diff --git a/pyformlang/cfg/tests/test_cfg.py b/pyformlang/cfg/tests/test_cfg.py
index f214572..c0e3cdd 100644
--- a/pyformlang/cfg/tests/test_cfg.py
+++ b/pyformlang/cfg/tests/test_cfg.py
@@ -377,7 +377,7 @@ def test_reverse(self) -> None:
         assert len(new_cfg.variables) == 1
         assert len(new_cfg.terminals) == 2
         assert len(new_cfg.productions) == 2
-        assert not (not new_cfg)
+        assert new_cfg
         assert new_cfg.contains([ter_b, ter_b, ter_a, ter_a])
 
     def test_emptiness(self) -> None:
@@ -974,7 +974,7 @@ def test_start_symbol(self) -> None:
         assert not cfg.start_symbol
 
 
-def get_example_text_duplicate():
+def get_example_text_duplicate() -> str:
     """Duplicate text."""
     text = """
         E  -> T E’
diff --git a/pyformlang/cfg/tests/test_llone_parser.py b/pyformlang/cfg/tests/test_llone_parser.py
index 4ef2a43..fa3db46 100644
--- a/pyformlang/cfg/tests/test_llone_parser.py
+++ b/pyformlang/cfg/tests/test_llone_parser.py
@@ -311,12 +311,12 @@ def test_get_llone_rightmost_derivation(self) -> None:
 
     def test_save_tree(self) -> None:
         text = """
-                    E  -> T E'
-                    E' -> + T E' | epsilon
-                    T  -> F T'
-                    T' -> * F T' | epsilon
-                    F  -> ( E ) | id
-                """
+            E  -> T E'
+            E' -> + T E' | epsilon
+            T  -> F T'
+            T' -> * F T' | epsilon
+            F  -> ( E ) | id
+        """
         cfg = CFG.from_text(text, start_symbol="E")
         llone_parser = LLOneParser(cfg)
         parse_tree = llone_parser.get_llone_parse_tree(
diff --git a/pyformlang/cfg/tests/test_recursive_decent_parser.py b/pyformlang/cfg/tests/test_recursive_decent_parser.py
index 176ae39..fcc5e5d 100644
--- a/pyformlang/cfg/tests/test_recursive_decent_parser.py
+++ b/pyformlang/cfg/tests/test_recursive_decent_parser.py
@@ -10,14 +10,14 @@
 
 
 @pytest.fixture
-def parser():
+def parser() -> RecursiveDecentParser:
     cfg = CFG.from_text("""
                 E -> S + S
                 E -> S * S
                 S -> ( E )
                 S -> int
             """)
-    yield RecursiveDecentParser(cfg)
+    return RecursiveDecentParser(cfg)
 
 
 class TestRecursiveDecentParser:
diff --git a/pyformlang/fcfg/tests/test_feature_structure.py b/pyformlang/fcfg/tests/test_feature_structure.py
index 8656870..9ebd8c8 100644
--- a/pyformlang/fcfg/tests/test_feature_structure.py
+++ b/pyformlang/fcfg/tests/test_feature_structure.py
@@ -10,7 +10,7 @@
 )
 
 
-def _get_agreement_subject_number_person():
+def _get_agreement_subject_number_person() -> FeatureStructure:
     fs2 = FeatureStructure.from_text(
         "AGREEMENT=(1)[NUMBER=sg, PERSON=3], SUBJECT=[AGREEMENT->(1)]"
     )
@@ -205,7 +205,7 @@ def test_copy(self) -> None:
         copy_of_copy = fs1_copy2.copy()
         self._assertions_test_copy(copy_of_copy)
 
-    def _assertions_test_copy(self, fs1_copy) -> None:
+    def _assertions_test_copy(self, fs1_copy: FeatureStructure) -> None:
         assert (
             fs1_copy.get_feature_by_path(["AGREEMENT", "NUMBER"]).value == "sg"
         )
diff --git a/pyformlang/finite_automaton/tests/test_deterministic_finite_automaton.py b/pyformlang/finite_automaton/tests/test_deterministic_finite_automaton.py
index 3ab5900..1b24df7 100644
--- a/pyformlang/finite_automaton/tests/test_deterministic_finite_automaton.py
+++ b/pyformlang/finite_automaton/tests/test_deterministic_finite_automaton.py
@@ -296,7 +296,7 @@ def test_dfa_generating_no_words(self) -> None:
         assert not accepted_words
 
 
-def get_example0():
+def get_example0() -> DeterministicFiniteAutomaton:
     """Gives a dfa."""
     dfa = DeterministicFiniteAutomaton()
     state0 = State(0)
@@ -317,7 +317,7 @@ def get_example0():
     return dfa
 
