Add Optional type hint for arguments with a default value of None (#556)

* Add Optional type hints

* Add Optional code style documentation

docs/development/code_guide.rst

@@ -49,6 +49,9 @@ Coding Style
 * For multiline imports, instead of using ``\`` to break lines, wrap the
   imported names in ``()`` parentheses and use consistent whitespace.
 
+* Use ``Optional[type]`` for type hinted keyword arguments with a default value of
+  ``None``.
+
 * There should be at most one pun per 1284 lines of code.
 
 Branches, commits, and pull requests

plasmapy/atomic/atomic.py

@@ -260,7 +260,7 @@ def particle_mass(
 
 
 @particle_input
-def isotopic_abundance(isotope: Particle, mass_numb: numbers.Integral = None) -> numbers.Real:
+def isotopic_abundance(isotope: Particle, mass_numb: Optional[numbers.Integral] = None) -> numbers.Real:
     """
     Return the isotopic abundances if known, and otherwise zero.
 
@@ -422,7 +422,7 @@ def electric_charge(particle: Particle) -> u.Quantity:
 
 
 @particle_input
-def is_stable(particle: Particle, mass_numb: numbers.Integral = None) -> bool:
+def is_stable(particle: Particle, mass_numb: Optional[numbers.Integral] = None) -> bool:
     """
     Return `True` for stable isotopes and particles and `False` for
     unstable isotopes.
@@ -476,7 +476,7 @@ def is_stable(particle: Particle, mass_numb: numbers.Integral = None) -> bool:
 
 
 @particle_input(any_of={'stable', 'unstable', 'isotope'})
-def half_life(particle: Particle, mass_numb: numbers.Integral = None) -> u.Quantity:
+def half_life(particle: Particle, mass_numb: Optional[numbers.Integral] = None) -> u.Quantity:
     """
     Return the half-life in seconds for unstable isotopes and particles,
     and numpy.inf in seconds for stable isotopes and particles.

plasmapy/atomic/nuclear.py

@@ -1,7 +1,7 @@
 """Functions that are related to nuclear reactions."""
 
 from astropy import units as u
-from typing import List, Union
+from typing import List, Union, Optional
 import re
 
 from plasmapy.utils import (
@@ -20,7 +20,7 @@
 
 
 @particle_input(any_of={'isotope', 'baryon'})
-def nuclear_binding_energy(particle: Particle, mass_numb: int = None) -> u.Quantity:
+def nuclear_binding_energy(particle: Particle, mass_numb: Optional[int] = None) -> u.Quantity:
     """
     Return the nuclear binding energy associated with an isotope.
 
@@ -79,7 +79,7 @@ def nuclear_binding_energy(particle: Particle, mass_numb: int = None) -> u.Quant
 
 
 @particle_input
-def mass_energy(particle: Particle, mass_numb: int = None) -> u.Quantity:
+def mass_energy(particle: Particle, mass_numb: Optional[int] = None) -> u.Quantity:
     """
     Return a particle's mass energy.  If the particle is an isotope or
     nuclide, return the nuclear mass energy only.

plasmapy/atomic/symbols.py

@@ -3,6 +3,8 @@
 and numbers.
 """
 
+from typing import Optional
+
 from .particle_class import Particle
 from .particle_input import particle_input
 import numbers
@@ -94,7 +96,7 @@ def atomic_symbol(element: Particle) -> str:
 
 
 @particle_input
-def isotope_symbol(isotope: Particle, mass_numb: numbers.Integral = None) -> str:
+def isotope_symbol(isotope: Particle, mass_numb: Optional[numbers.Integral] = None) -> str:
     """
     Return the symbol representing an isotope.
 

plasmapy/physics/parameters.py

@@ -3,6 +3,8 @@
 plasma frequency or Debye length.
 """
 
+from typing import Optional
+
 __all__ = [
     "mass_density",
     "Alfven_speed",
@@ -64,7 +66,7 @@ def _grab_charge(ion, z_mean=None):
     return Z
 
 
-def mass_density(density, particle: str = None, z_mean: numbers.Real = None) -> u.kg / u.m ** 3:
+def mass_density(density, particle: Optional[str] = None, z_mean: Optional[numbers.Real] = None) -> u.kg / u.m ** 3:
     """Utility function to merge two possible inputs for particle charge.
 
     Parameters