Add support for running as Python module

This change makes it easier to run pip-tools on systems which use multiple Python versions by enabling workflows like ``py -X.Y -m piptools ...`` on Windows and ``pythonX.Y -m piptools ...` elsewhere.`

README.md

@@ -61,6 +61,10 @@ $ pip-compile --upgrade-package flask --upgrade-package requests  # update both
 $ pip-compile -P flask -P requests==2.0.0  # update the flask package to the latest, and requests to v2.0.0
 ```
 
+If you use multiple Python versions, you can run ``pip-compile`` as ``py -X.Y
+-m piptools compile ...`` on Windows and ``pythonX.Y -m piptools compile ...``
+on other systems.
+
 Example usage for `pip-sync`
 ============================
 
@@ -87,3 +91,7 @@ To sync multiple `*.txt` dependency lists, just pass them in via command line ar
 $ pip-sync dev-requirements.txt requirements.txt
 ```
 Passing in empty arguments would cause it to default to `requirements.txt`.
+
+If you use multiple Python versions, you can run ``pip-sync`` as ``py -X.Y -m
+piptools sync ...`` on Windows and ``pythonX.Y -m piptools sync ...`` on other
+systems.

piptools/__main__.py

@@ -0,0 +1,16 @@
+import click
+from piptools.scripts import compile, sync
+
+
+@click.group()
+def cli():
+    pass
+
+
+cli.add_command(compile.cli, 'compile')
+cli.add_command(sync.cli, 'sync')
+
+
+# Enable ``python -m piptools ...``.
+if __name__ == '__main__':  # pragma: no cover
+    cli()

piptools/scripts/sync.py

@@ -28,6 +28,7 @@
 @click.option('--no-index', is_flag=True, help="Ignore package index (only looking at --find-links URLs instead)")
 @click.argument('src_files', required=False, type=click.Path(exists=True), nargs=-1)
 def cli(dry_run, force, find_links, index_url, extra_index_url, no_index, src_files):
+    """Synchronize virtual environment with requirements.txt."""
     if not src_files:
         if os.path.exists(DEFAULT_REQUIREMENTS_FILE):
             src_files = (DEFAULT_REQUIREMENTS_FILE,)

tests/test_cli.py

@@ -1,6 +1,7 @@
 import os
 from textwrap import dedent
 import subprocess
+import sys
 
 from click.testing import CliRunner
 
@@ -125,3 +126,46 @@ def test_realistic_complex_sub_dependencies(tmpdir):
 
         print(out.output)
         assert out.exit_code == 0
+
+
+def invoke(command):
+    """Invoke sub-process."""
+    try:
+        output = subprocess.check_output(
+            command,
+            stderr=subprocess.STDOUT,
+        )
+        status = 0
+    except subprocess.CalledProcessError as error:
+        output = error.output
+        status = error.returncode
+
+    return status, output
+
+
+def test_run_as_module_compile(tmpdir):
+    """piptools can be run as ``python -m piptools ...``."""
+
+    status, output = invoke([
+        sys.executable, '-m', 'piptools', 'compile', '--help',
+    ])
+
+    # Should have run pip-compile successfully.
+    output = output.decode('utf-8')
+    assert output.startswith('Usage:')
+    assert 'Compiles requirements.txt from requirements.in specs.' in output
+    assert status == 0
+
+
+def test_run_as_module_sync():
+    """piptools can be run as ``python -m piptools ...``."""
+
+    status, output = invoke([
+        sys.executable, '-m', 'piptools', 'sync', '--help',
+    ])
+
+    # Should have run pip-compile successfully.
+    output = output.decode('utf-8')
+    assert output.startswith('Usage:')
+    assert 'Synchronize virtual environment with requirements.txt.' in output
+    assert status == 0