CI: Check private function not used across modules

ci/code_checks.sh

@@ -116,6 +116,19 @@ if [[ -z "$CHECK" || "$CHECK" == "lint" ]]; then
     fi
     RET=$(($RET + $?)) ; echo $MSG "DONE"
 
+    MSG='Check for use of private functions across modules' ; echo $MSG
+    if [[ "$GITHUB_ACTIONS" == "true" ]]; then
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_function_across_module" --format="##[error]{source_path}:{line_number}:{msg}" pandas/core/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_function_across_module" --format="##[error]{source_path}:{line_number}:{msg}" pandas/tseries/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+    else
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_function_across_module" pandas/core/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+        $BASE_DIR/scripts/validate_unwanted_patterns.py --validation-type="private_function_across_module" pandas/tseries/
+        RET=$(($RET + $?)) ; echo $MSG "DONE"
+    fi
+
     echo "isort --version-number"
     isort --version-number
 

scripts/validate_unwanted_patterns.py

@@ -16,7 +16,7 @@
 import sys
 import token
 import tokenize
-from typing import IO, Callable, Iterable, List, Tuple
+from typing import IO, Callable, Iterable, List, Set, Tuple
 
 FILE_EXTENSIONS_TO_CHECK: Tuple[str, ...] = (".py", ".pyx", ".pxi.ini", ".pxd")
 PATHS_TO_IGNORE: Tuple[str, ...] = ("asv_bench/env",)
@@ -115,6 +115,55 @@ def bare_pytest_raises(file_obj: IO[str]) -> Iterable[Tuple[int, str]]:
                 )
 
 
+def private_function_across_module(file_obj: IO[str]) -> Iterable[Tuple[int, str]]:
+    """
+    Checking that a private function is not used across modules.
+
+    Parameters
+    ----------
+    file_obj : IO
+        File-like object containing the Python code to validate.
+
+    Yields
+    ------
+    line_number : int
+        Line number of the private function that is used across modules.
+    msg : str
+        Explenation of the error.
+    """
+    contents = file_obj.read()
+    tree = ast.parse(contents)
+
+    imported_modules: Set[str] = set()
+
+    for node in ast.walk(tree):
+        if isinstance(node, (ast.Import, ast.ImportFrom)):
+            for module in node.names:
+                module_fqdn = module.name if module.asname is None else module.asname
+                imported_modules.add(module_fqdn)
+
+        if not isinstance(node, ast.Call):
+            continue
+
+        try:
+            module_name = node.func.value.id
+            function_name = node.func.attr
+        except AttributeError:
+            continue
+
+        ### Exception section ###
+
+        # (Debatable) Class case
+        if module_name[0].isupper():
+            continue
+        # (Debatable) Dunder methods case
+        elif function_name.startswith("__") and function_name.endswith("__"):
+            continue
+
+        if module_name in imported_modules and function_name.startswith("_"):
+            yield (node.lineno, f"Private function '{module_name}.{function_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),
@@ -358,6 +407,7 @@ def main(
 if __name__ == "__main__":
     available_validation_types: List[str] = [
         "bare_pytest_raises",
+        "private_function_across_module",
         "strings_to_concatenate",
         "strings_with_wrong_placed_whitespace",
     ]