diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 13aaf64ab..4fa24373f 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -26,7 +26,7 @@ jobs:
           cache: "pip"
       - run: "pip install -e '.[dev]'"
       - run: mypy src
-  black:
+  ruff-format:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
@@ -35,7 +35,7 @@ jobs:
           python-version: "3.11"
           cache: "pip"
       - run: "pip install -e '.[dev]'"
-      - run: black --check .
+      - run: ruff format --check .
   pytest:
     runs-on: ubuntu-22.04
     services:
diff --git a/justfile b/justfile
index ccca7a793..d68a90512 100644
--- a/justfile
+++ b/justfile
@@ -22,7 +22,7 @@ check:
   (set -x; ruff . )
 
   echo
-  ( set -x; black --check . )
+  ( set -x; ruff format --check . )
 
   echo
   ( set -x; mypy src )
@@ -37,7 +37,7 @@ check:
 
 # Auto-fix code issues.
 fix:
-  black .
+  ruff format .
   ruff --fix .
 
 # Start a MongoDB instance in docker.
diff --git a/pyproject.toml b/pyproject.toml
index 763668ac3..fe9011eaa 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,12 +28,13 @@ readme = "README.rst"
 
 [project.optional-dependencies]
 dev = [
-  "black",
   "ruff",
   "moldoc",
   "mypy",
   "pip-tools",
-  "pytest",
+  # TODO: Remove pin when https://github.com/TvoroG/pytest-lazy-fixture/issues/65 is resolved.
+  # pytest-lazy-fixture 0.6.0 is incompatible with pytest 8.0.0
+  "pytest<8",
   "pytest-benchmark",
   "pytest-datadir",
   "pytest-lazy-fixture",
@@ -51,11 +52,11 @@ documentation = "https://stk.readthedocs.io"
 [tool.setuptools_scm]
 write_to = "src/stk/_version.py"
 
-[tool.black]
-line-length = 79
 
 [tool.ruff]
 line-length = 79
+
+[too.ruff.lint]
 extend-select = ["I"]
 
 [tool.pytest.ini_options]
diff --git a/src/stk/_internal/building_block.py b/src/stk/_internal/building_block.py
index e49c02b5e..f3f01f160 100644
--- a/src/stk/_internal/building_block.py
+++ b/src/stk/_internal/building_block.py
@@ -50,9 +50,11 @@ class BuildingBlock(Molecule):
     def __init__(
         self,
         smiles: str,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
         position_matrix: np.ndarray | None = None,
     ) -> None:
@@ -137,9 +139,11 @@ def __init__(
     def init_from_molecule(
         cls,
         molecule: Molecule,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
     ) -> typing.Self:
         """
@@ -200,9 +204,11 @@ def init_from_molecule(
     def init_from_vabene_molecule(
         cls,
         molecule: vabene.Molecule,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
         position_matrix: np.ndarray | None = None,
     ) -> typing.Self:
@@ -316,9 +322,11 @@ def init(
         atoms: Iterable[Atom],
         bonds: Iterable[Bond],
         position_matrix: np.ndarray,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
     ) -> typing.Self:
         """
@@ -380,11 +388,13 @@ def init(
             bonds=bonds,
             position_matrix=position_matrix,
         )
-        building_block._fg_repr = repr(functional_groups)
         functional_groups = building_block._extract_functional_groups(
             functional_groups=functional_groups,
         )
         building_block._with_functional_groups(functional_groups)
+        building_block._fg_repr = repr(  # type: ignore[has-type]
+            tuple(building_block.get_functional_groups())
+        )
         building_block._placer_ids = building_block._normalize_placer_ids(
             placer_ids=placer_ids,
             functional_groups=building_block._functional_groups,
@@ -400,9 +410,11 @@ def init(
     def init_from_file(
         cls,
         path: pathlib.Path | str,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
     ) -> typing.Self:
         """
@@ -479,9 +491,11 @@ def init_from_file(
     def init_from_rdkit_mol(
         cls,
         molecule: rdkit.Mol,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory] = (),
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ) = (),
         placer_ids: Iterable[int] | None = None,
     ) -> typing.Self:
         """
@@ -548,9 +562,11 @@ def init_from_rdkit_mol(
     def _init_from_rdkit_mol(
         self,
         molecule: rdkit.Mol,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory],
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ),
         placer_ids: Iterable[int] | None,
     ) -> None:
         """
@@ -595,7 +611,6 @@ def _init_from_rdkit_mol(
 
         """
 
