diff --git a/runtime-pyside6/tests/__init__.py b/runtime-pyside6/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/runtime-pyside6/tests/plugins/__init__.py b/runtime-pyside6/tests/plugins/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/runtime-pyside6/tests/plugins/natural/__init__.py b/runtime-pyside6/tests/plugins/natural/__init__.py
new file mode 100644
index 0000000..7a67869
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/__init__.py
@@ -0,0 +1,21 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+
+from .spy import Spy
+from .that import that
+
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/__init__.py b/runtime-pyside6/tests/plugins/natural/interfaces/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py b/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py
new file mode 100644
index 0000000..f2504a2
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py
@@ -0,0 +1,30 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+
+class Assertion(Exception):
+    def __init__(self, assertion: bool, message: str):
+        self.assertion = assertion
+        self.message = message
+
+    def __str__(self):
+        return self.message
+
+    def __bool__(self):
+        if not self.assertion:
+            raise self
+        return self.assertion
\ No newline at end of file
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/base.py b/runtime-pyside6/tests/plugins/natural/interfaces/base.py
new file mode 100644
index 0000000..88b9e06
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/base.py
@@ -0,0 +1,101 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+from typing import Self
+
+from tests.plugins.natural.interfaces.assertion import Assertion
+from tests.plugins.natural.interfaces.utils import repr_
+
+
+class AssertionInterface:
+    """
+    Most basic assertion interface.
+    You probably want to use BaseAssertionInterface
+    """
+    def __init__(self, value):
+        self._value = value
+
+    @property
+    def was(self) -> Self:
+        return self
+
+    @property
+    def be(self) -> Self:
+        return self
+
+    @property
+    def been(self) -> Self:
+        return self
+
+    @property
+    def have(self) -> Self:
+        return self
+
+    @property
+    def has(self) -> Self:
+        return self
+
+    @property
+    def a(self) -> Self:
+        return self
+
+    @property
+    def an(self) -> Self:
+        return self
+
+    def is_a(self, type_) -> Assertion:
+        """
+        Checks if the current value is equal to the provided value
+        """
+        value_type_name = type(self._value).__name__
+        return Assertion(isinstance(type_, type_), f"The value ({value_type_name} {repr_(self._value)}) is not a {type_.__name__}.")
+
+
+
+class EqualAssertionInterface(AssertionInterface):
+    """
+    Interface created for when its value should be checked for equality
+    """
+    def __init__(self, value):
+        super().__init__(value)
+
+    def __call__(self, value) -> Assertion:
+        return Assertion(value == self._value, f"The value ({repr_(self._value)}) is not equal to {repr(value)}.")
+
+    def to(self, value) -> Self:
+        return self(value)
+
+    def of(self, value) -> Self:
+        return self(value)
+
+
+
+class BaseAssertionInterface(AssertionInterface):
+
+    @property
+    def equals(self) -> EqualAssertionInterface:
+        """
+        Checks if the current value is equal to the provided value
+        """
+        return EqualAssertionInterface(self._value)
+
+    @property
+    def is_equal(self) -> EqualAssertionInterface:
+        """
+        Checks if the current value is equal to the provided value
+        """
+        return self.equals
\ No newline at end of file
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/basic.py b/runtime-pyside6/tests/plugins/natural/interfaces/basic.py
new file mode 100644
index 0000000..1d5a878
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/basic.py
@@ -0,0 +1,61 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+from tests.plugins.natural.interfaces.assertion import Assertion
