diff --git a/source/fab/build_config.py b/source/fab/build_config.py
index 4692c6c4..227abafa 100644
--- a/source/fab/build_config.py
+++ b/source/fab/build_config.py
@@ -34,7 +34,7 @@ class BuildConfig(object):
     """
 
     def __init__(self, project_label: str, source_root: Optional[Path] = None, steps: Optional[List[Step]] = None,
-                 multiprocessing: bool = True, n_procs: int = None, reuse_artefacts: bool = False,
+                 multiprocessing: bool = True, n_procs: Optional[int] = None, reuse_artefacts: bool = False,
                  fab_workspace: Optional[Path] = None, verbose: bool = False, prebuild_folder: Optional[Path] = None):
         """
         :param project_label:
diff --git a/source/fab/steps/analyse.py b/source/fab/steps/analyse.py
index cf8e3e4d..b3001f03 100644
--- a/source/fab/steps/analyse.py
+++ b/source/fab/steps/analyse.py
@@ -75,7 +75,7 @@ class Analyse(Step):
     """
     # todo: allow the user to specify a different output artefact collection name?
     def __init__(self,
-                 source: ArtefactsGetter = None,
+                 source: Optional[ArtefactsGetter] = None,
                  root_symbol: Optional[Union[str, List[str]]] = None,  # todo: iterable is more correct
                  std: str = "f2008",
                  special_measure_analysis_results: Optional[List[AnalysedFile]] = None,
diff --git a/source/fab/steps/archive_objects.py b/source/fab/steps/archive_objects.py
index ff997443..e68205b8 100644
--- a/source/fab/steps/archive_objects.py
+++ b/source/fab/steps/archive_objects.py
@@ -10,7 +10,7 @@
 
 import logging
 from string import Template
-from typing import Dict
+from typing import Dict, Optional
 
 from fab.constants import OBJECT_FILES, OBJECT_ARCHIVES
 from fab.steps import Step
@@ -67,7 +67,7 @@ class ArchiveObjects(Step):
 
     """
     # todo: the output path should not be an abs fpath, it should be relative to the proj folder
-    def __init__(self, source: ArtefactsGetter = None, archiver='ar',
+    def __init__(self, source: Optional[ArtefactsGetter] = None, archiver='ar',
                  output_fpath=None, output_collection=OBJECT_ARCHIVES, name='archive objects'):
         """
         :param source:
diff --git a/source/fab/steps/c_pragma_injector.py b/source/fab/steps/c_pragma_injector.py
index 83417181..1aba6aed 100644
--- a/source/fab/steps/c_pragma_injector.py
+++ b/source/fab/steps/c_pragma_injector.py
@@ -31,7 +31,7 @@ class CPragmaInjector(Step):
     including in paths relative to the c file.
 
     """
-    def __init__(self, source: ArtefactsGetter = None, output_name=None, name="c pragmas"):
+    def __init__(self, source: Optional[ArtefactsGetter] = None, output_name=None, name="c pragmas"):
         """
         :param source:
             An :class:`~fab.artefacts.ArtefactsGetter` which give us our c files to process.
diff --git a/source/fab/steps/compile_c.py b/source/fab/steps/compile_c.py
index b261ccb8..8d71a5ee 100644
--- a/source/fab/steps/compile_c.py
+++ b/source/fab/steps/compile_c.py
@@ -10,7 +10,7 @@
 import logging
 import os
 from collections import defaultdict
-from typing import List, Dict
+from typing import List, Dict, Optional
 
 from fab.build_config import FlagsConfig
 from fab.constants import OBJECT_FILES
@@ -38,8 +38,8 @@ class CompileC(Step):
 
     """
     # todo: tell the compiler (and other steps) which artefact name to create?
-    def __init__(self, compiler: str = None, common_flags: List[str] = None, path_flags: List = None,
-                 source: ArtefactsGetter = None, name="compile c"):
+    def __init__(self, compiler: Optional[str] = None, common_flags: Optional[List[str]] = None,
+                 path_flags: Optional[List] = None, source: Optional[ArtefactsGetter] = None, name="compile c"):
         """
         :param compiler:
             The command line compiler to call. Defaults to `gcc -c`.
diff --git a/source/fab/steps/compile_fortran.py b/source/fab/steps/compile_fortran.py
index bd235d4b..8447fb90 100644
--- a/source/fab/steps/compile_fortran.py
+++ b/source/fab/steps/compile_fortran.py
@@ -13,7 +13,7 @@
 import zlib
 from collections import defaultdict
 from pathlib import Path
-from typing import List, Set, Dict, Tuple
+from typing import List, Set, Dict, Tuple, Optional
 
 from fab.build_config import FlagsConfig
 from fab.constants import OBJECT_FILES
@@ -40,8 +40,9 @@ class CompileFortran(Step):
     The files are compiled in multiple passes, with each pass enabling further files to be compiled in the next pass.
 