-        self._fg_repr = repr(functional_groups)
         atoms = tuple(
             Atom(a.GetIdx(), a.GetAtomicNum(), a.GetFormalCharge())
             for a in molecule.GetAtoms()
@@ -620,6 +635,7 @@ def _init_from_rdkit_mol(
                 functional_groups=functional_groups,
             )
         )
+        self._fg_repr = repr(self._functional_groups)
         self._placer_ids = self._normalize_placer_ids(
             placer_ids=placer_ids,
             functional_groups=self._functional_groups,
@@ -714,9 +730,11 @@ def _get_core_ids(
 
     def _extract_functional_groups(
         self,
-        functional_groups: FunctionalGroup
-        | FunctionalGroupFactory
-        | Iterable[FunctionalGroup | FunctionalGroupFactory],
+        functional_groups: (
+            FunctionalGroup
+            | FunctionalGroupFactory
+            | Iterable[FunctionalGroup | FunctionalGroupFactory]
+        ),
     ) -> Iterator[FunctionalGroup]:
         """
         Yield functional groups.
diff --git a/src/stk/_internal/ea/evolutionary_algorithm/implementations/implementation.py b/src/stk/_internal/ea/evolutionary_algorithm/implementations/implementation.py
index f616fbce4..7eaa7b7d9 100644
--- a/src/stk/_internal/ea/evolutionary_algorithm/implementations/implementation.py
+++ b/src/stk/_internal/ea/evolutionary_algorithm/implementations/implementation.py
@@ -138,7 +138,7 @@ def get_key(record: T) -> str:
             population, keys = dedupe(
                 items=(
                     molecule_record
-                    for molecule_record, in self._generation_selector.select(
+                    for (molecule_record,) in self._generation_selector.select(
                         population=normalized_fitness_values
                     )
                 ),
diff --git a/src/stk/_internal/ea/fitness_normalizers/add.py b/src/stk/_internal/ea/fitness_normalizers/add.py
index 7f0dca334..dbbba349d 100644
--- a/src/stk/_internal/ea/fitness_normalizers/add.py
+++ b/src/stk/_internal/ea/fitness_normalizers/add.py
@@ -106,9 +106,8 @@ class Add(FitnessNormalizer[T]):
     def __init__(
         self,
         number: float | Iterable[float],
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -134,8 +133,10 @@ def __init__(
 
     def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         return {
-            record: np.add(self._number, fitness_value)
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                np.add(self._number, fitness_value)
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/divide_by_mean.py b/src/stk/_internal/ea/fitness_normalizers/divide_by_mean.py
index 58b8c4473..a4c45fdc8 100644
--- a/src/stk/_internal/ea/fitness_normalizers/divide_by_mean.py
+++ b/src/stk/_internal/ea/fitness_normalizers/divide_by_mean.py
@@ -85,9 +85,8 @@ class DivideByMean(FitnessNormalizer[T]):
 
     def __init__(
         self,
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -116,8 +115,10 @@ def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         logger.debug(f"Means used: {mean}")
 
         return {
-            record: np.divide(fitness_value, mean)
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                np.divide(fitness_value, mean)
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/multiply.py b/src/stk/_internal/ea/fitness_normalizers/multiply.py
index 697ae19ba..ca7754ebf 100644
--- a/src/stk/_internal/ea/fitness_normalizers/multiply.py
+++ b/src/stk/_internal/ea/fitness_normalizers/multiply.py
@@ -107,9 +107,8 @@ class Multiply(FitnessNormalizer[T]):
     def __init__(
         self,
         coefficient: float | Iterable[float],
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -135,8 +134,10 @@ def __init__(
 
     def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         return {
-            record: np.multiply(self._coefficient, fitness_value)
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                np.multiply(self._coefficient, fitness_value)
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/power.py b/src/stk/_internal/ea/fitness_normalizers/power.py
index b42e6a21e..fd3dc594c 100644
--- a/src/stk/_internal/ea/fitness_normalizers/power.py
+++ b/src/stk/_internal/ea/fitness_normalizers/power.py
@@ -141,9 +141,8 @@ class Power(FitnessNormalizer[T]):
     def __init__(
         self,
         power: float | Iterable[float],
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -169,8 +168,10 @@ def __init__(
 
     def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         return {
-            record: np.float_power(fitness_value, self._power)
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                np.float_power(fitness_value, self._power)
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/replace_fitness.py b/src/stk/_internal/ea/fitness_normalizers/replace_fitness.py
index 611c8f7a5..477265a87 100644
--- a/src/stk/_internal/ea/fitness_normalizers/replace_fitness.py
+++ b/src/stk/_internal/ea/fitness_normalizers/replace_fitness.py
@@ -68,9 +68,8 @@ def get_minimum_fitness_value(fitness_values):
     def __init__(
         self,
         get_replacement: Callable[[dict[T, Any]], Any],
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -97,8 +96,10 @@ def __init__(
     def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         replacement = self._get_replacement(fitness_values)
         return {
-            record: replacement
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                replacement
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/shift_up.py b/src/stk/_internal/ea/fitness_normalizers/shift_up.py
index 1e8d3c98d..eba2fcad7 100644
--- a/src/stk/_internal/ea/fitness_normalizers/shift_up.py
+++ b/src/stk/_internal/ea/fitness_normalizers/shift_up.py
@@ -144,9 +144,8 @@ class ShiftUp(FitnessNormalizer[T]):
 
     def __init__(
         self,
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -184,8 +183,10 @@ def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
                 shift[i] = 1 - min_
 
         return {
-            record: fitness_value + shift
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                fitness_value + shift
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/ea/fitness_normalizers/sum.py b/src/stk/_internal/ea/fitness_normalizers/sum.py
index 97cc1391a..1af0d963d 100644
--- a/src/stk/_internal/ea/fitness_normalizers/sum.py
+++ b/src/stk/_internal/ea/fitness_normalizers/sum.py
@@ -86,9 +86,8 @@ class Sum(FitnessNormalizer[T]):
 
