Add support to the installer for parallizing dependency installs

tox_poetry_installer/installer.py

@@ -2,7 +2,10 @@
 # Silence this one globally to support the internal function imports for the proxied poetry module.
 # See the docstring in 'tox_poetry_installer._poetry' for more context.
 # pylint: disable=import-outside-toplevel
+import concurrent.futures
+import contextlib
 import typing
+from typing import Optional
 from typing import Sequence
 from typing import Set
 
@@ -18,13 +21,18 @@ if typing.TYPE_CHECKING:
 
 
 def install(
-    poetry: "_poetry.Poetry", venv: ToxVirtualEnv, packages: Sequence[PoetryPackage]
+    poetry: "_poetry.Poetry",
+    venv: ToxVirtualEnv,
+    packages: Sequence[PoetryPackage],
+    parallels: Optional[int] = None,
 ):
     """Install a bunch of packages to a virtualenv
 
     :param poetry: Poetry object the packages were sourced from
     :param venv: Tox virtual environment to install the packages to
     :param packages: List of packages to install to the virtual environment
+    :param parallels: Number of parallel processes to use for installing dependency packages, or
+                      ``None`` to disable parallelization.
     """
     from tox_poetry_installer import _poetry
 
@@ -40,14 +48,34 @@ def install(
 
     installed: Set[PoetryPackage] = set()
 
+    @contextlib.contextmanager
+    def _optional_parallelize():
+        """A bit of cheat, really
+
+        A context manager that exposes a common interface for the caller that optionally
+        enables/disables the usage of the parallel thread pooler depending on the value of
+        the ``parallels`` parameter.
+        """
+        if parallels:
+            with concurrent.futures.ThreadPoolExecutor(
+                max_workers=parallels
+            ) as executor:
+                yield executor.submit
+        else:
+            yield lambda func, arg: func(arg)
+
+    with _optional_parallelize() as executor:
         for dependency in packages:
             if dependency not in installed:
+                installed.add(dependency)
                 tox.reporter.verbosity2(
                     f"{constants.REPORTER_PREFIX} Installing {dependency}"
                 )
-            pip.install(dependency)
+                executor(pip.install, dependency)
-            installed.add(dependency)
             else:
                 tox.reporter.verbosity2(
                     f"{constants.REPORTER_PREFIX} Skipping {dependency}, already installed"
                 )
+        tox.reporter.verbosity2(
+            f"{constants.REPORTER_PREFIX} Waiting for installs to finish..."
+        )