     """
-    def __init__(self, compiler: str = None, common_flags: List[str] = None, path_flags: List = None,
-                 source: ArtefactsGetter = None, two_stage_flag=None, name='compile fortran'):
+    def __init__(self, compiler: Optional[str] = None, common_flags: Optional[List[str]] = None,
+                 path_flags: Optional[List] = None, source: Optional[ArtefactsGetter] = None,
+                 two_stage_flag=None, name='compile fortran'):
         """
         :param compiler:
             The command line compiler to call. Defaults to `gfortran -c`.
@@ -335,7 +336,7 @@ def compile_file(self, analysed_file, flags, output_fpath):
 
 
 # todo: generalise this for the preprocessor, we see flags in FPP
-def get_compiler(compiler: str = None) -> Tuple[str, List[str]]:
+def get_compiler(compiler: Optional[str] = None) -> Tuple[str, List[str]]:
     """
     Separate the compiler and flags from the given string (or `FC` environment variable), like `gfortran -c`.
 
diff --git a/source/fab/steps/grab.py b/source/fab/steps/grab.py
index 5f5f2bcb..151e092a 100644
--- a/source/fab/steps/grab.py
+++ b/source/fab/steps/grab.py
@@ -11,7 +11,7 @@
 import os
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import Dict, Union
+from typing import Dict, Union, Optional
 
 try:
     import svn  # type: ignore
@@ -33,7 +33,7 @@ class GrabSourceBase(Step, ABC):
     Unlike most steps, grab steps don't need to read or create artefact collections.
 
     """
-    def __init__(self, src: str, dst: str = None, name=None):
+    def __init__(self, src: str, dst: Optional[str] = None, name=None):
         """
         :param src:
             The source location to grab. The nature of this parameter is depends on the subclass.
@@ -72,7 +72,7 @@ class GrabFolder(GrabSourceBase):
 
     """
 
-    def __init__(self, src: Union[Path, str], dst: str = None, name=None):
+    def __init__(self, src: Union[Path, str], dst: Optional[str] = None, name=None):
         """
         :param src:
             The source location to grab. The nature of this parameter is depends on the subclass.
@@ -100,7 +100,7 @@ class GrabFcm(GrabSourceBase):
     Grab an FCM repo folder to the project workspace.
 
     """
-    def __init__(self, src: str, dst: str = None, revision=None, name=None):
+    def __init__(self, src: str, dst: Optional[str] = None, revision=None, name=None):
         """
         :param src:
             Such as `fcm:jules.xm_tr/src`.
diff --git a/source/fab/steps/link.py b/source/fab/steps/link.py
index fdea4c0c..0e7c91a6 100644
--- a/source/fab/steps/link.py
+++ b/source/fab/steps/link.py
@@ -11,7 +11,7 @@
 import os
 from abc import ABC
 from string import Template
-from typing import List
+from typing import List, Optional
 
 from fab.constants import OBJECT_FILES, OBJECT_ARCHIVES, EXECUTABLES
 from fab.steps import Step
@@ -45,7 +45,7 @@ class LinkerBase(Step, ABC):
     compiler steps.
 
     """
-    def __init__(self, linker: str = None, flags=None, source: ArtefactsGetter = None, name='link'):
+    def __init__(self, linker: Optional[str] = None, flags=None, source: Optional[ArtefactsGetter] = None, name='link'):
         """
         :param linker:
             E.g 'gcc' or 'ld'.
@@ -107,8 +107,8 @@ class LinkSharedObject(LinkExe):
     We can assume the list of object files is the entire project source, compiled.
 
     """
-    def __init__(self, output_fpath: str, linker: str = None, flags=None, source: ArtefactsGetter = None,
-                 name='link shared object'):
+    def __init__(self, output_fpath: str, linker: Optional[str] = None, flags=None,
+                 source: Optional[ArtefactsGetter] = None, name='link shared object'):
         """
         Params are as for :class:`~fab.steps.link_exe.LinkerBase`, with the addition of:
 
diff --git a/source/fab/steps/preprocess.py b/source/fab/steps/preprocess.py
index f79eca71..cfa3f70b 100644
--- a/source/fab/steps/preprocess.py
+++ b/source/fab/steps/preprocess.py
@@ -10,7 +10,7 @@
 import logging
 import os
 from pathlib import Path
-from typing import List
+from typing import List, Optional
 
 from fab.build_config import FlagsConfig
 from fab.constants import PRAGMAD_C
@@ -34,9 +34,9 @@ class PreProcessor(Step):
     LABEL: str
 
     def __init__(self,
-                 source: ArtefactsGetter = None, output_collection=None, output_suffix=None,
-                 preprocessor: str = None, common_flags: List[str] = None, path_flags: List = None,
-                 name=None):
+                 source: Optional[ArtefactsGetter] = None, output_collection=None, output_suffix=None,
+                 preprocessor: Optional[str] = None, common_flags: Optional[List[str]] = None,
+                 path_flags: Optional[List] = None, name=None):
         """
         :param source:
             Defines the files to preprocess. Defaults to DEFAULT_SOURCE.