tmthydvnprt - 1 year ago 93
Python Question

# Frozen Distribution

In
`scipy.stats`
you can create a frozen distribution that allows the parameterization (shape, location & scale) of the distribution to be permanently set for that instance.

For example, you can create an gamma distribution (
`scipy.stats.gamma`
) with
`a`
,
`loc`
and
`scale`
parameters and freeze them so they do not have to be passed around every time that distribution is needed.

``````import scipy.stats as stats

# Parameters for this particular gamma distribution
a, loc, scale = 3.14, 5.0, 2.0

# Do something with the general distribution parameterized
print 'gamma stats:', stats.gamma(a, loc=loc, scale=scale).stats()

# Create frozen distribution
rv = stats.gamma(a, loc=loc, scale=scale)

# Do something with the specific, already parameterized, distribution
print 'rv stats   :', rv.stats()
``````

``````gamma stats: (array(11.280000000000001), array(12.56))
rv stats   : (array(11.280000000000001), array(12.56))
``````

## Accessible `rv` parameters?

Since the parameters will most likely not be passed around as a result of this feature, is there a way to get those values back from only the frozen distribution,
`rv`
, later on?

# Accessing `rv` frozen parameters

Yes, the parameters used to create a frozen distribution are available within the instance of the distribution. They are stored within the `args` & `kwds` attribute. This will be dependent on if the distribution's instance was created with positional arguments or keyword arguments.

``````import scipy.stats as stats

# Parameters for this particular alpha distribution
a, loc, scale = 3.14, 5.0, 2.0

# Create frozen distribution
rv1 = stats.gamma(a, loc, scale)
rv2 = stats.gamma(a, loc=loc, scale=scale)

# Do something with frozen parameters
print 'positional and keyword'
print 'frozen args : {}'.format(rv1.args)
print 'frozen kwds : {}'.format(rv1.kwds)
print
print 'positional only'
print 'frozen args : {}'.format(rv2.args)
print 'frozen kwds : {}'.format(rv2.kwds)
``````

``````positional and keyword
frozen args : (3.14, 5.0, 2.0)
frozen kwds : {}

positional only
frozen args : (3.14,)
frozen kwds : {'loc': 5.0, 'scale': 2.0}
``````

## Bonus: Private method that handles both `args` and `kwds`

There is an private method, `.dist._parse_args()`, which handles both cases of parameter passing and will return a consistent result.

``````# Get the original parameters regardless of argument type
shape1, loc1, scale1 = rv1.dist._parse_args(*rv1.args, **rv1.kwds)
shape2, loc2, scale2 = rv2.dist._parse_args(*rv2.args, **rv2.kwds)

print 'positional and keyword'
print 'frozen parameters: shape={}, loc={}, scale={}'.format(shape1, loc1, scale1)
print
print 'positional only'
print 'frozen parameters: shape={}, loc={}, scale={}'.format(shape2, loc2, scale2)
``````

``````positional and keyword
frozen parameters: shape=(3.14,), loc=5.0, scale=2.0

positional only
frozen parameters: shape=(3.14,), loc=5.0, scale=2.0
``````

### Caveat

Granted, using private methods is typically bad practice because technically internal APIs can always change, however, sometimes they provide nice features, would be easy to re-implement should things change and nothing is really private in Python :).

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