diff --git a/utils/tests_fetcher.py b/utils/tests_fetcher.py
index 5d0518b00f..3344ae960a 100644
--- a/utils/tests_fetcher.py
+++ b/utils/tests_fetcher.py
@@ -18,20 +18,34 @@ Welcome to tests_fetcher V2.
 This util is designed to fetch tests to run on a PR so that only the tests impacted by the modifications are run, and
 when too many models are being impacted, only run the tests of a subset of core models. It works like this.
 
-Stage 1: Identify the modified files. This takes all the files from the branching point to the current commit (so
-all modifications in a PR, not just the last commit) but excludes modifications that are on docstrings or comments
-only.
+Stage 1: Identify the modified files. For jobs that run on the main branch, it's just the diff with the last commit.
+On a PR, this takes all the files from the branching point to the current commit (so all modifications in a PR, not
+just the last commit) but excludes modifications that are on docstrings or comments only.
 
 Stage 2: Extract the tests to run. This is done by looking at the imports in each module and test file: if module A
 imports module B, then changing module B impacts module A, so the tests using module A should be run. We thus get the
 dependencies of each model and then recursively builds the 'reverse' map of dependencies to get all modules and tests
-impacted by a given file. We then only keep the tests (and only the code models tests if there are too many modules).
+impacted by a given file. We then only keep the tests (and only the core models tests if there are too many modules).
 
 Caveats:
   - This module only filters tests by files (not individual tests) so it's better to have tests for different things
     in different files.
   - This module assumes inits are just importing things, not really building objects, so it's better to structure
     them this way and move objects building in separate submodules.
+
+Usage:
+
+Base use to fetch the tests in a pull request
+
+```bash
+python utils/tests_fetcher.py
+```
+
+Base use to fetch the tests on a the main branch (with diff from the last commit):
+
+```bash
+python utils/tests_fetcher.py --diff_with_last_commit
+```
 """
 
 import argparse
@@ -41,6 +55,7 @@ import os
 import re
 from contextlib import contextmanager
 from pathlib import Path
+from typing import Dict, List, Optional, Tuple, Union
 
 from git import Repo
 
@@ -80,9 +95,13 @@ IMPORTANT_MODELS = [
 
 
 @contextmanager
-def checkout_commit(repo, commit_id):
+def checkout_commit(repo: Repo, commit_id: str):
     """
-    Context manager that checks out a commit in the repo.
+    Context manager that checks out a given commit when entered, but gets back to the reference it was at on exit.
+
+    Args:
+        repo (`git.Repo`): A git repository (for instance the Transformers repo).
+        commit_id (`str`): The commit reference to checkout inside the context manager.
     """
     current_head = repo.head.commit if repo.head.is_detached else repo.head.ref
 
@@ -94,10 +113,19 @@ def checkout_commit(repo, commit_id):
         repo.git.checkout(current_head)
 
 
-def clean_code(content):
+def clean_code(content: str) -> str:
     """
-    Remove docstrings, empty line or comments from `content`.
+    Remove docstrings, empty line or comments from some code (used to detect if a diff is real or only concern
+    comments or docstings).
+
+    Args:
+        content (`str`): The code to clean
+
+    Returns:
+        `str`: The cleaned code.
     """
+    # We need to deactivate autoformatting here to write escaped triple quotes (we cannot use real triple quotes or
+    # this would mess up the result if this function applied to this particular file).
     # fmt: off
     # Remove docstrings by splitting on triple " then triple ':
     splits = content.split('\"\"\"')
@@ -111,15 +139,22 @@ def clean_code(content):
     for line in content.split("\n"):
         # remove anything that is after a # sign.
         line = re.sub("#.*$", "", line)
-        if len(line) == 0 or line.isspace():
-            continue
-        lines_to_keep.append(line)
+        # remove white lines
+        if len(line) != 0 and not line.isspace():
+            lines_to_keep.append(line)
     return "\n".join(lines_to_keep)
 