 
-def get_example0_bis():
+def get_example0_bis() -> DeterministicFiniteAutomaton:
     """Gives a dfa."""
     dfa = DeterministicFiniteAutomaton()
     dfa.add_start_state(0)
@@ -330,7 +330,7 @@ def get_example0_bis():
     return dfa
 
 
-def get_dfa_example():
+def get_dfa_example() -> DeterministicFiniteAutomaton:
     """An example of DFA."""
     dfa1 = DeterministicFiniteAutomaton()
     dfa1.add_transitions(
@@ -341,7 +341,7 @@ def get_dfa_example():
     return dfa1
 
 
-def get_dfa_example_for_word_generation():
+def get_dfa_example_for_word_generation() -> DeterministicFiniteAutomaton:
     """DFA example for the word generation test."""
     dfa = DeterministicFiniteAutomaton()
     states = [State(x) for x in range(4)]
@@ -364,7 +364,7 @@ def get_dfa_example_for_word_generation():
     return dfa
 
 
-def get_cyclic_dfa_example():
+def get_cyclic_dfa_example() -> DeterministicFiniteAutomaton:
     """Gets DFA example with several cycles on path to final."""
     dfa = DeterministicFiniteAutomaton(start_state=0, final_states={3})
     dfa.add_transitions(
@@ -380,7 +380,7 @@ def get_cyclic_dfa_example():
     return dfa
 
 
-def get_dfa_example_without_accepted_words():
+def get_dfa_example_without_accepted_words() -> DeterministicFiniteAutomaton:
     """DFA example accepting no words."""
     dfa = DeterministicFiniteAutomaton()
     states = [State(x) for x in range(4)]
diff --git a/pyformlang/finite_automaton/tests/test_epsilon_nfa.py b/pyformlang/finite_automaton/tests/test_epsilon_nfa.py
index db8e3c3..e480389 100644
--- a/pyformlang/finite_automaton/tests/test_epsilon_nfa.py
+++ b/pyformlang/finite_automaton/tests/test_epsilon_nfa.py
@@ -1,5 +1,6 @@
 """Tests for epsilon NFA."""
 
+from typing import List, Tuple
 import copy
 import networkx
 
@@ -101,6 +102,7 @@ def test_union0(self) -> None:
 
     def test_union1(self) -> None:
         """Tests the union of three ENFAs.
+
         Union is (a*b)|(ab+)|c.
         """
         enfa0 = get_enfa_example0()
@@ -133,6 +135,7 @@ def test_concatenate0(self) -> None:
 
     def test_concatenate1(self) -> None:
         """Tests the concatenation of three ENFAs.
+
         Concatenation is a*bc((ab+)|c).
         """
         enfa0 = get_enfa_example0()
@@ -162,6 +165,7 @@ def test_kleene0(self) -> None:
 
     def test_kleene1(self) -> None:
         """Tests the kleene star of an ENFA.
+
         Expression is ((ab+)|c)*.
         """
         enfa = get_enfa_example2()
@@ -560,7 +564,8 @@ def test_max_length_zero_not_accepting_empty_string(self) -> None:
         assert not accepted_words
 
 
-def get_digits_enfa():
+def get_digits_enfa() -> Tuple[EpsilonNFA, List[Symbol],
+                               Symbol, Symbol, Symbol, Symbol]:
     """An epsilon NFA to recognize digits."""
     epsilon = Epsilon()
     plus = Symbol("+")
@@ -593,8 +598,9 @@ def get_digits_enfa():
     return enfa, digits, epsilon, plus, minus, point
 
 
-def get_enfa_example0():
-    """Gives an example ENFA
+def get_enfa_example0() -> EpsilonNFA:
+    """Gives an example ENFA.
+
     Accepts a*b.
     """
     enfa0 = EpsilonNFA()
@@ -611,8 +617,9 @@ def get_enfa_example0():
     return enfa0
 
 
-def get_enfa_example1():
-    """Gives an example ENFA
+def get_enfa_example1() -> EpsilonNFA:
+    """Gives an example ENFA.
+
     Accepts c.
     """
     enfa1 = EpsilonNFA()
@@ -625,8 +632,9 @@ def get_enfa_example1():
     return enfa1
 
 
-def get_enfa_example2():
-    """Gives an example ENFA
+def get_enfa_example2() -> EpsilonNFA:
+    """Gives an example ENFA.
+
     Accepts (ab+)|c.
     """
     enfa = EpsilonNFA(start_states={0, 3}, final_states={2, 4})
@@ -634,7 +642,7 @@ def get_enfa_example2():
     return enfa
 
 
-def get_enfa_example0_bis():
+def get_enfa_example0_bis() -> EpsilonNFA:
     """A non minimal NFA, equivalent to example0."""
     enfa0 = EpsilonNFA()
     state3 = State(3)
@@ -657,7 +665,7 @@ def get_enfa_example0_bis():
     return enfa0
 