     def __init__(
         self,
-        filter: Callable[
-            [dict[T, Any], T], bool
-        ] = lambda fitness_values, record: True,
+        filter: Callable[[dict[T, Any], T], bool] = lambda fitness_values,
+        record: True,
     ) -> None:
         """
         Parameters:
@@ -106,8 +105,10 @@ def __init__(
 
     def normalize(self, fitness_values: dict[T, Any]) -> dict[T, Any]:
         return {
-            record: sum(fitness_value)
-            if self._filter(fitness_values, record)
-            else fitness_value
+            record: (
+                sum(fitness_value)
+                if self._filter(fitness_values, record)
+                else fitness_value
+            )
             for record, fitness_value in fitness_values.items()
         }
diff --git a/src/stk/_internal/functional_group_factories/amide_factory.py b/src/stk/_internal/functional_group_factories/amide_factory.py
index b7e2e3561..25966c779 100644
--- a/src/stk/_internal/functional_group_factories/amide_factory.py
+++ b/src/stk/_internal/functional_group_factories/amide_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/bromo_factory.py b/src/stk/_internal/functional_group_factories/bromo_factory.py
index c7e151f94..cbd37f8d0 100644
--- a/src/stk/_internal/functional_group_factories/bromo_factory.py
+++ b/src/stk/_internal/functional_group_factories/bromo_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/carboxylic_acid_factory.py b/src/stk/_internal/functional_group_factories/carboxylic_acid_factory.py
index a6ef8e576..cf2d8bd27 100644
--- a/src/stk/_internal/functional_group_factories/carboxylic_acid_factory.py
+++ b/src/stk/_internal/functional_group_factories/carboxylic_acid_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/dibromo_factory.py b/src/stk/_internal/functional_group_factories/dibromo_factory.py
index a9d0dce76..12e3aaa93 100644
--- a/src/stk/_internal/functional_group_factories/dibromo_factory.py
+++ b/src/stk/_internal/functional_group_factories/dibromo_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/difluoro_factory.py b/src/stk/_internal/functional_group_factories/difluoro_factory.py
index 7c7f284e1..d660df2e0 100644
--- a/src/stk/_internal/functional_group_factories/difluoro_factory.py
+++ b/src/stk/_internal/functional_group_factories/difluoro_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/diol_factory.py b/src/stk/_internal/functional_group_factories/diol_factory.py
index fc612bc7e..e2b26444f 100644
--- a/src/stk/_internal/functional_group_factories/diol_factory.py
+++ b/src/stk/_internal/functional_group_factories/diol_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/fluoro_factory.py b/src/stk/_internal/functional_group_factories/fluoro_factory.py
index c3e41a67b..d291c15ed 100644
--- a/src/stk/_internal/functional_group_factories/fluoro_factory.py
+++ b/src/stk/_internal/functional_group_factories/fluoro_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/iodo_factory.py b/src/stk/_internal/functional_group_factories/iodo_factory.py
index 7d4c1e7fe..b01ee800f 100644
--- a/src/stk/_internal/functional_group_factories/iodo_factory.py
+++ b/src/stk/_internal/functional_group_factories/iodo_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/primary_amino_factory.py b/src/stk/_internal/functional_group_factories/primary_amino_factory.py
index 1669bd88d..486cc5037 100644
--- a/src/stk/_internal/functional_group_factories/primary_amino_factory.py
+++ b/src/stk/_internal/functional_group_factories/primary_amino_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/ring_amine_factory.py b/src/stk/_internal/functional_group_factories/ring_amine_factory.py
index 22e78dfd3..6363764db 100644
--- a/src/stk/_internal/functional_group_factories/ring_amine_factory.py
+++ b/src/stk/_internal/functional_group_factories/ring_amine_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/secondary_amino_factory.py b/src/stk/_internal/functional_group_factories/secondary_amino_factory.py
index 4447d8094..2aaddf344 100644
--- a/src/stk/_internal/functional_group_factories/secondary_amino_factory.py
+++ b/src/stk/_internal/functional_group_factories/secondary_amino_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/smarts.py b/src/stk/_internal/functional_group_factories/smarts.py
index 7e1485cf1..595dcedd0 100644
--- a/src/stk/_internal/functional_group_factories/smarts.py
+++ b/src/stk/_internal/functional_group_factories/smarts.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/terminal_alkene_factory.py b/src/stk/_internal/functional_group_factories/terminal_alkene_factory.py
index 87f72908c..8c0aee31b 100644
--- a/src/stk/_internal/functional_group_factories/terminal_alkene_factory.py
+++ b/src/stk/_internal/functional_group_factories/terminal_alkene_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/thioacid_factory.py b/src/stk/_internal/functional_group_factories/thioacid_factory.py
index abd1a4646..f966ef200 100644
--- a/src/stk/_internal/functional_group_factories/thioacid_factory.py
+++ b/src/stk/_internal/functional_group_factories/thioacid_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_group_factories/thiol_factory.py b/src/stk/_internal/functional_group_factories/thiol_factory.py
index 8b33a45b8..d0b84b983 100644
--- a/src/stk/_internal/functional_group_factories/thiol_factory.py
+++ b/src/stk/_internal/functional_group_factories/thiol_factory.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.functional_group_factories.functional_group_factory import (
     FunctionalGroupFactory,
 )
diff --git a/src/stk/_internal/functional_groups/generic_functional_group.py b/src/stk/_internal/functional_groups/generic_functional_group.py
index b73e583d5..66f93e827 100644
--- a/src/stk/_internal/functional_groups/generic_functional_group.py
+++ b/src/stk/_internal/functional_groups/generic_functional_group.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from __future__ import annotations
 