+from tests.plugins.natural.interfaces.base import BaseAssertionInterface
+from tests.plugins.natural.interfaces.int import IntInterface
+from tests.plugins.natural.interfaces.utils import repr_
+
+
+class FixedIteratorInterface(BaseAssertionInterface):
+    @property
+    def length(self) -> IntInterface:
+        return IntInterface(len(self._value))
+
+    def elements(self, *elements) -> Assertion:
+        tests = [elem for elem in elements if elem in self._value]
+        return Assertion(
+            len(tests) == 0,
+            f"This value ({repr_(self._value)}) does not have elements ({repr_(tests)})"
+        )
+
+    def element(self, element) -> Assertion:
+        return Assertion(
+            element in self._value,
+            f"This value ({repr_(self._value)}) does not have element ({repr_(element)})"
+        )
+
+    def contains(self, *elements) -> Assertion:
+        return self.elements(*elements)
+
+class BoolInterface(BaseAssertionInterface):
+    @property
+    def is_true(self):
+        return Assertion(
+            self._value == True,
+            f"The value ({repr_(self._value)}) is not True."
+        )
+
+    @property
+    def is_false(self):
+        return Assertion(
+            self._value == False,
+            f"The value ({repr_(self._value)}) is not False."
+        )
+
+class StringInterface(LengthInterface):
+    pass
\ No newline at end of file
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/int.py b/runtime-pyside6/tests/plugins/natural/interfaces/int.py
new file mode 100644
index 0000000..aaf0511
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/int.py
@@ -0,0 +1,229 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+from typing import Self
+
+from tests.plugins.natural.interfaces.assertion import Assertion
+from tests.plugins.natural.interfaces.base import BaseAssertionInterface, EqualAssertionInterface, AssertionInterface
+from tests.plugins.natural.interfaces.utils import repr_
+
+
+class IntComparisonAssertionInterface(AssertionInterface):
+    def __init__(self, value):
+        super().__init__(value)
+        self._compare_to = None
+
+    def _compare(self) -> Assertion:
+        raise RuntimeError(f"No comparison method defined in {type(self).__name__}.")
+
+    def __bool__(self) -> bool:
+        return bool(self._compare())
+
+    def __call__(self, compare_to: int) -> Self:
+        if self._compare_to is None:
+            self._compare_to = int(compare_to)
+        else:
+            self._compare_to *= int(compare_to)
+        return self
+
+    @property
+    def time(self) -> Self:
+        return self
+
+    @property
+    def times(self) -> Self:
+        return self
+
+    @property
+    def never(self) -> Self:
+        return self(0)
+
+    @property
+    def once(self) -> Self:
+        return self(1)
+
+    @property
+    def twice(self) -> Self:
+        return self(2)
+
+    @property
+    def thrice(self) -> Self:
+        return self(3)
+
+    @property
+    def zero(self) -> Self:
+        return self(0)
+
+    @property
+    def one(self) -> Self:
+        return self(1)
+
+    @property
+    def two(self) -> Self:
+        return self(2)
+
+    @property
+    def three(self) -> Self:
+        return self(3)
+
+    @property
+    def four(self) -> Self:
+        return self(4)
+
+    @property
+    def five(self) -> Self:
+        return self(5)
+
+    @property
+    def six(self) -> Self:
+        return self(6)
+
+    @property
+    def seven(self) -> Self:
+        return self(7)
+
+    @property
+    def eight(self) -> Self:
+        return self(8)
+
+    @property
+    def nine(self) -> Self:
+        return self(9)
+
+    @property
+    def ten(self) -> Self:
+        return self(10)
+
+    @property
+    def twenty(self) -> Self:
+        return self(20)
+
+    @property
+    def thirty(self) -> Self:
+        return self(30)
+
+    @property
+    def forty(self) -> Self:
+        return self(40)
+
+    @property
