Help on return type(?)

[Steven D'Aprano]

On Sun, 10 Jan 2016 12:45 pm, Robert wrote:

> Hi,
> 
> I see below code snippet. The return line is not as the usual type.
> 
> 
> 
> def make_cov(cov_type, n_comp, n_fea):
>     mincv = 0.1
>     rand = np.random.random
>     return {
>         'spherical': (mincv + mincv * np.dot(rand((n_components, 1)),
>                                              np.ones((1, n_features)))) **
>                                              2,
>         'tied': (make_spd_matrix(n_features)
>                  + mincv * np.eye(n_features)),
>         'diag': (mincv + mincv * rand((n_components, n_features))) ** 2,
>         'full': np.array([(make_spd_matrix(n_features)
>                            + mincv * np.eye(n_features))
>                           for x in range(n_components)])
>     }[cov_type]
> 
> Specifically, could you explain the meaning of
> 
> {
> ...    }[cov_type]
> 
> to me?

It is a dictionary lookup. { ... } sets up a dictionary with keys

    'spherical'
    'tied'
    'diag'
    'full'

then { ... }[cov_type] extracts one of the values depending on whether
cov_type is 'spherical', 'tied', 'diag', or 'full'.




-- 
Steven