 
-def keep_doc_examples_only(content):
+def keep_doc_examples_only(content: str) -> str:
     """
-    Remove code, docstring that is not code example, empty line or comments from `content`.
+    Remove everything from the code content except the doc examples (used to determined if a diff should trigger doc
+    tests or not).
+
+    Args:
+        content (`str`): The code to clean
+
+    Returns:
+        `str`: The cleaned code.
     """
     # Keep doc examples only by splitting on triple "`"
     splits = content.split("```")
@@ -131,17 +166,18 @@ def keep_doc_examples_only(content):
     for line in content.split("\n"):
         # remove anything that is after a # sign.
         line = re.sub("#.*$", "", line)
-        if len(line) == 0 or line.isspace():
-            continue
-        lines_to_keep.append(line)
+        # remove white lines
+        if len(line) != 0 and not line.isspace():
+            lines_to_keep.append(line)
     return "\n".join(lines_to_keep)
 
 
-def get_all_tests():
+def get_all_tests() -> List[str]:
     """
-    Return a list of paths to all test folders and files under `tests`. All paths are rooted at `tests`.
+    Walks the `tests` folder to return a list of files/subfolders. This is used to split the tests to run when using
+    paralellism. The split is:
 
-    - folders under `tests`: `tokenization`, `pipelines`, etc. The folder `models` is excluded.
+    - folders under `tests`: (`tokenization`, `pipelines`, etc) except the subfolder `models` is excluded.
     - folders under `tests/models`: `bert`, `gpt2`, etc.
     - test files under `tests`: `test_modeling_common.py`, `test_tokenization_common.py`, etc.
     """
@@ -165,9 +201,17 @@ def get_all_tests():
     return tests
 
 
-def diff_is_docstring_only(repo, branching_point, filename):
+def diff_is_docstring_only(repo: Repo, branching_point: str, filename: str) -> bool:
     """
-    Check if the diff is only in docstrings in a filename.
+    Check if the diff is only in docstrings (or comments and whitespace) in a filename.
+
+    Args:
+        repo (`git.Repo`): A git repository (for instance the Transformers repo).
+        branching_point (`str`): The commit reference of where to compare for the diff.
+        filename (`str`): The filename where we want to know if the diff isonly in docstrings/comments.
+
+    Returns:
+        `bool`: Whether the diff is docstring/comments only or not.
     """
     folder = Path(repo.working_dir)
     with checkout_commit(repo, branching_point):
@@ -183,9 +227,17 @@ def diff_is_docstring_only(repo, branching_point, filename):
     return old_content_clean == new_content_clean
 
 
-def diff_contains_doc_examples(repo, branching_point, filename):
+def diff_contains_doc_examples(repo: Repo, branching_point: str, filename: str) -> bool:
     """
-    Check if the diff is only in code in a filename.
+    Check if the diff is only in code examples of the doc in a filename.
+
+    Args:
+        repo (`git.Repo`): A git repository (for instance the Transformers repo).
+        branching_point (`str`): The commit reference of where to compare for the diff.
+        filename (`str`): The filename where we want to know if the diff is only in codes examples.
+
+    Returns:
+        `bool`: Whether the diff is only in code examples of the doc or not.
     """
     folder = Path(repo.working_dir)
     with checkout_commit(repo, branching_point):
@@ -201,9 +253,22 @@ def diff_contains_doc_examples(repo, branching_point, filename):
     return old_content_clean != new_content_clean
 
 
-def get_diff(repo, base_commit, commits):
+def get_diff(repo: Repo, base_commit: str, commits: List[str]) -> List[str]:
     """
-    Get's the diff between one or several commits and the head of the repository.
+    Get the diff between a base commit and one or several commits.
+
+    Args:
+        repo (`git.Repo`):
+            A git repository (for instance the Transformers repo).
+        base_commit (`str`):
+            The commit reference of where to compare for the diff. This is the current commit, not the branching point!
+        commits (`List[str]`):
+            The list of commits with which to compare the repo at `base_commit` (so the branching point).
+
+    Returns:
+        `List[str]`: The list of Python files with a diff (files added, renamed or deleted are always returned, files
+        modified are returned if the diff in the file is not only in docstrings or comments, see
+        `diff_is_docstring_only`).
     """
     print("\n### DIFF ###\n")
     code_diff = []
@@ -230,12 +295,17 @@ def get_diff(repo, base_commit, commits):
     return code_diff
 
