Add option to install only dev dependencies

Add an option to poetry install to install only dev dependencies,
e.g. poetry install --dev-only.

Fixes https://github.com/python-poetry/poetry/issues/2572

docs/docs/cli.md

@@ -109,6 +109,13 @@ the `--no-dev` option.
 poetry install --no-dev
 ```
 
+Conversely, you can specify to the command that you only want to install the development dependencies
+by passing the `--dev-only` option. Note that `--no-dev` takes priority if both options are passed.
+
+```bash
+poetry install --dev-only
+```
+
 If you want to remove old dependencies no longer present in the lock file, use the
 `--remove-untracked` option.
 
@@ -142,9 +149,13 @@ If you want to skip this installation, use the `--no-root` option.
 poetry install --no-root
 ```
 
+Installation of your project's package is also skipped when the `--dev-only`
+option is passed.
+
 ### Options
 
 * `--no-dev`: Do not install dev dependencies.
+* `--dev-only`: Only install dev dependencies.
 * `--no-root`: Do not install the root package (your project).
 * `--dry-run`: Output the operations but do not execute anything (implicitly enables --verbose).
 * `--remove-untracked`: Remove dependencies not presented in the lock file

poetry/console/commands/install.py

@@ -10,6 +10,7 @@ class InstallCommand(InstallerCommand):
 
     options = [
         option("no-dev", None, "Do not install the development dependencies."),
+        option("dev-only", None, "Only install the development dependencies."),
         option(
             "no-root", None, "Do not install the root package (the current project)."
         ),
@@ -64,6 +65,7 @@ dependencies and not including the current project, run the command with the
 
         self._installer.extras(extras)
         self._installer.dev_mode(not self.option("no-dev"))
+        self._installer.dev_only(self.option("dev-only"))
         self._installer.dry_run(self.option("dry-run"))
         self._installer.remove_untracked(self.option("remove-untracked"))
         self._installer.verbose(self._io.is_verbose())
@@ -73,7 +75,7 @@ dependencies and not including the current project, run the command with the
         if return_code != 0:
             return return_code
 
-        if self.option("no-root"):
+        if self.option("no-root") or self.option("dev-only"):
             return 0
 
         try:

poetry/installation/installer.py

@@ -47,6 +47,7 @@ class Installer:
         self._verbose = False
         self._write_lock = True
         self._dev_mode = True
+        self._dev_only = False
         self._execute_operations = True
         self._lock = False
 
@@ -136,6 +137,14 @@ class Installer:
     def is_dev_mode(self):  # type: () -> bool
         return self._dev_mode
 
+    def dev_only(self, dev_only=False):  # type: (bool) -> Installer
+        self._dev_only = dev_only
+
+        return self
+
+    def is_dev_only(self):  # type: () -> bool
+        return self._dev_only
+
     def update(self, update=True):  # type: (bool) -> Installer
         self._update = update
 
@@ -270,6 +279,9 @@ class Installer:
         if not self.is_dev_mode():
             root = root.clone()
             del root.dev_requires[:]
+        elif self.is_dev_only():
+            root = root.clone()
+            del root.requires[:]
 
         if self._io.is_verbose():
             self._io.write_line("")

tests/installation/test_installer.py

@@ -267,7 +267,10 @@ def test_run_update_after_removing_dependencies(
     assert 1 == installer.executor.removals_count
 
 
-def test_run_install_no_dev(installer, locker, repo, package, installed):
+def _configure_run_install_dev(locker, repo, package, installed):
+    """
+    Perform common test setup for `test_run_install_*dev*()` methods.
+    """
     locker.locked(True)
     locker.mock_lock_data(
         {
@@ -323,7 +326,34 @@ def test_run_install_no_dev(installer, locker, repo, package, installed):
     package.add_dependency(Factory.create_dependency("B", "~1.1"))
     package.add_dependency(Factory.create_dependency("C", "~1.2", category="dev"))
 
+
+def test_run_install_no_dev(installer, locker, repo, package, installed):
+    _configure_run_install_dev(locker, repo, package, installed)
+
+    installer.dev_mode(False)
+    installer.run()
+
+    assert 0 == installer.executor.installations_count
+    assert 0 == installer.executor.updates_count
+    assert 1 == installer.executor.removals_count
+
+
+def test_run_install_dev_only(installer, locker, repo, package, installed):
+    _configure_run_install_dev(locker, repo, package, installed)
+
+    installer.dev_only(True)
+    installer.run()
+
+    assert 0 == installer.executor.installations_count
+    assert 0 == installer.executor.updates_count
+    assert 2 == installer.executor.removals_count
+
+
+def test_run_install_no_dev_and_dev_only(installer, locker, repo, package, installed):
+    _configure_run_install_dev(locker, repo, package, installed)
+
     installer.dev_mode(False)
+    installer.dev_only(True)
     installer.run()
 
     assert 0 == installer.executor.installations_count