Enable comparing Elements for equality

[lmmentel]

fixes #42

[mattwthompson]

This handles my use case perfectly, thanks!

The things I can think of - basic behavior of == and !=, ability to still use as a key in a dict - work as I would intuitively expect. I'd be happy to consider other details if that would be of help, this is just what comes to mind.

There appear to be many attributes left over after to_drop is applied, so I would not be worried about a potential hash collision.

In [1]: from mendeleev import element, O

In [2]: assert element(8) == O

In [3]: assert element(8) == element(8)

In [4]: assert O == O

In [5]: def f():
   ...:     return element(8)
   ...:

In [6]: assert f() == element(8) == O

In [7]: element_dict = {O: 0}

In [8]: element_dict[O] = 1

In [9]: element_dict[O]
Out[9]: 1

In [10]:         to_drop = [
    ...:             "_ionization_energies",
    ...:             "_oxidation_states",
    ...:             "_sa_instance_state",
    ...:             "_series",
    ...:             "_series_id",
    ...:             "ec",
    ...:             "group",
    ...:             "ionic_radii",
    ...:             "isotopes",
    ...:             "screening_constants",
    ...:         ]

In [11]: [(k, v) for k, v in O.__dict__.items() if k not in to_drop]
Out[11]:
[('covalent_radius_cordero', 66.0),
 ('en_ghosh', 0.304575),
 ('mendeleev_number', 99),
 ('covalent_radius_pyykko', 63.0),
 ('en_pauling', 3.44),
 ('metallic_radius', None),
 ('covalent_radius_pyykko_double', 57.0),
 ('evaporation_heat', None),
 ('metallic_radius_c12', None),
 ('covalent_radius_pyykko_triple', 53.0),
 ('fusion_heat', None),
 ('molcas_gv_color', '#f32e42'),
 ('c6', 15.6),
 ('gas_basicity', 459.6),
 ('name', 'Oxygen'),
 ('c6_gb', 16.7),
 ('geochemical_class', 'major'),
 ('name_origin', 'Greek: oxys and genes, (acid former).'),
 ('cpk_color', '#f00000'),
 ('glawe_number', 97),
 ('period', 2),
 ('density', 0.0013080000000000001),
 ('goldschmidt_class', 'litophile'),
 ('pettifor_number', 101),
 ('group_id', 16),
 ('proton_affinity', 485.2),
 ('description',
  "A colourless, odourless gaseous element belonging to group 16 of the periodic table. It is the most abundant element present in the earth's crust. It also makes up 20.8% of the Earth's atmosphere. For industrial purposes, it is separated from liquid air by fractional distillation. It is used in high temperature welding, and in breathing. It commonly comes in the form of Oxygen, but is found as Ozone in the upper atmosphere. It was discovered by Priestley in 1774."),
 ('dipole_polarizability', 5.3),
 ('heat_of_formation', 249.229),
 ('sources',
  'Obtained primarily from liquid air by fractional distillation. Small amounts are made in the laboratory by electrolysis of water or heating potassium chlorate (KClO3) with manganese dioxide (MnO2) catalyst.'),
 ('dipole_polarizability_unc', 0.2),
 ('is_monoisotopic', None),
 ('specific_heat', None),
 ('discoverers', 'Joseph Priestly, Carl Wilhelm Scheele'),
 ('is_radioactive', False),
 ('symbol', 'O'),
 ('discovery_location', 'England/Sweden'),
 ('jmol_color', '#ff0d0d'),
 ('thermal_conductivity', 0.027),
 ('discovery_year', 1774),
 ('lattice_constant', 6.83),
 ('uses',
  'Used in steel making, welding, and supporting life. Naturally occuring ozone (O3) in the upper atmosphere shields the earth from ultraviolet radiation.'),
 ('electron_affinity', 1.4611135),
 ('lattice_structure', 'CUB'),
 ('vdw_radius', 152.0),
 ('en_allen', 21.36),
 ('melting_point', 54.8),
 ('vdw_radius_alvarez', 150.0),
 ('vdw_radius_bondi', 152.0),
 ('econf', '[He] 2s2 2p4'),
 ('vdw_radius_truhlar', None),
 ('vdw_radius_rt', 158.0),
 ('abundance_crust', 461000.0),
 ('vdw_radius_batsanov', 155.0),
 ('abundance_sea', 857000.0),
 ('vdw_radius_dreiding', 340.46),
 ('annotation', 'density(@ -183C), '),
 ('vdw_radius_uff', 350.0),
 ('atomic_number', 8),
 ('vdw_radius_mm3', 182.0),
 ('atomic_radius', 60.0),
 ('atomic_radius_rahm', 171.0),
 ('atomic_volume', 14.0),
 ('atomic_weight', 15.999),
 ('atomic_weight_uncertainty', None),
 ('block', 'p'),
 ('boiling_point', 90.19),
 ('cas', '7782-44-7'),
 ('covalent_radius_bragg', 65.0)]

In [12]: assert O != element(7)

In [13]: assert element(8) != element(7)

[lmmentel]

Thanks for the feedback @mattwthompson!

[lmmentel]

Thanks for the comments @kalvdans! If you feel something should still be reworked please submit an issue.

[kalvdans]

Yes, I feel that the hash is misused for equality, I've opened #47 with my suggestions :)