+    def fifty(self) -> Self:
+        return self(50)
+
+    @property
+    def sixty(self) -> Self:
+        return self(60)
+
+    @property
+    def seventy(self) -> Self:
+        return self(70)
+
+    @property
+    def eighty(self) -> Self:
+        return self(70)
+
+    @property
+    def ninety(self) -> Self:
+        return self(70)
+
+    @property
+    def hundred(self) -> Self:
+        return self(100)
+
+    @property
+    def thousand(self) -> Self:
+        return self(1_000)
+
+    @property
+    def million(self) -> Self:
+        return self(1_000_000)
+
+    @property
+    def billion(self) -> Self:
+        return self(1_000_000_000)
+
+
+class LessThanComparisonInterface(IntComparisonAssertionInterface):
+    def _compare(self) -> Assertion:
+        return Assertion(
+            self._value < self._compare_to,
+            f"The value ({repr_(self._value)}) is not less than to {repr_(self._compare_to)}."
+        )
+
+class MoreThanComparisonInterface(IntComparisonAssertionInterface):
+    def _compare(self) -> Assertion:
+        return Assertion(
+            self._value > self._compare_to,
+            f"The value ({repr_(self._value)}) is not more than to {repr_(self._compare_to)}."
+        )
+
+class AtLeastComparisonInterface(IntComparisonAssertionInterface):
+    def _compare(self) -> Assertion:
+        return Assertion(
+            self._value >= self._compare_to,
+            f"The value ({repr_(self._value)}) is not at least to {repr_(self._compare_to)}."
+        )
+
+class AtMostComparisonInterface(IntComparisonAssertionInterface):
+    def _compare(self) -> Assertion:
+        return Assertion(
+            self._value <= self._compare_to,
+            f"The value ({repr_(self._value)}) is not at least to {repr_(self._compare_to)}."
+        )
+
+class EqualComparisonInterface(IntComparisonAssertionInterface):
+    def _compare(self) -> Assertion:
+        return Assertion(
+            self._value == self._compare_to,
+            f"The value ({repr_(self._value)}) is not equal to {repr_(self._compare_to)}."
+        )
+
+    def to(self) -> Self:
+        return self
+
+    def of(self) -> Self:
+        return self
+
+
+class IntInterface(AssertionInterface):
+    def less_than(self) -> LessThanComparisonInterface:
+        return LessThanComparisonInterface(self._value)
+
+    @property
+    def more_than(self) -> MoreThanComparisonInterface:
+        return MoreThanComparisonInterface(self._value)
+
+    @property
+    def at_least(self) -> AtLeastComparisonInterface:
+        return AtLeastComparisonInterface(self._value)
+
+    @property
+    def at_most(self) -> AtMostComparisonInterface:
+        return AtMostComparisonInterface(self._value)
+
+    @property
+    def equals(self) -> EqualComparisonInterface:
+        return EqualComparisonInterface(self._value)
+
+    @property
+    def is_equal(self) -> EqualComparisonInterface:
+        return EqualComparisonInterface(self._value)
+
+    @property
+    def exactly(self) -> EqualComparisonInterface:
+        return EqualComparisonInterface(self._value)
+
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/utils.py b/runtime-pyside6/tests/plugins/natural/interfaces/utils.py
new file mode 100644
index 0000000..50e3777
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/utils.py
@@ -0,0 +1,27 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+
+from PySide6.QtQml import QJSValue
+
+
+def repr_(data):
+    if isinstance(data, QJSValue):
+        variant = data.toVariant()
+        return f"QJSValue<{type(variant).__name__}>({repr(variant)})"
+    else:
+        return repr(data)
\ No newline at end of file
diff --git a/runtime-pyside6/tests/spy.py b/runtime-pyside6/tests/plugins/natural/spy.py
similarity index 69%
rename from runtime-pyside6/tests/spy.py
rename to runtime-pyside6/tests/plugins/natural/spy.py
index 01f9aa0..1d77629 100644
--- a/runtime-pyside6/tests/spy.py
+++ b/runtime-pyside6/tests/plugins/natural/spy.py
@@ -17,35 +17,29 @@
 """
 from typing import Callable, Self
 