 
-def get_modified_python_files(diff_with_last_commit=False):
+def get_modified_python_files(diff_with_last_commit: bool = False) -> List[str]:
     """
     Return a list of python files that have been modified between:
 
     - the current head and the main branch if `diff_with_last_commit=False` (default)
     - the current head and its parent commit otherwise.
+
+    Returns:
+        `List[str]`: The list of Python files with a diff (files added, renamed or deleted are always returned, files
+        modified are returned if the diff in the file is not only in docstrings or comments, see
+        `diff_is_docstring_only`).
     """
     repo = Repo(PATH_TO_REPO)
 
@@ -255,23 +325,34 @@ def get_modified_python_files(diff_with_last_commit=False):
         return get_diff(repo, repo.head.commit, parent_commits)
 
 
-def get_diff_for_doctesting(repo, base_commit, commits):
+def get_diff_for_doctesting(repo: Repo, base_commit: str, commits: List[str]) -> List[str]:
     """
-    Get's the diff between one or several commits and the head of the repository where some doc example(s) are changed.
+    Get the diff in doc examples between a base commit and one or several commits.
+
+    Args:
+        repo (`git.Repo`):
+            A git repository (for instance the Transformers repo).
+        base_commit (`str`):
+            The commit reference of where to compare for the diff. This is the current commit, not the branching point!
+        commits (`List[str]`):
+            The list of commits with which to compare the repo at `base_commit` (so the branching point).
+
+    Returns:
+        `List[str]`: The list of Python and Markdown files with a diff (files added or renamed are always returned, files
+        modified are returned if the diff in the file is only in doctest examples).
     """
     print("\n### DIFF ###\n")
     code_diff = []
     for commit in commits:
         for diff_obj in commit.diff(base_commit):
+            # We only consider Python files and doc files.
+            if not diff_obj.b_path.endswith(".py") and not diff_obj.b_path.endswith(".md"):
+                continue
             # We always add new python/md files
-            if diff_obj.change_type in ["A"] and (diff_obj.b_path.endswith(".py") or diff_obj.b_path.endswith(".md")):
+            if diff_obj.change_type in ["A"]:
                 code_diff.append(diff_obj.b_path)
             # Now for modified files
-            elif (
-                diff_obj.change_type in ["M", "R"]
-                and diff_obj.b_path.endswith(".py")
-                or diff_obj.b_path.endswith(".md")
-            ):
+            elif diff_obj.change_type in ["M", "R"]:
                 # In case of renames, we'll look at the tests using both the old and new name.
                 if diff_obj.a_path != diff_obj.b_path:
                     code_diff.extend([diff_obj.a_path, diff_obj.b_path])
@@ -285,12 +366,16 @@ def get_diff_for_doctesting(repo, base_commit, commits):
     return code_diff
 
 
-def get_doctest_files(diff_with_last_commit=False):
+def get_doctest_files(diff_with_last_commit: bool = False) -> List[str]:
     """
-    Return a list of python and mdx files where some doc example(s) in them have been modified between:
+    Return a list of python and Markdown files where doc example have been modified between:
 
     - the current head and the main branch if `diff_with_last_commit=False` (default)
     - the current head and its parent commit otherwise.
+
+    Returns:
+        `List[str]`: The list of Python and Markdown files with a diff (files added or renamed are always returned, files
+        modified are returned if the diff in the file is only in doctest examples).
     """
     repo = Repo(PATH_TO_REPO)
 
@@ -313,7 +398,7 @@ def get_doctest_files(diff_with_last_commit=False):
     # This is the full list of doctest tests
     with open("utils/documentation_tests.txt") as fp:
         documentation_tests = set(fp.read().strip().split("\n"))
-    # Not to run slow doctest tests
+    # Do not run slow doctest tests
     with open("utils/slow_documentation_tests.txt") as fp:
         slow_documentation_tests = set(fp.read().strip().split("\n"))
 
