Warn about packages/modules included as package data

1 files changed, 54 insertions, 2 deletions

diff --git a/setuptools/command/build_py.py b/setuptools/command/build_py.py
index c3fdc092..ba7b7259 100644
--- a/setuptools/command/build_py.py
+++ b/setuptools/command/build_py.py
@@ -8,6 +8,9 @@ import io
 import distutils.errors
 import itertools
 import stat
+import warnings
+from pathlib import Path
+from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
 from setuptools.extern.more_itertools import unique_everseen
 
 
@@ -129,6 +132,7 @@ class build_py(orig.build_py):
             src_dirs[assert_relative(self.get_package_dir(package))] = package
 
         self.run_command('egg_info')
+        checker = _IncludePackageDataAbuse()
         ei_cmd = self.get_finalized_command('egg_info')
         for path in ei_cmd.filelist.files:
             d, f = os.path.split(assert_relative(path))
@@ -139,8 +143,13 @@ class build_py(orig.build_py):
                 d, df = os.path.split(d)
                 f = os.path.join(df, f)
             if d in src_dirs:
-                if path.endswith('.py') and f == oldf:
-                    continue  # it's a module, not data
+                if f == oldf:
+                    if checker.is_module(f):
+                        continue  # it's a module, not data
+                else:
+                    importable = checker.importable_item(src_dirs[d], f)
+                    if importable:
+                        checker.warn(importable)
                 mf.setdefault(src_dirs[d], []).append(path)
 
     def get_data_files(self):
@@ -240,3 +249,46 @@ def assert_relative(path):
         % path
     )
     raise DistutilsSetupError(msg)
+
+
+class _IncludePackageDataAbuse:
+    """Inform users that package or module is included as 'data file'"""
+
+    MESSAGE = """\
+    !!\n\n
+    ###################################
+    # Package/module would be ignored #
+    ###################################
+    Python recognizes {importable!r} as an importable package or module, however
+    it is included in the distribution as "data".
+    This behavior is likely to change in future versions of setuptools (and
+    therefore is considered deprecated).
+
+    Please make sure that {importable!r} is recognized as a package/module by using
+    setuptools' `packages` configuration field or the proper package discovery methods.
+
+    To find more information, look for "package discovery" and "data files" on
+    setuptools documentation page.
+    \n\n!!
+    """
+
+    def __init__(self):
+        self._already_warned = set()
+
+    def is_module(self, file):
+        return file.endswith(".py") and file[:-len(".py")].isidentifier()
+
+    def importable_item(self, pkg, file):
+        path = Path(file)
+        parents = path.parent.parts
+        module = [path.stem] if tuple(path.suffixes) == (".py",) else []
+        parts = list(itertools.takewhile(str.isidentifier, [*parents, *module]))
+        if parts:
+            return ".".join([pkg, *parts])
+        return None
+
+    def warn(self, importable):
+        if importable not in self._already_warned:
+            msg = textwrap.dedent(self.MESSAGE).format(importable=importable)
+            warnings.warn(msg, SetuptoolsDeprecationWarning, stacklevel=2)
+            self._already_warned.add(importable)