charm_build: install binary packages with no special options

Option `--prefer-binary` is only available in pip 20.x and bionic
installs 9.x. Options are to update pip to a newer version and keep
using `--prefer-binary`; change the option to `--only-binary` (in
this case installation of packages only available in source form
or depending on packages only available in source form would fail); or
not specify any option related to binary installation (which would
result in installation from source if tha latest version of a package
is only available in source form, which is a possible situation but
likely to be a corner case). This commit implements the third
solution.

Signed-off-by: Claudio Matsuoka <[email protected]>

charmcraft/charm_builder.py

@@ -232,8 +232,8 @@ def handle_dependencies(self):
             _process_run([pip_cmd, "--version"])
 
             if self.binary_python_packages:
-                # install binary python packages
-                cmd = [pip_cmd, "install", "--upgrade", "--prefer-binary"]  # base command
+                # install python packages, allowing binary packages
+                cmd = [pip_cmd, "install", "--upgrade"]  # base command
                 for pkg in self.binary_python_packages:
                     cmd.append(pkg)  # the python package to install
                 _process_run(cmd)

charmcraft/parts.py

@@ -71,8 +71,8 @@ class CharmPlugin(plugins.Plugin):
       - ``charm-binary-python-packages``
         (list of strings)
         A list of python packages to install from PyPI before installing
-        requirements. Binary packages are preferred, but they may also be
-        installed from sources if a package is only available in source format.
+        requirements. Binary packages are allowed, but they may also be
+        installed from sources if a package is only available in source form.
 
       - ``charm-python-packages``
         (list of strings)

tests/test_charm_builder.py

@@ -730,7 +730,7 @@ def test_build_dependencies_virtualenv_binary_packages(tmp_path):
     assert mock.mock_calls == [
         call(["python3", "-m", "venv", str(tmp_path / STAGING_VENV_DIRNAME)]),
         call([pip_cmd, "--version"]),
-        call([pip_cmd, "install", "--upgrade", "--prefer-binary", "pkg1", "pkg2"]),
+        call([pip_cmd, "install", "--upgrade", "pkg1", "pkg2"]),
     ]
 
     site_packages_dir = charm_builder._find_venv_site_packages(pathlib.Path(STAGING_VENV_DIRNAME))
@@ -762,7 +762,7 @@ def test_build_dependencies_virtualenv_all(tmp_path):
     assert mock.mock_calls == [
         call(["python3", "-m", "venv", str(tmp_path / STAGING_VENV_DIRNAME)]),
         call([pip_cmd, "--version"]),
-        call([pip_cmd, "install", "--upgrade", "--prefer-binary", "pkg1", "pkg2"]),
+        call([pip_cmd, "install", "--upgrade", "pkg1", "pkg2"]),
         call([pip_cmd, "install", "--upgrade", "--no-binary", ":all:", "pkg3", "pkg4"]),
         call(
             [