def precision(key):
_check_obsolete(key)<EOL>return physical_constants[key][<NUM_LIT:2>] / physical_constants[key][<NUM_LIT:0>]<EOL>
Relative precision in physical_constants indexed by key

Parameters
----------
key : Python string or unicode
    Key in dictionary `physical_constants`

Returns
-------
prec : float
    Relative precision in `physical_constants` corresponding to `key`

See Also
--------
codata : Contains the description of `physical_constants`, which, as a
    dictionary literal object, does not itself possess a docstring.

Examples
--------
>>> from scipy.constants import codata
>>> codata.precision(u'proton mass')
4.96226989798e-08