# Decorators¶

## Passing Particle objects to functions and methods¶

When calculating plasma parameters, we very frequently need to access the properties of the particles that make up that plasma. The particle_input decorator allows functions and methods to easily access properties of different particles.

The particle_input decorator takes valid representations of particles given in arguments to functions and passes through the corresponding Particle object. The arguments must be annotated with Particle so that the decorator knows to create the Particle object. The decorated function can then access particle properties by using Particle attributes. This decorator will raise an InvalidParticleError if the input does not correspond to a valid particle.

Here is an example of a decorated function.

from plasmapy.particles import Particle, particle_input

@particle_input
def particle_mass(particle: Particle):
return particle.mass


This function can now accept either Particle objects or valid representations of particles.

>>> particle_mass('p+')  # string input
<Quantity 1.67262192e-27 kg>
>>> proton = Particle("proton")
>>> particle_mass(proton)  # Particle object input
<Quantity 1.67262192e-27 kg>


If only one positional or keyword argument is annotated with Particle, then the keywords mass_numb and Z may be used when the decorated function is called.

@particle_input
def integer_charge(particle: Particle, Z: int = None, mass_numb: int = None) -> int:
return particle.integer_charge


The above example includes optional type hint annotations for Z and mass_numb and the returned value. The particle_input decorator may be used in methods in classes as well:

class ExampleClass:
@particle_input
def particle_symbol(self, particle: Particle) -> str:
return particle.particle


On occasion it is necessary for a function to accept only certain categories of particles. The particle_input decorator enables several ways to allow this.

If an annotated keyword is named element, isotope, or ion; then particle_input will raise an InvalidElementError, InvalidIsotopeError, or InvalidIonError if the particle is not associated with an element, isotope, or ion; respectively.

@particle_input
def capitalized_element_name(element: Particle):
return element.element_name

@particle_input
def number_of_neutrons(isotope: Particle):
return isotope.mass_number - isotope.atomic_number

@particle_input
def number_of_bound_electrons(ion: Particle):
return ion.atomic_number - ion.integer_charge


The keywords require, any_of, and exclude to the decorator allow further customization of the particle categories allowed as inputs. These keywords are used as in is_category.

@particle_input(require='charged')
def sign_of_charge(charged_particle: Particle):
"""Require a charged particle."""
return '+' if charged_particle.integer_charge > 0 else '-'

@particle_input(any_of=['charged', 'uncharged'])
def integer_charge(particle: Particle) -> int:
"""Accept only particles with charge information."""
return particle.integer_charge

@particle_input(exclude={'antineutrino', 'neutrino'})
def particle_mass(particle: Particle):
"""
Exclude neutrinos/antineutrinos because these particles have
weakly constrained masses.
"""
return particle.mass