            DependencyType::Optional(extra) => {
+                toml.ensure_optional_dependency(extra)?;
+            }
+            _ => {}
+        }
+    }
+
     // Validate any indexes that were provided on the command-line to ensure
     // they point to existing non-empty directories when using path URLs.
     let mut valid_indexes = Vec::with_capacity(indexes.len());

crates/uv/tests/it/edit.rs

@@ -4954,6 +4954,112 @@ fn add_virtual_dependency_group() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn add_empty_requirements_group() -> Result<()> {
+    // Test that `uv add -r requirements.txt --group <name>` creates an empty group
+    // when the requirements file is empty
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("")?;
+
+    uv_snapshot!(context.filters(), context.add()
+        .arg("-r").arg("requirements.txt")
+        .arg("--group").arg("user"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Requirements file `requirements.txt` does not contain any dependencies
+    Resolved 1 package in [TIME]
+    Audited in [TIME]
+    ");
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+
+        [dependency-groups]
+        user = []
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+#[test]
+fn add_empty_requirements_optional() -> Result<()> {
+    // Test that `uv add -r requirements.txt --optional <extra>` creates an empty extra
+    // when the requirements file is empty
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("")?;
+
+    uv_snapshot!(context.filters(), context.add()
+        .arg("-r").arg("requirements.txt")
+        .arg("--optional").arg("extra"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Requirements file `requirements.txt` does not contain any dependencies
+    Resolved 1 package in [TIME]
+    Audited in [TIME]
+    ");
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+
+        [project.optional-dependencies]
+        extra = []
+        "###
+        );
+    });
+
+    Ok(())
+}
+
 #[test]
 fn remove_virtual_empty() -> Result<()> {
     // testing how `uv remove` reacts to a pyproject with no `[project]` and nothing useful to it