@@ -348,10 +433,21 @@ _re_single_line_direct_imports = re.compile(r"(?:^|\n)\s*from\s+transformers(\S*
 _re_multi_line_direct_imports = re.compile(r"(?:^|\n)\s*from\s+transformers(\S*)\s+import\s+\(([^\)]+)\)")
 
 
-def extract_imports(module_fname, cache=None):
+def extract_imports(module_fname: str, cache: Dict[str, List[str]] = None) -> List[str]:
     """
-    Get the imports a given module makes. This takes a module filename and returns the list of module filenames
-    imported in the module with the objects imported in that module filename.
+    Get the imports a given module makes.
+
+    Args:
+        module_fname (`str`):
+            The name of the file of the module where we want to look at the imports (given relative to the root of
+            the repo).
+        cache (Dictionary `str` to `List[str]`, *optional*):
+            To speed up this function if it was previously called on `module_fname`, the cache of all previously
+            computed results.
+
+    Returns:
+        `List[str]`: The list of module filenames imported in the input `module_fname` (a submodule we import from that
+        is a subfolder will give its init file).
     """
     if cache is not None and module_fname in cache:
         return cache[module_fname]
@@ -359,7 +455,8 @@ def extract_imports(module_fname, cache=None):
     with open(PATH_TO_REPO / module_fname, "r", encoding="utf-8") as f:
         content = f.read()
 
-    # Filter out all docstrings to not get imports in code examples.
+    # Filter out all docstrings to not get imports in code examples. As before we need to deactivate formatting to
+    # keep this as escaped quotes and avoid this function failing on this file.
     # fmt: off
     splits = content.split('\"\"\"')
     # fmt: on
@@ -376,6 +473,7 @@ def extract_imports(module_fname, cache=None):
     multiline_relative_imports = _re_multi_line_relative_imports.findall(content)
     relative_imports += [(mod, imp) for mod, imp in multiline_relative_imports if "# tests_ignore" not in imp]
 
+    # We need to remove parts of the module name depending on the depth of the relative imports.
     for module, imports in relative_imports:
         level = 0
         while module.startswith("."):
@@ -395,13 +493,15 @@ def extract_imports(module_fname, cache=None):
     multiline_direct_imports = _re_multi_line_direct_imports.findall(content)
     direct_imports += [(mod, imp) for mod, imp in multiline_direct_imports if "# tests_ignore" not in imp]
 
+    # We need to find the relative path of those imports.
     for module, imports in direct_imports:
-        import_parts = module.split(".")[1:]  # ignore the first .
+        import_parts = module.split(".")[1:]  # ignore the name of the repo since we add it below.
         dep_parts = ["src", "transformers"] + import_parts
         imported_module = os.path.sep.join(dep_parts)
         imported_modules.append((imported_module, [imp.strip() for imp in imports.split(",")]))
 
     result = []
+    # Double check we get proper modules (either a python file or a folder with an init).
     for module_file, imports in imported_modules:
         if (PATH_TO_REPO / f"{module_file}.py").is_file():
             module_file = f"{module_file}.py"
@@ -417,15 +517,30 @@ def extract_imports(module_fname, cache=None):
     return result
 
 
-def get_module_dependencies(module_fname, cache=None):
+def get_module_dependencies(module_fname: str, cache: Dict[str, List[str]] = None) -> List[str]:
     """
-    Get the dependencies of a module from the module filename as a list of module filenames. This will resolve any
-    __init__ we pass: if we import from a submodule utils, the dependencies will be utils/foo.py and utils/bar.py (if
-    the objects imported actually come from utils.foo and utils.bar) not utils/__init__.py.
+    Refines the result of `extract_imports` to remove subfolders and get a proper list of module filenames: if a file
+    as an import `from utils import Foo, Bar`, with `utils` being a subfolder containing many files, this will traverse
+    the `utils` init file to check where those dependencies come from: for instance the files utils/foo.py and utils/bar.py.
+
+    Warning: This presupposes that all intermediate inits are properly built (with imports from the respective
+    submodules) and work better if objects are defined in submodules and not the intermediate init (otherwise the
+    intermediate init is added, and inits usually have a lot of dependencies).
+
+    Args:
+        module_fname (`str`):
+            The name of the file of the module where we want to look at the imports (given relative to the root of
+            the repo).
+        cache (Dictionary `str` to `List[str]`, *optional*):
+            To speed up this function if it was previously called on `module_fname`, the cache of all previously
+            computed results.
+
+    Returns:
+        `List[str]`: The list of module filenames imported in the input `module_fname` (with submodule imports refined).
     """
     dependencies = []
     imported_modules = extract_imports(module_fname, cache=cache)
-    # The while loop is to recursively traverse all inits we may encounter.
+    # The while loop is to recursively traverse all inits we may encounter: we will add things as we go.
     while len(imported_modules) > 0:
         new_modules = []
         for module, imports in imported_modules:
@@ -461,7 +576,7 @@ def get_module_dependencies(module_fname, cache=None):
     return dependencies
 