 import typing
diff --git a/src/stk/_internal/reaction_factories/generic_reaction_factory.py b/src/stk/_internal/reaction_factories/generic_reaction_factory.py
index 2743f9a93..a9c658a3d 100644
--- a/src/stk/_internal/reaction_factories/generic_reaction_factory.py
+++ b/src/stk/_internal/reaction_factories/generic_reaction_factory.py
@@ -4,6 +4,8 @@
 
 """
 
+import textwrap
+
 from stk._internal.functional_groups.aldehyde import Aldehyde
 from stk._internal.functional_groups.alkene import Alkene
 from stk._internal.functional_groups.alkyne import Alkyne
@@ -159,7 +161,20 @@ def get_reaction(self, construction_state, edge_group):
                 edge_group=edge_group,
             )
         )
-        functional_group1, functional_group2 = functional_groups
+
+        try:
+            functional_group1, functional_group2 = functional_groups
+        except ValueError as ex:
+            ex.add_note(
+                textwrap.dedent(
+                    f"""\
+    Two functional groups not found during reaction.
+    This suggests an issue with edge id {next(edge_group.get_edge_ids())}
+    in your topology graph. (edge group: {edge_group})"""
+                )
+            )
+            raise
+
         edge = construction_state.get_edge(
             edge_id=next(edge_group.get_edge_ids()),
         )
diff --git a/src/stk/_internal/reactions/ring_amine_reaction.py b/src/stk/_internal/reactions/ring_amine_reaction.py
index 1dfbaffb6..a36c21270 100644
--- a/src/stk/_internal/reactions/ring_amine_reaction.py
+++ b/src/stk/_internal/reactions/ring_amine_reaction.py
@@ -99,17 +99,17 @@ def _get_new_bonds(self):
         nc2h1 = C(-8)
         nc2h2 = C(-9)
 
-        yield Bond(n1, n_joiner, 1),
-        yield Bond(n_joiner, n2, 1, self._periodicity),
-        yield Bond(n_joiner, nh1, 1),
-        yield Bond(n_joiner, nh2, 1),
-        yield Bond(c1, nc_joiner1, 1),
-        yield Bond(nc_joiner1, n2, 1, self._periodicity),
-        yield Bond(nc_joiner1, nc1h1, 1),
-        yield Bond(nc_joiner1, nc1h2, 1),
-        yield Bond(nc_joiner2, c2, 1, self._periodicity),
-        yield Bond(n1, nc_joiner2, 1),
-        yield Bond(nc_joiner2, nc2h1, 1),
+        yield (Bond(n1, n_joiner, 1),)
+        yield (Bond(n_joiner, n2, 1, self._periodicity),)
+        yield (Bond(n_joiner, nh1, 1),)
+        yield (Bond(n_joiner, nh2, 1),)
+        yield (Bond(c1, nc_joiner1, 1),)
+        yield (Bond(nc_joiner1, n2, 1, self._periodicity),)
+        yield (Bond(nc_joiner1, nc1h1, 1),)
+        yield (Bond(nc_joiner1, nc1h2, 1),)
+        yield (Bond(nc_joiner2, c2, 1, self._periodicity),)
+        yield (Bond(n1, nc_joiner2, 1),)
+        yield (Bond(nc_joiner2, nc2h1, 1),)
         yield Bond(nc_joiner2, nc2h2, 1)
 
     def _get_deleted_atoms(self):
diff --git a/src/stk/_internal/topology_graphs/cage/cage.py b/src/stk/_internal/topology_graphs/cage/cage.py
index d1e43f4aa..b6cd66b48 100644
--- a/src/stk/_internal/topology_graphs/cage/cage.py
+++ b/src/stk/_internal/topology_graphs/cage/cage.py
@@ -966,8 +966,9 @@ def __init_subclass__(cls, **kwargs) -> None:
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         vertex_alignments: dict[int, int] | None = None,
         vertex_positions: dict[int, np.ndarray] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
@@ -1160,11 +1161,13 @@ def _normalize_edge_prototypes(
     @classmethod
     def _normalize_building_blocks(
         cls,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[
-            BuildingBlock,
-            tuple[int, ...],
-        ],
+        building_blocks: (
+            abc.Iterable[BuildingBlock]
+            | dict[
+                BuildingBlock,
+                tuple[int, ...],
+            ]
+        ),
     ) -> dict[BuildingBlock, abc.Sequence[_CageVertex]]:
         # Use tuple here because it prints nicely.
         allowed_degrees = tuple(cls._vertices_of_degree.keys())
diff --git a/src/stk/_internal/topology_graphs/cage/m3l3_triangle.py b/src/stk/_internal/topology_graphs/cage/m3l3_triangle.py
index 98073d797..8004c78cf 100644
--- a/src/stk/_internal/topology_graphs/cage/m3l3_triangle.py
+++ b/src/stk/_internal/topology_graphs/cage/m3l3_triangle.py
@@ -145,10 +145,12 @@ class M3L3Triangle(Cage):
 
