feat: add apt.is_installed() function

src/wuttamess/apt.py

@@ -51,6 +51,19 @@ def install(c, *packages, **kwargs):
     return c.run(f'DEBIAN_FRONTEND={frontend} apt-get --assume-yes install {packages}')
 
 
+def is_installed(c, package):
+    """
+    Check if the given APT package is installed.
+
+    :param c: Fabric connection.
+
+    :param package: Name of package to be checked.
+
+    :returns: ``True`` if package is installed, else ``False``.
+    """
+    return c.run(f'dpkg-query -s {package}', warn=True).ok
+
+
 def update(c):
     """
     Update the APT package lists.  Essentially this runs:

tests/test_apt.py

@@ -25,6 +25,21 @@ class TestInstall(TestCase):
         c.run.assert_called_once_with('DEBIAN_FRONTEND=noninteractive apt-get --assume-yes install postfix')
 
 
+class TestIsInstalled(TestCase):
+
+    def test_already_installed(self):
+        c = MagicMock()
+        c.run.return_value.ok = True
+        self.assertTrue(mod.is_installed(c, 'postfix'))
+        c.run.assert_called_once_with('dpkg-query -s postfix', warn=True)
+
+    def test_not_installed(self):
+        c = MagicMock()
+        c.run.return_value.ok = False
+        self.assertFalse(mod.is_installed(c, 'postfix'))
+        c.run.assert_called_once_with('dpkg-query -s postfix', warn=True)
+
+
 class TestUpdate(TestCase):
 
     def test_basic(self):