 
-def create_reverse_dependency_tree():
+def create_reverse_dependency_tree() -> List[Tuple[str, str]]:
     """
     Create a list of all edges (a, b) which mean that modifying a impacts b with a going over all module and test files.
     """
@@ -473,10 +588,17 @@ def create_reverse_dependency_tree():
     return list(set(edges))
 
 
-def get_tree_starting_at(module, edges):
+def get_tree_starting_at(module: str, edges: List[Tuple[str, str]]) -> List[Union[str, List[str]]]:
     """
-    Returns the tree starting at a given module following all edges in the following format: [module, [list of edges
-    starting at module], [list of edges starting at the preceding level], ...]
+    Returns the tree starting at a given module following all edges.
+
+    Args:
+        module (`str`): The module that will be the root of the subtree we want.
+        eges (`List[Tuple[str, str]]`): The list of all edges of the tree.
+
+    Returns:
+        `List[Union[str, List[str]]]`: The tree to print in the following format: [module, [list of edges
+        starting at module], [list of edges starting at the preceding level], ...]
     """
     vertices_seen = [module]
     new_edges = [edge for edge in edges if edge[0] == module and edge[1] != module and "__init__.py" not in edge[1]]
@@ -497,6 +619,11 @@ def get_tree_starting_at(module, edges):
 def print_tree_deps_of(module, all_edges=None):
     """
     Prints the tree of modules depending on a given module.
+
+    Args:
+        module (`str`): The module that will be the root of the subtree we want.
+        all_eges (`List[Tuple[str, str]]`, *optional*):
+            The list of all edges of the tree. Will be set to `create_reverse_dependency_tree()` if not passed.
     """
     if all_edges is None:
         all_edges = create_reverse_dependency_tree()
@@ -522,16 +649,24 @@ def print_tree_deps_of(module, all_edges=None):
         print(line[0])
 
 
-def init_test_examples_dependencies():
+def init_test_examples_dependencies() -> Tuple[Dict[str, List[str]], List[str]]:
     """
     The test examples do not import from the examples (which are just scripts, not modules) so we need som extra
-    care initializing the dependency map there.
+    care initializing the dependency map, which is the goal of this function. It initializes the dependency map for
+    example files by linking each example to the example test file for the example framework.
+
+    Returns:
+        `Tuple[Dict[str, List[str]], List[str]]`: A tuple with two elements: the initialized dependency map which is a
+        dict test example file to list of example files potentially tested by that test file, and the list of all
+        example files (to avoid recomputing it later).
     """
     test_example_deps = {}
     all_examples = []
     for framework in ["flax", "pytorch", "tensorflow"]:
         test_files = list((PATH_TO_EXAMPLES / framework).glob("test_*.py"))
         all_examples.extend(test_files)
+        # Remove the files at the root of examples/framework since they are not proper examples (they are eith utils
+        # or example test files).
         examples = [
             f for f in (PATH_TO_EXAMPLES / framework).glob("**/*.py") if f.parent != PATH_TO_EXAMPLES / framework
         ]
@@ -539,24 +674,33 @@ def init_test_examples_dependencies():
         for test_file in test_files:
             with open(test_file, "r", encoding="utf-8") as f:
                 content = f.read()
+            # Map all examples to the test files found in examples/framework.
             test_example_deps[str(test_file.relative_to(PATH_TO_REPO))] = [
                 str(e.relative_to(PATH_TO_REPO)) for e in examples if e.name in content
             ]
+            # Also map the test files to themselves.
             test_example_deps[str(test_file.relative_to(PATH_TO_REPO))].append(
                 str(test_file.relative_to(PATH_TO_REPO))
             )
     return test_example_deps, all_examples
 