     def __init__(
         self,
-        corners: Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
-        linkers: Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        corners: (
+            Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
+        linkers: (
+            Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         vertex_alignments: dict[int, int] | None = None,
         vertex_positions: dict[int, np.ndarray] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cage/m4l4_square.py b/src/stk/_internal/topology_graphs/cage/m4l4_square.py
index f05fca434..dc8cc0772 100644
--- a/src/stk/_internal/topology_graphs/cage/m4l4_square.py
+++ b/src/stk/_internal/topology_graphs/cage/m4l4_square.py
@@ -149,10 +149,12 @@ class M4L4Square(Cage):
 
     def __init__(
         self,
-        corners: Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
-        linkers: Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        corners: (
+            Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
+        linkers: (
+            Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         vertex_alignments: dict[int, int] | None = None,
         vertex_positions: dict[int, np.ndarray] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cage/one_plus_one.py b/src/stk/_internal/topology_graphs/cage/one_plus_one.py
index f0357807c..06f519bfe 100644
--- a/src/stk/_internal/topology_graphs/cage/one_plus_one.py
+++ b/src/stk/_internal/topology_graphs/cage/one_plus_one.py
@@ -176,8 +176,9 @@ class OnePlusOne(Cage):
 
     def __init__(
         self,
-        building_blocks: Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
         num_processes: int = 1,
diff --git a/src/stk/_internal/topology_graphs/cage/six_plus_twelve.py b/src/stk/_internal/topology_graphs/cage/six_plus_twelve.py
index 258abd134..40a883ec8 100644
--- a/src/stk/_internal/topology_graphs/cage/six_plus_twelve.py
+++ b/src/stk/_internal/topology_graphs/cage/six_plus_twelve.py
@@ -3,6 +3,7 @@
 ===============
 
 """
+
 import numpy as np
 
 from stk._internal.topology_graphs.edge import Edge
diff --git a/src/stk/_internal/topology_graphs/cage/ten_plus_twenty.py b/src/stk/_internal/topology_graphs/cage/ten_plus_twenty.py
index 94abc6a01..8eb51da44 100644
--- a/src/stk/_internal/topology_graphs/cage/ten_plus_twenty.py
+++ b/src/stk/_internal/topology_graphs/cage/ten_plus_twenty.py
@@ -3,6 +3,7 @@
 ===============
 
 """
+
 import numpy as np
 
 from stk._internal.topology_graphs.edge import Edge
diff --git a/src/stk/_internal/topology_graphs/cof/cof.py b/src/stk/_internal/topology_graphs/cof/cof.py
index 31130a850..9391e7fa8 100644
--- a/src/stk/_internal/topology_graphs/cof/cof.py
+++ b/src/stk/_internal/topology_graphs/cof/cof.py
@@ -336,8 +336,9 @@ def __init_subclass__(cls, **kwargs):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         periodic: bool = False,
         vertex_alignments: dict[int, int] | None = None,
@@ -457,12 +458,10 @@ def __init__(
                 for building_block, ids in building_blocks.items()
             }
         else:
-            building_block_vertices = (
-                self._get_building_block_vertices(  # type: ignore[assignment]
-                    building_blocks=building_blocks,
-                    vertices=vertices,
-                    edges=edges,
-                )
+            building_block_vertices = self._get_building_block_vertices(  # type: ignore[assignment]
+                building_blocks=building_blocks,
+                vertices=vertices,
+                edges=edges,
             )
 
         building_block_vertices = self._with_unaligning_vertices(
diff --git a/src/stk/_internal/topology_graphs/cof/periodic_hexagonal.py b/src/stk/_internal/topology_graphs/cof/periodic_hexagonal.py
index 488c0d684..6ceeabe40 100644
--- a/src/stk/_internal/topology_graphs/cof/periodic_hexagonal.py
+++ b/src/stk/_internal/topology_graphs/cof/periodic_hexagonal.py
@@ -119,8 +119,9 @@ class PeriodicHexagonal(Cof):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cof/periodic_honeycomb.py b/src/stk/_internal/topology_graphs/cof/periodic_honeycomb.py
index a6ee84a6e..c2564e476 100644
--- a/src/stk/_internal/topology_graphs/cof/periodic_honeycomb.py
+++ b/src/stk/_internal/topology_graphs/cof/periodic_honeycomb.py
@@ -121,8 +121,9 @@ class PeriodicHoneycomb(Cof):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cof/periodic_kagome.py b/src/stk/_internal/topology_graphs/cof/periodic_kagome.py
index c0da9f208..28f243ca7 100644
--- a/src/stk/_internal/topology_graphs/cof/periodic_kagome.py
+++ b/src/stk/_internal/topology_graphs/cof/periodic_kagome.py
@@ -133,8 +133,9 @@ class PeriodicKagome(Cof):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cof/periodic_linkerless_honeycomb.py b/src/stk/_internal/topology_graphs/cof/periodic_linkerless_honeycomb.py
index 7bea9e4c3..aa8bbdb16 100644
--- a/src/stk/_internal/topology_graphs/cof/periodic_linkerless_honeycomb.py
+++ b/src/stk/_internal/topology_graphs/cof/periodic_linkerless_honeycomb.py
@@ -116,8 +116,9 @@ class PeriodicLinkerlessHoneycomb(Cof):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/cof/periodic_square.py b/src/stk/_internal/topology_graphs/cof/periodic_square.py
index 9eb744643..832b5f548 100644
--- a/src/stk/_internal/topology_graphs/cof/periodic_square.py
+++ b/src/stk/_internal/topology_graphs/cof/periodic_square.py
@@ -125,8 +125,9 @@ class PeriodicSquare(Cof):
 
