(WIP) CI: Check that private functions are not used across modules

ci/code_checks.sh

@@ -116,6 +116,21 @@ if [[ -z "$CHECK" || "$CHECK" == "lint" ]]; then
     fi
     RET=$(($RET + $?)) ; echo $MSG "DONE"
 
+    MSG='Check for use of private import across modules' ; echo $MSG
+    if [[ "$GITHUB_ACTIONS" == "true" ]]; then
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_import_across_module" --format="##[error]{source_path}:{line_number}:{msg}" pandas/core/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_import_across_module" --format="##[error]{source_path}:{line_number}:{msg}" pandas/tseries/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+    else
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_import_across_module" pandas/core/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_import_across_module" pandas/tseries/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+    fi
+
     echo "isort --version-number"
     isort --version-number
 

scripts/validate_unwanted_patterns.py

@@ -16,10 +16,27 @@
 import sys
 import token
 import tokenize
-from typing import IO, Callable, FrozenSet, Iterable, List, Tuple
+from typing import IO, Callable, FrozenSet, Iterable, List, Set, Tuple
 
 PATHS_TO_IGNORE: Tuple[str, ...] = ("asv_bench/env",)
 
+PRIVATE_IMPORTS_TO_IGNORE: Set[str] = {
+    "_extension_array_shared_docs",
+    "_index_shared_docs",
+    "_merge_doc",
+    "_shared_docs",
+    "_new_Index",
+    "_new_PeriodIndex",
+    "_doc_template",
+    "_interval_shared_docs",
+    "_apply_docs",
+    "_arith_doc_FRAME",
+    "_flex_comp_doc_FRAME",
+    "_make_flex_doc",
+    "_op_descriptions",
+    "_pipe_template",
+}
+
 
 def _get_literal_string_prefix_len(token_string: str) -> int:
     """
@@ -114,6 +131,38 @@ def bare_pytest_raises(file_obj: IO[str]) -> Iterable[Tuple[int, str]]:
                 )
 
 
+def private_import_across_module(file_obj: IO[str]) -> Iterable[Tuple[int, str]]:
+    """
+    Checking that a private function is not imported across modules.
+
+    Parameters
+    ----------
+    file_obj : IO
+        File-like object containing the Python code to validate.
+
+    Yields
+    ------
+    line_number : int
+        Line number of import statement, that imports the private function.
+    msg : str
+        Explenation of the error.
+    """
+    contents = file_obj.read()
+    tree = ast.parse(contents)
+
+    for node in ast.walk(tree):
+        if not (isinstance(node, ast.Import) or isinstance(node, ast.ImportFrom)):
+            continue
+
+        for module in node.names:
+            module_name = module.name.split(".")[-1]
+            if module_name in PRIVATE_IMPORTS_TO_IGNORE:
+                continue
+
+            if module_name.startswith("_"):
+                yield (node.lineno, f"Import of internal function {repr(module_name)}")
+
+
 def strings_to_concatenate(file_obj: IO[str]) -> Iterable[Tuple[int, str]]:
     """
     This test case is necessary after 'Black' (https://github.com/psf/black),
@@ -362,6 +411,7 @@ def main(
 if __name__ == "__main__":
     available_validation_types: List[str] = [
         "bare_pytest_raises",
+        "private_import_across_module",
         "strings_to_concatenate",
         "strings_with_wrong_placed_whitespace",
     ]