-from PySide6.QtQml import QJSValue
+from tests.plugins.natural.interfaces.base import Assertion, repr_, AssertionInterface
+from tests.plugins.natural.interfaces.int import IntComparisonAssertionInterface
 
 PRINT_PREFIX = (" " * 24)
 
-class SpyAssertionFailed(Exception):
-    def __init__(self, message, calls):
-        self.message = message + "\n"
+
+class SpyAssertion(Assertion):
+    def __init__(self, assertion: bool, message: str, calls: list):
+        super().__init__(assertion, message + "\n")
         if len(calls) > 0:
             self.message += self.render_calls(calls)
         else:
             self.message += f"{PRINT_PREFIX}0 registered calls."
 
-    def repr(self, data):
-        if isinstance(data, QJSValue):
-            variant = data.toVariant()
-            return f"QJSValue<{type(variant).__name__}>({repr(variant)})"
-        else:
-            return repr(data)
 
     def render_calls(self, calls):
         lines = [f"{PRINT_PREFIX}{len(calls)} registered call(s):"]
         for call in calls:
-            repr_args = [self.repr(arg) for arg in call[0]]
-            repr_kwargs =[f"{key}={self.repr(arg)}" for key, arg in call[1].items()]
+            repr_args = [repr_(arg) for arg in call[0]]
+            repr_kwargs = [f"{key}={repr_(arg)}" for key, arg in call[1].items()]
             lines.append(f"  - {', '.join([*repr_args, *repr_kwargs])}")
         return ("\n" + PRINT_PREFIX).join(lines)
 
-    def __str__(self):
-        return self.message
 
 class Methods:
     AT_LEAST_ONCE = "AT_LEAST_ONCE"
@@ -55,18 +49,19 @@ class Methods:
     MORE_THAN = "AT_LEAST"
     LESS_THAN = "AT_MOST"
 
-class CalledInterface:
+
+class CalledInterface(IntComparisonAssertionInterface):
     """
     Internal class generated by Spy.was_called.
     """
 
     def __init__(self, calls: list[tuple[list, dict]]):
+        super().__init__(len(calls))
         self.__calls = calls
         self.__method = Methods.AT_LEAST_ONCE
-        self.__times = None
 
     def __apply_method(self, calls):
-        required = self.__times
+        required = self._compare_to
         calls_count = len(calls)
         match self.__method:
             case Methods.AT_LEAST_ONCE:
@@ -91,61 +86,59 @@ class CalledInterface:
                 raise RuntimeError(f"Unknown method {self.__method}.")
         return compare, error
 
-    def __bool__(self):
+    def __bool__(self) -> bool:
         """
         Converts to boolean on assertion.
         """
         compare, error = self.__apply_method(self.__calls)
-        if not compare:
-            raise SpyAssertionFailed(error+".")
-        return compare
-
+        return bool(SpyAssertion(compare, error + ".", self.__calls))
 
     """
     Chaining methods
     """
+
     def __call__(self, *args, **kwargs) -> Self:
         if len(args) != 1:
             raise RuntimeError("Cannot call called interface with more than one argument.")
-        self.__times = int(args[0])
+        self._compare_to = int(args[0])
         if self.__method == Methods.AT_LEAST_ONCE:
             self.__method = Methods.EXACTLY
         return self
 
-    @property
-    def never(self) -> Self:
-        return self(0)
-
-    @property
-    def once(self) -> Self:
-        return self(1)
-
-    @property
-    def twice(self) -> Self:
-        return self(2)
-
-    @property
-    def thrice(self) -> Self:
-        return self(3)
-
     @property
     def at_least(self) -> Self:
-        self.__method = Methods.AT_LEAST
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.AT_LEAST
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
         return self
 
     @property
     def at_most(self) -> Self:
-        self.__method = Methods.AT_MOST
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.AT_MOST
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
         return self
 
     @property
     def more_than(self) -> Self:
-        self.__method = Methods.MORE_THAN
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.MORE_THAN
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.MORE_THAN}")
         return self
 