 
-def get_example_non_minimal():
+def get_example_non_minimal() -> EpsilonNFA:
     """A non minimal example a.a*.b."""
     enfa0 = EpsilonNFA()
     state0 = State(0)
@@ -684,7 +692,7 @@ def get_example_non_minimal():
     return enfa0
 
 
-def get_enfa_example_for_word_generation():
+def get_enfa_example_for_word_generation() -> EpsilonNFA:
     """ENFA example for the word generation test."""
     enfa = EpsilonNFA()
     states = [State(x) for x in range(9)]
@@ -717,7 +725,7 @@ def get_enfa_example_for_word_generation():
     return enfa
 
 
-def get_cyclic_enfa_example():
+def get_cyclic_enfa_example() -> EpsilonNFA:
     """ENFA example with a cycle on the path to the final state."""
     enfa = EpsilonNFA()
     states = [State(x) for x in range(4)]
@@ -738,7 +746,7 @@ def get_cyclic_enfa_example():
     return enfa
 
 
-def get_epsilon_cycle_enfa_example():
+def get_epsilon_cycle_enfa_example() -> EpsilonNFA:
     """ENFA example with an epsilon cycle."""
     enfa = EpsilonNFA()
     states = [State(x) for x in range(4)]
diff --git a/pyformlang/finite_automaton/tests/test_nondeterministic_finite_automaton.py b/pyformlang/finite_automaton/tests/test_nondeterministic_finite_automaton.py
index 7f36944..4d77f6b 100644
--- a/pyformlang/finite_automaton/tests/test_nondeterministic_finite_automaton.py
+++ b/pyformlang/finite_automaton/tests/test_nondeterministic_finite_automaton.py
@@ -163,10 +163,8 @@ def test_start_state_at_the_end_generation(self) -> None:
         assert len(accepted_words) == 5
 
 
-def get_nfa_example_for_word_generation():
-    """Gets Nondeterministic Finite Automaton \
-    example for the word generation test.
-    """
+def get_nfa_example_for_word_generation() -> NondeterministicFiniteAutomaton:
+    """Gets NFA example for the word generation test."""
     nfa = NondeterministicFiniteAutomaton(
         start_states={0, 4}, final_states={3, 4, 6, 8}
     )
@@ -186,7 +184,7 @@ def get_nfa_example_for_word_generation():
     return nfa
 
 
-def get_nfa_example_with_duplicates():
+def get_nfa_example_with_duplicates() -> NondeterministicFiniteAutomaton:
     """Gets NFA example with duplicate word chains."""
     nfa = NondeterministicFiniteAutomaton(
         start_states={0, 1, 5, 6}, final_states={3, 4, 8}
@@ -205,7 +203,7 @@ def get_nfa_example_with_duplicates():
     return nfa
 
 
-def get_cyclic_nfa_example():
+def get_cyclic_nfa_example() -> NondeterministicFiniteAutomaton:
     """Gets NFA example with several cycles on path to final."""
     nfa = NondeterministicFiniteAutomaton(start_states={0, 5}, final_states={4})
     nfa.add_transitions(
@@ -223,7 +221,8 @@ def get_cyclic_nfa_example():
     return nfa
 
 
-def get_nfa_example_with_final_state_at_start():
+def get_nfa_example_with_final_state_at_start() \
+        -> NondeterministicFiniteAutomaton:
     """Gets NFA example with final state at start."""
     nfa = NondeterministicFiniteAutomaton(start_states={0, 5}, final_states={0})
     nfa.add_transitions(
@@ -239,7 +238,8 @@ def get_nfa_example_with_final_state_at_start():
     return nfa
 
 
-def get_nfa_example_with_start_state_at_the_end():
+def get_nfa_example_with_start_state_at_the_end() \
+        -> NondeterministicFiniteAutomaton:
     """Gets NFA example with start state at the end."""
     nfa = NondeterministicFiniteAutomaton(
         start_states={0, 3, 4}, final_states={3}
diff --git a/pyformlang/fst/tests/test_fst.py b/pyformlang/fst/tests/test_fst.py
index 707e52a..578ba49 100644
--- a/pyformlang/fst/tests/test_fst.py
+++ b/pyformlang/fst/tests/test_fst.py
@@ -9,21 +9,21 @@
 
 
 @pytest.fixture
-def fst0():
-    fst0 = FST()
-    fst0.add_start_state("q0")
-    fst0.add_transition("q0", "a", "q1", ["b"])
-    fst0.add_final_state("q1")
-    yield fst0
+def fst0() -> FST:
+    fst = FST()
+    fst.add_start_state("q0")
+    fst.add_transition("q0", "a", "q1", ["b"])
+    fst.add_final_state("q1")
+    return fst
 
