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
 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
 If you want to remove old dependencies no longer present in the lock file, use the
 `--remove-untracked` option.
 `--remove-untracked` option.
 
 
@@ -142,9 +149,13 @@ If you want to skip this installation, use the `--no-root` option.
 poetry install --no-root
 poetry install --no-root
 ```
 ```
 
 
+Installation of your project's package is also skipped when the `--dev-only`
+option is passed.
+
 ### Options
 ### Options
 
 
 * `--no-dev`: Do not install dev dependencies.
 * `--no-dev`: Do not install dev dependencies.
+* `--dev-only`: Only install dev dependencies.
 * `--no-root`: Do not install the root package (your project).
 * `--no-root`: Do not install the root package (your project).
 * `--dry-run`: Output the operations but do not execute anything (implicitly enables --verbose).
 * `--dry-run`: Output the operations but do not execute anything (implicitly enables --verbose).
 * `--remove-untracked`: Remove dependencies not presented in the lock file
 * `--remove-untracked`: Remove dependencies not presented in the lock file

poetry/console/commands/install.py

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

poetry/installation/installer.py

@@ -47,6 +47,7 @@ class Installer:
         self._verbose = False
         self._verbose = False
         self._write_lock = True
         self._write_lock = True
         self._dev_mode = True
         self._dev_mode = True
+        self._dev_only = False
         self._execute_operations = True
         self._execute_operations = True
         self._lock = False
         self._lock = False
 
 
@@ -136,6 +137,14 @@ class Installer:
     def is_dev_mode(self):  # type: () -> bool
     def is_dev_mode(self):  # type: () -> bool
         return self._dev_mode
         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
     def update(self, update=True):  # type: (bool) -> Installer
         self._update = update
         self._update = update
 
 
@@ -270,6 +279,9 @@ class Installer:
         if not self.is_dev_mode():
         if not self.is_dev_mode():
             root = root.clone()
             root = root.clone()
             del root.dev_requires[:]
             del root.dev_requires[:]
+        elif self.is_dev_only():
+            root = root.clone()
+            del root.requires[:]
 
 
         if self._io.is_verbose():
         if self._io.is_verbose():
             self._io.write_line("")
             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
     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.locked(True)
     locker.mock_lock_data(
     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("B", "~1.1"))
     package.add_dependency(Factory.create_dependency("C", "~1.2", category="dev"))
     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_mode(False)
+    installer.dev_only(True)
     installer.run()
     installer.run()
 
 
     assert 0 == installer.executor.installations_count
     assert 0 == installer.executor.installations_count