     @property
     def less_than(self) -> Self:
-        self.__method = Methods.LESS_THAN
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.LESS_THAN
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.LESS_THAN}")
+        return self
+
+    @property
+    def time(self) -> Self:
         return self
 
     @property
@@ -155,6 +148,7 @@ class CalledInterface:
     """
     Class properties.
     """
+
     def __match_calls_for_condition(self, condition: Callable[[list, dict], bool]) -> tuple[bool, str]:
         calls = []
         for call in self.__calls:
@@ -163,12 +157,12 @@ class CalledInterface:
         compare, error = self.__apply_method(calls)
         return compare, error
 
-
-    def with_arguments(self, *args, **kwargs):
+    def with_arguments(self, *args, **kwargs) -> SpyAssertion:
         """
         Checks if the Spy has been called the given number of times
         with at least the given arguments.
         """
+
         def some_args_matched(a, kw):
             args_matched = all((
                 arg in a
@@ -179,55 +173,49 @@ class CalledInterface:
                 for key, arg in kwargs.items()
             ))
             return args_matched and kwargs_matched
+
         compare, error = self.__match_calls_for_condition(some_args_matched)
-        if not compare:
-            repr_args = ', '.join([repr(arg) for arg in args])
-            repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
-            raise SpyAssertionFailed(f"{error} with arguments ({repr_args}) and keyword arguments ({repr_kwargs}).", self.__calls)
-        return compare
+        repr_args = ', '.join([repr(arg) for arg in args])
+        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
+        msg = f"{error} with arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
+        return SpyAssertion(compare, msg, self.__calls)
 
-
-    def with_arguments_matching(self, test_condition: Callable[[list, dict], bool]):
+    def with_arguments_matching(self, test_condition: Callable[[list, dict], bool]) -> SpyAssertion:
         """
         Checks if the Spy has been called the given number of times
         with arguments matching the given conditions.
         """
         compare, error = self.__match_calls_for_condition(test_condition)
-        if not compare:
-            raise SpyAssertionFailed(f"{error} with arguments matching given conditions.", self.__calls)
-        return compare
+        msg = f"{error} with arguments matching given conditions."
+        return SpyAssertion(compare, msg, self.__calls)
 
-
-    def with_exact_arguments(self, *args, **kwargs):
+    def with_exact_arguments(self, *args, **kwargs) -> SpyAssertion:
         """
         Checks if the Spy has been called the given number of times
         with all the given arguments.
         """
         compare, error = self.__match_calls_for_condition(lambda a, kw: a == args and kw == kwargs)
-        if not compare:
-            repr_args = ', '.join([repr(arg) for arg in args])
-            repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
-            raise SpyAssertionFailed(f"{error} with exact arguments ({repr_args}) and keyword arguments ({repr_kwargs}).", self.__calls)
-        return compare
+        repr_args = ', '.join([repr(arg) for arg in args])
+        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
+        msg = f"{error} with exact arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
+        return SpyAssertion(compare, msg, self.__calls)
 
-    def with_no_argument(self):
+    def with_no_argument(self) -> SpyAssertion:
         """
         Checks if the Spy has been called the given number of times
         with all the given arguments.
         """
         compare, error = self.__match_calls_for_condition(lambda a, kw: len(a) == 0 and len(kw) == 0)
-        if not compare:
-            raise SpyAssertionFailed(f"{error} with no arguments.", self.__calls)
-        return compare
+        return SpyAssertion(compare, f"{error} with no arguments.", self.__calls)
 
 
-
-class Spy:
+class Spy(AssertionInterface):
     """
     Class to spy into method calls with natural language expressions.
     """
 
     def __init__(self, function: Callable = None):
+        super().__init__(function)
         self.function = function
         self.calls = []
 
@@ -237,7 +225,7 @@ class Spy:
             self.function(*args, **kwargs)
 