 
-def create_reverse_dependency_map():
+def create_reverse_dependency_map() -> Dict[str, List[str]]:
     """
-    Create the dependency map from module/test filename to the list of modules/tests that depend on it (even
-    recursively).
+    Create the dependency map from module/test filename to the list of modules/tests that depend on it recursively.
+
+    Returns:
+        `Dict[str, List[str]]`: The reverse dependency map as a dictionary mapping filenames to all the filenames
+        depending on it recursively. This way the tests impacted by a change in file A are the test files in the list
+        corresponding to key A in this result.
     """
     cache = {}
+    # Start from the example deps init.
     example_deps, examples = init_test_examples_dependencies()
+    # Add all modules and all tests to all examples
     all_modules = list(PATH_TO_TRANFORMERS.glob("**/*.py")) + list(PATH_TO_TESTS.glob("**/*.py")) + examples
     all_modules = [str(mod.relative_to(PATH_TO_REPO)) for mod in all_modules]
+    # Compute the direct dependencies of all modules.
     direct_deps = {m: get_module_dependencies(m, cache=cache) for m in all_modules}
     direct_deps.update(example_deps)
 
@@ -566,6 +710,8 @@ def create_reverse_dependency_map():
         something_changed = False
         for m in all_modules:
             for d in direct_deps[m]:
+                # We stop recursing at an init (cause we always end up in the main init and we don't want to add all
+                # files which the main init imports)
                 if d.endswith("__init__.py"):
                     continue
                 if d not in direct_deps:
@@ -581,6 +727,8 @@ def create_reverse_dependency_map():
         for d in direct_deps[m]:
             reverse_map[d].append(m)
 
+    # For inits, we don't do the reverse deps but the direct deps: if modifying an init, we want to make sure we test
+    # all the modules impacted by that init.
     for m in [f for f in all_modules if f.endswith("__init__.py")]:
         direct_deps = get_module_dependencies(m, cache=cache)
         deps = sum([reverse_map[d] for d in direct_deps if not d.endswith("__init__.py")], direct_deps)
@@ -589,13 +737,26 @@ def create_reverse_dependency_map():
     return reverse_map
 
 
-def create_module_to_test_map(reverse_map=None, filter_models=False):
+def create_module_to_test_map(
+    reverse_map: Dict[str, List[str]] = None, filter_models: bool = False
+) -> Dict[str, List[str]]:
     """
     Extract the tests from the reverse_dependency_map and potentially filters the model tests.
+
+    Args:
+        reverse_map (`Dict[str, List[str]]`, *optional*):
+            The reverse dependency map as created by `create_reverse_dependency_map`. Will default to the result of
+            that function if not provided.
+        filter_models (`bool`, *optional*, defaults to `False`):
+            Whether or not to filter model tests to only include core models if a file impacts a lot of models.
+
+    Returns:
+        `Dict[str, List[str]]`: A dictionary that maps each file to the tests to execute if that file was modified.
     """
     if reverse_map is None:
         reverse_map = create_reverse_dependency_map()
 
+    # Utility that tells us if a given file is a test (taking test examples into account)
     def is_test(fname):
         if fname.startswith("tests"):
             return True
@@ -603,14 +764,17 @@ def create_module_to_test_map(reverse_map=None, filter_models=False):
             return True
         return False
 
+    # Build the test map
     test_map = {module: [f for f in deps if is_test(f)] for module, deps in reverse_map.items()}
 
     if not filter_models:
         return test_map
 
+    # Now we deal with the filtering if `filter_models` is True.
     num_model_tests = len(list(PATH_TO_TESTS.glob("models/*")))
 
     def has_many_models(tests):
+        # We filter to core models when a given file impacts more than half the model tests.
         model_tests = {Path(t).parts[2] for t in tests if t.startswith("tests/models/")}
         return len(model_tests) > num_model_tests // 2
 