 
 @pytest.fixture
-def fst1():
-    fst1 = FST()
-    fst1.add_start_state("q1")
-    fst1.add_transition("q1", "b", "q2", ["c"])
-    fst1.add_final_state("q2")
-    yield fst1
+def fst1() -> FST:
+    fst = FST()
+    fst.add_start_state("q1")
+    fst.add_transition("q1", "b", "q2", ["c"])
+    fst.add_final_state("q2")
+    return fst
 
 
 class TestFST:
@@ -93,14 +93,14 @@ def test_translate(self) -> None:
         assert ["b", "c"] in translation
         assert ["b"] + ["c"] * 9 in translation
 
-    def test_union(self, fst0, fst1) -> None:
+    def test_union(self, fst0: FST, fst1: FST) -> None:
         """Tests the union."""
         fst_union = fst0.union(fst1)
         self._make_test_fst_union(fst_union)
         fst_union = fst0 | fst1
         self._make_test_fst_union(fst_union)
 
-    def _make_test_fst_union(self, fst_union) -> None:
+    def _make_test_fst_union(self, fst_union: FST) -> None:
         assert len(fst_union.start_states) == 2
         assert len(fst_union.final_states) == 2
         assert fst_union.get_number_transitions() == 2
@@ -111,7 +111,7 @@ def _make_test_fst_union(self, fst_union) -> None:
         translation = list(fst_union.translate(["a", "b"]))
         assert translation == []
 
-    def test_concatenate(self, fst0, fst1) -> None:
+    def test_concatenate(self, fst0: FST, fst1: FST) -> None:
         """Tests the concatenation."""
         fst_concatenate = fst0 + fst1
         translation = list(fst_concatenate.translate(["a", "b"]))
@@ -121,7 +121,7 @@ def test_concatenate(self, fst0, fst1) -> None:
         translation = list(fst_concatenate.translate(["b"]))
         assert translation == []
 
-    def test_concatenate2(self, fst0, fst1) -> None:
+    def test_concatenate2(self, fst0: FST, fst1: FST) -> None:
         """Tests the concatenation."""
         fst_concatenate = fst0 + fst1 + fst1
         translation = list(fst_concatenate.translate(["a", "b", "b"]))
@@ -131,7 +131,7 @@ def test_concatenate2(self, fst0, fst1) -> None:
         translation = list(fst_concatenate.translate(["b"]))
         assert translation == []
 
-    def test_kleene_start(self, fst0) -> None:
+    def test_kleene_start(self, fst0: FST) -> None:
         """Tests the kleene star on a fst."""
         fst_star = fst0.kleene_star()
         translation = list(fst_star.translate(["a"]))
diff --git a/pyformlang/indexed_grammar/tests/test_indexed_grammar.py b/pyformlang/indexed_grammar/tests/test_indexed_grammar.py
index 8ef444d..29d9be2 100644
--- a/pyformlang/indexed_grammar/tests/test_indexed_grammar.py
+++ b/pyformlang/indexed_grammar/tests/test_indexed_grammar.py
@@ -1,6 +1,8 @@
 """Testing of indexed grammar, with manual rules."""
 
-from pyformlang.indexed_grammar import Rules
+from typing import Iterable
+
+from pyformlang.indexed_grammar import Rules, ReducedRule
 from pyformlang.indexed_grammar import ConsumptionRule
 from pyformlang.indexed_grammar import EndRule
 from pyformlang.indexed_grammar import ProductionRule
@@ -343,7 +345,8 @@ def test_removal_useless(self) -> None:
         )
 
     def test_intersection0(self) -> None:
-        """Tests the intersection of indexed grammar with regex
+        """Tests the intersection of indexed grammar with regex.
+
         Long to run!
         """
         l_rules = [
@@ -389,7 +392,7 @@ def test_intersection1(self) -> None:
         assert not intersection.is_empty()
 
 
-def get_example_rules():
+def get_example_rules() -> Iterable[ReducedRule]:
     """Duplicate example of rules."""
     l_rules = [  # Initialization rules
         ProductionRule("S", "Cinit", "end"),
diff --git a/pyformlang/regular_expression/tests/test_python_regex.py b/pyformlang/regular_expression/tests/test_python_regex.py
index 5808e41..55b6a1a 100644
--- a/pyformlang/regular_expression/tests/test_python_regex.py
+++ b/pyformlang/regular_expression/tests/test_python_regex.py
@@ -201,7 +201,7 @@ def test_shortcut_word(self) -> None:
         assert regex.accepts(["a", "A"])
         assert regex.accepts(["a", "f"])
 
-    def _test_compare(self, regex, s_test) -> None:
+    def _test_compare(self, regex: str, s_test: str) -> None:
         r_pyformlang = PythonRegex(regex)
         r_python = re.compile(regex)
         assert (r_python.fullmatch(s_test) is not None) == r_pyformlang.accepts(