     @property
-    def was_called(self) -> CalledInterface:
+    def called(self) -> CalledInterface:
         """
         Returns a boolean-able interface to check conditions for a given number of
         time the spy was called.
@@ -245,7 +233,7 @@ class Spy:
         return CalledInterface(self.calls)
 
     @property
-    def was_not_called(self) -> CalledInterface:
+    def not_called(self) -> CalledInterface:
         """
         Returns a boolean-able interface to check that conditions were never fulfilled
         in the times the spy was called.
diff --git a/runtime-pyside6/tests/plugins/natural/that.py b/runtime-pyside6/tests/plugins/natural/that.py
new file mode 100644
index 0000000..38ee54e
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/that.py
@@ -0,0 +1,57 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+from typing import overload, Generic, TypeVar
+
+from .spy import Spy
+from .interfaces.base import AssertionInterface, BaseAssertionInterface
+from .interfaces.basic import StringInterface, BoolInterface
+from .interfaces.int import IntInterface
+
+Interface = TypeVar("Interface", bound=AssertionInterface)
+
+MATCHES = [
+    (str, StringInterface),
+    (int, IntInterface),
+    (bool, BoolInterface),
+]
+
+
+@overload
+def that(value: str) -> StringInterface: ...
+
+
+@overload
+def that(value: int) -> IntInterface: ...
+
+
+@overload
+def that(value: bool) -> BoolInterface: ...
+
+
+@overload
+def that[Interface](value: Interface) -> Interface: ...
+
+
+def that(value: any) -> AssertionInterface:
+    if not isinstance(value, AssertionInterface):
+        interface = next((i for t, i in MATCHES if isinstance(value, t)), None)
+        if interface is not None:
+            value = interface(value)
+        else:
+            value = BaseAssertionInterface(value)
+    return value
diff --git a/runtime-pyside6/tests/test_promise.py b/runtime-pyside6/tests/test_promise.py
index 06c0b8a..a97f864 100644
--- a/runtime-pyside6/tests/test_promise.py
+++ b/runtime-pyside6/tests/test_promise.py
@@ -17,11 +17,9 @@
 """
 from time import sleep
 
-import pytest
-from PySide6.QtCore import QObject
 from PySide6.QtQml import QJSValue
 
-from spy import Spy
+from tests.plugins.natural import that, Spy
 from LogarithmPlotter.util.js import PyJSValue
 from LogarithmPlotter.util.promise import PyPromise
 
@@ -95,10 +93,10 @@ class TestPyPromise:
         # Check on our spy.
         with qtbot.waitSignal(promise.fulfilled, timeout=10000):
                 pass
-        assert spy_fulfilled.was_called.once
-        assert spy_fulfilled.was_not_called.with_arguments(3)
-        assert spy_fulfilled.was_called.with_arguments_matching(check_promise_result(3))
-        assert spy_rejected.was_not_called
+        assert that(spy_fulfilled).was.called.once
+        assert that(spy_fulfilled).was.not_called.with_arguments(3)
+        assert that(spy_fulfilled).was.called.with_arguments_matching(check_promise_result(3))
+        assert spy_rejected.was.not_called
 
     def test_rejected(self, qtbot):
         spy_fulfilled = Spy()
@@ -106,10 +104,10 @@ class TestPyPromise:
         promise = PyPromise(async_throw)
         then_res = promise.then(spy_fulfilled, spy_rejected)
         # Check if the return value is the same promise (so we can chain then)
-        assert then_res == promise
+        assert that(then_res).is_equal.to(promise)
         # Check on our spies.
         with qtbot.waitSignal(promise.rejected, timeout=10000):
             pass
-        assert spy_rejected.was_called.once
-        assert spy_rejected.was_called.with_arguments("Exception('aaaa')")
-        assert spy_fulfilled.was_not_called
+        assert that(spy_rejected).was.called.once
+        assert that(spy_rejected).was.called.with_arguments("Exception('aaaa')")
+        assert that(spy_fulfilled).was.not_called