@@ -623,7 +787,7 @@ def create_module_to_test_map(reverse_map=None, filter_models=False):
 def check_imports_all_exist():
     """
     Isn't used per se by the test fetcher but might be used later as a quality check. Putting this here for now so the
-    code is not lost.
+    code is not lost. This checks all imports in a given file do exist.
     """
     cache = {}
     all_modules = list(PATH_TO_TRANFORMERS.glob("**/*.py")) + list(PATH_TO_TESTS.glob("**/*.py"))
@@ -636,11 +800,21 @@ def check_imports_all_exist():
                 print(f"{module} has dependency on {dep} which does not exist.")
 
 
-def _print_list(l):
+def _print_list(l) -> str:
+    """
+    Pretty print a list of elements with one line per element and a - starting each line.
+    """
     return "\n".join([f"- {f}" for f in l])
 
 
-def create_json_map(test_files_to_run, json_output_file):
+def create_json_map(test_files_to_run: List[str], json_output_file: str):
+    """
+    Creates a map from a list of tests to run to easily split them by category, when running parallelism of slow tests.
+
+    Args:
+        test_files_to_run (`List[str]`): The list of tests to run.
+        json_output_file (`str`): The path where to store the built json map.
+    """
     if json_output_file is None:
         return
 
@@ -673,7 +847,34 @@ def create_json_map(test_files_to_run, json_output_file):
         json.dump(test_map, fp, ensure_ascii=False)
 
 
-def infer_tests_to_run(output_file, diff_with_last_commit=False, filter_models=True, json_output_file=None):
+def infer_tests_to_run(
+    output_file: str,
+    diff_with_last_commit: bool = False,
+    filter_models: bool = True,
+    json_output_file: Optional[str] = None,
+):
+    """
+    The main function called by the test fetcher. Determines the tests to run from the diff.
+
+    Args:
+        output_file (`str`):
+            The path where to store the summary of the test fetcher analysis. Other files will be stored in the same
+            folder:
+
+            - examples_test_list.txt: The list of examples tests to run.
+            - test_repo_utils.txt: Will indicate if the repo utils tests should be run or not.
+            - doctest_list.txt: The list of doctests to run.
+
+        diff_with_last_commit (`bool`, *optional*, defaults to `False`):
+            Whether to analyze the diff with the last commit (for use on the main branch after a PR is merged) or with
+            the branching point from main (for use on each PR).
+        filter_models (`bool`, *optional*, defaults to `True`):
+            Whether or not to filter the tests to core models only, when a file modified results in a lot of model
+            tests.
+        json_output_file (`str`, *optional*):
+            The path where to store the json file mapping categories of tests to tests to run (used for parallelism or
+            the slow tests).
+    """
     modified_files = get_modified_python_files(diff_with_last_commit=diff_with_last_commit)
     print(f"\n### MODIFIED FILES ###\n{_print_list(modified_files)}")
 
@@ -751,7 +952,7 @@ def infer_tests_to_run(output_file, diff_with_last_commit=False, filter_models=T
             f.write(" ".join(doctest_list))
 
 
-def filter_tests(output_file, filters):
+def filter_tests(output_file: str, filters: List[str]):
     """
     Reads the content of the output file and filters out all the tests in a list of given folders.
 
@@ -778,11 +979,16 @@ def filter_tests(output_file, filters):
         f.write(" ".join(test_files))
 
 
-def parse_commit_message(commit_message):
+def parse_commit_message(commit_message: str) -> Dict[str, bool]:
     """
     Parses the commit message to detect if a command is there to skip, force all or part of the CI.
 
-    Returns a dictionary of strings to bools with keys skip, test_all_models and test_all.
+    Args:
+        commit_message (`str`): The commit message of the current commit.
+
+    Returns:
+        `Dict[str, bool]`: A dictionary of strings to bools with keys the following keys: `"skip"`,
+        `"test_all_models"` and `"test_all"`.
     """
     if commit_message is None:
         return {"skip": False, "no_filter": False, "test_all": False}