     def __init__(
         self,
-        building_blocks: abc.Iterable[BuildingBlock]
-        | dict[BuildingBlock, tuple[int, ...]],
+        building_blocks: (
+            abc.Iterable[BuildingBlock] | dict[BuildingBlock, tuple[int, ...]]
+        ),
         lattice_size: tuple[int, int, int],
         vertex_alignments: dict[int, int] | None = None,
         reaction_factory: ReactionFactory = GenericReactionFactory(),
diff --git a/src/stk/_internal/topology_graphs/host_guest/vertices.py b/src/stk/_internal/topology_graphs/host_guest/vertices.py
index df772d836..0aea6b288 100644
--- a/src/stk/_internal/topology_graphs/host_guest/vertices.py
+++ b/src/stk/_internal/topology_graphs/host_guest/vertices.py
@@ -4,7 +4,6 @@
 
 """
 
-
 import numpy as np
 
 from stk._internal.topology_graphs.vertex import Vertex
diff --git a/src/stk/_internal/topology_graphs/metal_complex/metal_complex.py b/src/stk/_internal/topology_graphs/metal_complex/metal_complex.py
index 6fb01afb6..333ae47d6 100644
--- a/src/stk/_internal/topology_graphs/metal_complex/metal_complex.py
+++ b/src/stk/_internal/topology_graphs/metal_complex/metal_complex.py
@@ -609,11 +609,13 @@ def __init__(
 
     def _normalize_metals(
         self,
-        metals: BuildingBlock
-        | dict[
-            BuildingBlock,
-            tuple[int, ...],
-        ],
+        metals: (
+            BuildingBlock
+            | dict[
+                BuildingBlock,
+                tuple[int, ...],
+            ]
+        ),
     ) -> dict[BuildingBlock, abc.Sequence[Vertex]]:
         """
         Return a map between metals and vertices.
diff --git a/src/stk/_internal/topology_graphs/rotaxane/nrotaxane.py b/src/stk/_internal/topology_graphs/rotaxane/nrotaxane.py
index b3d3e4dfc..87ce9a093 100644
--- a/src/stk/_internal/topology_graphs/rotaxane/nrotaxane.py
+++ b/src/stk/_internal/topology_graphs/rotaxane/nrotaxane.py
@@ -489,7 +489,8 @@ def __init__(
 
         # Save the chosen orientations for __repr__.
         self._orientations = tuple(
-            int(vertex.get_flip()) for vertex in vertices[1:]  # type:ignore
+            int(vertex.get_flip())  # type: ignore[union-attr]
+            for vertex in vertices[1:]
         )
 
         super().__init__(
diff --git a/src/stk/_internal/topology_graphs/topology_graph/utilities.py b/src/stk/_internal/topology_graphs/topology_graph/utilities.py
index 772edb09b..886c6eddf 100644
--- a/src/stk/_internal/topology_graphs/topology_graph/utilities.py
+++ b/src/stk/_internal/topology_graphs/topology_graph/utilities.py
@@ -4,7 +4,6 @@
 
 """
 
-
 import typing
 
 import numpy as np
diff --git a/src/stk/_internal/utilities/updaters/mae.py b/src/stk/_internal/utilities/updaters/mae.py
index 4a8cf8310..6cdd9e11b 100644
--- a/src/stk/_internal/utilities/updaters/mae.py
+++ b/src/stk/_internal/utilities/updaters/mae.py
@@ -4,7 +4,6 @@
 
 """
 
-
 from stk._internal.utilities.utilities import mol_from_mae_file
 
 
diff --git a/tests/drop_test_database.py b/tests/drop_test_database.py
index e0a0335ba..3fdab1d63 100644
--- a/tests/drop_test_database.py
+++ b/tests/drop_test_database.py
@@ -6,7 +6,6 @@
 
 """
 
-
 import pymongo
 
 
diff --git a/tests/molecular/molecules/building_block/case_data.py b/tests/molecular/molecules/building_block/case_data.py
index 8655eb877..0fb4a3a44 100644
--- a/tests/molecular/molecules/building_block/case_data.py
+++ b/tests/molecular/molecules/building_block/case_data.py
@@ -11,6 +11,9 @@ class CaseData:
         The functional groups :attr:`.building_block` should be
         holding.
 
