Atomic weights in Python variables and a nearly-trivial REPL, for weird people like me who decide to do high-school chemistry homework using Python as a calculator.

atomicweights.py

from __future__ import division, print_function
H=1.008;                                                                                                                                                                                                       He=4.002602;
Li=6.94;       Be=9.012182;                                                                                                                       B=10.81;      C=12.011;  N=14.007;    O=15.999; F=18.9984032;Ne=20.1797;
Na=22.98976928;Mg=24.3050;                                                                                                                        Al=26.9815386;Si=28.085; P=30.973762; S=32.06;  Cl=35.45;    Ar=39.948;
K=39.0983;     Ca=40.078; Sc=44.955912;Ti=47.867;V=50.9415;   Cr=51.9961;Mn=54.938045;Fe=55.845; Co=58.933195;Ni=58.6934;Cu=63.546;    Zn=65.38;  Ga=69.723;    Ge=72.63;  As=74.92160; Se=78.96; Br=79.904;   Kr=83.798;
Rb=85.4678;    Sr=87.62;  Y=88.90585;  Zr=91.224;Nb=92.90638; Mo=95.96;  Tc=98.0;     Ru=101.07; Rh=102.90550;Pd=106.42; Ag=107.8682;  Cd=112.411;In=114.818;   Sn=118.710;Sb=121.760;  Te=127.60;I=126.90447; Xe=131.293;
Cs=132.9054519;Ba=137.327;             Hf=178.49;Ta=180.94788;W=183.84;  Re=186.207;  Os=190.23; Ir=192.217;  Pt=195.084;Au=196.966569;Hg=200.59; Tl=204.38;    Pb=207.2;  Bi=208.98040;Po=209.0; At=210.0;    Rn=222.0;
Fr=223.0;      Ra=226.0;               Rf=267.0; Db=268.0;    Sg=269.0;  Bh=270.0;    Hs=269.0;  Mt=278.0;    Ds=281.0;  Rg=281.0;     Cn=285.0;  Uut=286.0;    Fl=289.0;  Uup=288.0;   Lv=293.0; Uus=294.0;   Uuo=294.0;

"Lathanides";La=138.90547;Ce=140.116;  Pr=140.90765;Nd=144.242; Pm=145.0;Sm=150.36;Eu=151.964;Gd=157.25;Tb=158.92535;Dy=162.500;Ho=164.93032;Er=167.259;Tm=168.93421;Yb=173.054;Lu=174.9668;
"Actinides"; Ac=227.0;    Th=232.03806;Pa=231.03588;U=238.02891;Np=237.0;Pu=244.0; Am=243.0;  Cm=247.0; Bk=247.0;    Cf=251.0;  Es=252.0;    Fm=257.0;  Md=258.0;    No=259.0;  Lr=262.0;
if __name__ == "__main__":
	# random Python-2/3-agnostic hack from StackOverflow somewhere
	try: input = raw_input
	except NameError: pass
	while True:
		try:
			_ = eval(input("> "))
			print(_)
		except Exception as e: print("Error: " + str(e))
		except KeyboardInterrupt: print(); break