+    known_repr : str
+        The representation of the building block.
+
     core_atom_ids : :class:`tuple` of :class:`int`
         The correct core atom ids for :attr:`.building_block`.
 
@@ -23,6 +26,7 @@ def __init__(
         self,
         building_block,
         functional_groups,
+        known_repr,
         core_atom_ids,
         placer_ids,
     ):
@@ -37,6 +41,9 @@ def __init__(
         functional_groups : :class:`tuple` of :class:`.FunctionalGroup`
             The functional groups `building_block` should be holding.
 
+        known_repr : str
+            The representation of the building block.
+
         core_atom_ids : :class:`tuple` of :class:`int`
             The correct core atom ids for `building_block`.
 
@@ -47,5 +54,6 @@ def __init__(
 
         self.building_block = building_block
         self.functional_groups = functional_groups
+        self.known_repr = known_repr
         self.core_atom_ids = core_atom_ids
         self.placer_ids = placer_ids
diff --git a/tests/molecular/molecules/building_block/fixtures/default_init.py b/tests/molecular/molecules/building_block/fixtures/default_init.py
index 41fe19292..9122c4d16 100644
--- a/tests/molecular/molecules/building_block/fixtures/default_init.py
+++ b/tests/molecular/molecules/building_block/fixtures/default_init.py
@@ -10,6 +10,7 @@
         lambda: CaseData(
             building_block=stk.BuildingBlock("Br[C+2][C+2]Br"),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
@@ -32,6 +33,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(1, 2),
         ),
@@ -41,6 +48,7 @@
                 placer_ids=(1, 2),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(1, 2),
         ),
@@ -64,6 +72,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(0, 3),
         ),
@@ -73,6 +87,7 @@
                 functional_groups=[stk.IodoFactory()],
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
@@ -95,6 +110,11 @@
                     deleters=(stk.Br(2),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2]Br', (Bromo(Br(0), C(1, charge=2), bond"
+                "ers=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(2), C(1, "
+                "charge=2), bonders=(C(1, charge=2),), deleters=(Br(2),))))"
+            ),
             core_atom_ids=(1,),
             placer_ids=(1,),
         ),
diff --git a/tests/molecular/molecules/building_block/fixtures/init.py b/tests/molecular/molecules/building_block/fixtures/init.py
index 747b22b62..4fd4c941d 100644
--- a/tests/molecular/molecules/building_block/fixtures/init.py
+++ b/tests/molecular/molecules/building_block/fixtures/init.py
@@ -26,6 +26,7 @@
                 ),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
@@ -61,6 +62,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(1, 2),
         ),
@@ -83,6 +90,7 @@
                 placer_ids=(1, 2),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(1, 2),
         ),
@@ -119,6 +127,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(0, 3),
         ),
@@ -141,6 +155,7 @@
                 functional_groups=[stk.IodoFactory()],
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
diff --git a/tests/molecular/molecules/building_block/fixtures/init_from_file.py b/tests/molecular/molecules/building_block/fixtures/init_from_file.py
index 175cd0d3b..606d0be86 100644
--- a/tests/molecular/molecules/building_block/fixtures/init_from_file.py
+++ b/tests/molecular/molecules/building_block/fixtures/init_from_file.py
@@ -43,6 +43,9 @@ class InitFromFileData:
         The ids of *placer* atoms the initialized building block should
         have.
 
+    known_repr : str
+        The representation of the building block.
+
     """
 
     def __init__(
@@ -53,6 +56,7 @@ def __init__(
         case_data_functional_groups,
         case_data_core_atom_ids,
         case_data_placer_ids,
+        known_repr,
     ):
         """
         Initialize a :class:`.InitFromData` instance.
@@ -83,6 +87,9 @@ def __init__(
             The ids of *placer* atoms the initialized building block
             should have.
 
+        known_repr : str
+            The representation of the building block.
+
         """
 
         self.building_block = building_block
@@ -91,6 +98,7 @@ def __init__(
         self.case_data_functional_groups = case_data_functional_groups
         self.case_data_core_atom_ids = case_data_core_atom_ids
         self.case_data_placer_ids = case_data_placer_ids
+        self.known_repr = known_repr
 
 
 @pytest.fixture(
@@ -103,6 +111,7 @@ def __init__(
             case_data_functional_groups=(),
             case_data_core_atom_ids=(0, 1, 2, 3),
             case_data_placer_ids=(0, 1, 2, 3),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
         ),
         lambda: InitFromFileData(
             building_block=stk.BuildingBlock("Br[C+2][C+2]Br"),
@@ -124,6 +133,12 @@ def __init__(
             ),
             case_data_core_atom_ids=(1, 2),
             case_data_placer_ids=(1, 2),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
         ),
         lambda: InitFromFileData(
             building_block=stk.BuildingBlock("Br[C+2][C+2]Br"),
@@ -132,6 +147,7 @@ def __init__(
             case_data_functional_groups=(),
             case_data_core_atom_ids=(0, 1, 2, 3),
             case_data_placer_ids=(1, 2),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
         ),
         lambda: InitFromFileData(
             building_block=stk.BuildingBlock("Br[C+2][C+2]Br"),
@@ -153,6 +169,12 @@ def __init__(
             ),
             case_data_core_atom_ids=(1, 2),
             case_data_placer_ids=(0, 3),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
         ),
         lambda: InitFromFileData(
             building_block=stk.BuildingBlock("Br[C+2][C+2]Br"),
@@ -161,6 +183,7 @@ def __init__(
             case_data_functional_groups=(),
             case_data_core_atom_ids=(0, 1, 2, 3),
             case_data_placer_ids=(0, 1, 2, 3),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
         ),
     ),
 )
@@ -191,4 +214,5 @@ def init_from_file(path, init_from_file_data):
         functional_groups=data.case_data_functional_groups,
         core_atom_ids=data.case_data_core_atom_ids,
         placer_ids=data.case_data_placer_ids,
+        known_repr=data.known_repr,
     )
diff --git a/tests/molecular/molecules/building_block/fixtures/init_from_molecule.py b/tests/molecular/molecules/building_block/fixtures/init_from_molecule.py
index 6bb49d045..0d1355a20 100644
--- a/tests/molecular/molecules/building_block/fixtures/init_from_molecule.py
+++ b/tests/molecular/molecules/building_block/fixtures/init_from_molecule.py
@@ -12,6 +12,7 @@
                 molecule=stk.BuildingBlock("Br[C+2][C+2]Br"),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
@@ -34,6 +35,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(1, 2),
         ),
@@ -43,6 +50,7 @@
                 placer_ids=(1, 2),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(1, 2),
         ),
@@ -66,6 +74,12 @@
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(0, 3),
         ),
@@ -75,6 +89,7 @@
                 functional_groups=[stk.IodoFactory()],
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
diff --git a/tests/molecular/molecules/building_block/fixtures/init_from_rdkit_mol.py b/tests/molecular/molecules/building_block/fixtures/init_from_rdkit_mol.py
index 00ec7547e..a9d96cf27 100644
--- a/tests/molecular/molecules/building_block/fixtures/init_from_rdkit_mol.py
+++ b/tests/molecular/molecules/building_block/fixtures/init_from_rdkit_mol.py
@@ -19,6 +19,7 @@ def _get_rdkit_molecule() -> rdkit.Mol:
                 molecule=_get_rdkit_molecule(),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
@@ -41,6 +42,12 @@ def _get_rdkit_molecule() -> rdkit.Mol:
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(1, 2),
         ),
@@ -50,6 +57,7 @@ def _get_rdkit_molecule() -> rdkit.Mol:
                 placer_ids=(1, 2),
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(1, 2),
         ),
@@ -73,6 +81,12 @@ def _get_rdkit_molecule() -> rdkit.Mol:
                     deleters=(stk.Br(3),),
                 ),
             ),
+            known_repr=(
+                "BuildingBlock('Br[C+2][C+2]Br', (Bromo(Br(0), C(1, charge=2),"
+                " bonders=(C(1, charge=2),), deleters=(Br(0),)), Bromo(Br(3), "
+                "C(2, charge=2), bonders=(C(2, charge=2),), deleters=(Br(3),))"
+                "))"
+            ),
             core_atom_ids=(1, 2),
             placer_ids=(0, 3),
         ),
@@ -82,6 +96,7 @@ def _get_rdkit_molecule() -> rdkit.Mol:
                 functional_groups=[stk.IodoFactory()],
             ),
             functional_groups=(),
+            known_repr="BuildingBlock('Br[C+2][C+2]Br', ())",
             core_atom_ids=(0, 1, 2, 3),
             placer_ids=(0, 1, 2, 3),
         ),
diff --git a/tests/molecular/molecules/building_block/test_repr.py b/tests/molecular/molecules/building_block/test_repr.py
new file mode 100644
index 000000000..ae8735aa5
--- /dev/null
+++ b/tests/molecular/molecules/building_block/test_repr.py
@@ -0,0 +1,41 @@
+def test_repr(case_data):
+    """
+    Test :meth:`.BuildingBlock.__repr__`.
+
+    Parameters
+    ----------
+    case_data : :class:`.CaseData`
+        A test case. Holds the building block to test and the correct
+        functional groups.
+
+    Returns
+    -------
+    None : :class:`NoneType`
+
+    """
+
+    _test_repr(
+        building_block=case_data.building_block,
+        known_repr=case_data.known_repr,
+    )
+
+
+def _test_repr(building_block, known_repr):
+    """
+    Test :meth:`.BuildingBlock.get_functional_groups`.
+
+    Parameters
+    ----------
+    building_block : :class:`.BuildingBlock`
+        The building block to test.
+
+    functional_groups : :class:`tuple` of :class:`.FunctionalGroup`
+        The correct functional groups.
+
+    Returns
+    -------
+    None : :class:`NoneType`
+
+    """
+    print(building_block.__repr__(), known_repr)
+    assert building_block.__repr__() == known_repr