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filters.py
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filters.py
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"""Provide filters for querying close approaches and limit the generated results.
The `create_filters` function produces a collection of objects that is used by
the `query` method to generate a stream of `CloseApproach` objects that match
all of the desired criteria. The arguments to `create_filters` are provided by
the main module and originate from the user's command-line options.
This function can be thought to return a collection of instances of subclasses
of `AttributeFilter` - a 1-argument callable (on a `CloseApproach`) constructed
from a comparator (from the `operator` module), a reference value, and a class
method `get` that subclasses can override to fetch an attribute of interest from
the supplied `CloseApproach`.
The `limit` function simply limits the maximum number of values produced by an
iterator.
You'll edit this file in Tasks 3a and 3c.
"""
import operator
class UnsupportedCriterionError(NotImplementedError):
"""A filter criterion is unsupported."""
class AttributeFilter:
"""A general superclass for filters on comparable attributes.
An `AttributeFilter` represents the search criteria pattern comparing some
attribute of a close approach (or its attached NEO) to a reference value. It
essentially functions as a callable predicate for whether a `CloseApproach`
object satisfies the encoded criterion.
It is constructed with a comparator operator and a reference value, and
calling the filter (with __call__) executes `get(approach) OP value` (in
infix notation).
Concrete subclasses can override the `get` classmethod to provide custom
behavior to fetch a desired attribute from the given `CloseApproach`.
"""
def __init__(self, op, value):
"""Construct a new `AttributeFilter` from an binary predicate and a reference value.
The reference value will be supplied as the second (right-hand side)
argument to the operator function. For example, an `AttributeFilter`
with `op=operator.le` and `value=10` will, when called on an approach,
evaluate `some_attribute <= 10`.
:param op: A 2-argument predicate comparator (such as `operator.le`).
:param value: The reference value to compare against.
"""
self.op = op
self.value = value
def __call__(self, approach):
"""Invoke `self(approach)`."""
return self.op(self.get(approach), self.value)
@classmethod
def get(cls, approach):
"""Get an attribute of interest from a close approach.
Concrete subclasses must override this method to get an attribute of
interest from the supplied `CloseApproach`.
:param approach: A `CloseApproach` on which to evaluate this filter.
:return: The value of an attribute of interest, comparable to `self.value` via `self.op`.
"""
raise UnsupportedCriterionError
def __repr__(self):
"""Return `repr(self)`, a computer-readable string representation of this object."""
return f"{self.__class__.__name__}(op=operator.{self.op.__name__}, value={self.value})"
class DateFilter(AttributeFilter):
"""Sub-class of the `AttributeFilter` class. Especialized to filter the date property of an approach."""
@classmethod
def get(cls, approach):
"""Return the date part of the time property (date time) of the approach."""
return approach.time.date()
class DistanceFilter(AttributeFilter):
"""Sub-class of the `AttributeFilter` class. Especialized to filter the distance property of an approach."""
@classmethod
def get(cls, approach):
"""Return the distance property of the approach."""
return approach.distance
class VelocityFilter(AttributeFilter):
"""Sub-class of the `AttributeFilter` class. Especialized to filter the velocity property of an approach."""
@classmethod
def get(cls, approach):
"""Return the velocity property of the approach."""
return approach.velocity
class DiameterFilter(AttributeFilter):
"""Sub-class of the `AttributeFilter` class. Especialized to filter the diameter property of an approach's neo."""
@classmethod
def get(cls, approach):
"""Return the diameter property of the approach's neo."""
return approach.neo.diameter
class HazardousFilter(AttributeFilter):
"""Sub-class of the `AttributeFilter` class. Especialized to filter the hazardous property of an approach's neo."""
@classmethod
def get(cls, approach):
"""Return the hazardous property of the approach's neo."""
return approach.neo.hazardous
def create_filters(
date=None, start_date=None, end_date=None,
distance_min=None, distance_max=None,
velocity_min=None, velocity_max=None,
diameter_min=None, diameter_max=None,
hazardous=None
):
"""Create a collection of filters from user-specified criteria.
Each of these arguments is provided by the main module with a value from the
user's options at the command line. Each one corresponds to a different type
of filter. For example, the `--date` option corresponds to the `date`
argument, and represents a filter that selects close approaches that occurred
on exactly that given date. Similarly, the `--min-distance` option
corresponds to the `distance_min` argument, and represents a filter that
selects close approaches whose nominal approach distance is at least that
far away from Earth. Each option is `None` if not specified at the command
line (in particular, this means that the `--not-hazardous` flag results in
`hazardous=False`, not to be confused with `hazardous=None`).
The return value must be compatible with the `query` method of `NEODatabase`
because the main module directly passes this result to that method. For now,
this can be thought of as a collection of `AttributeFilter`s.
:param date: A `date` on which a matching `CloseApproach` occurs.
:param start_date: A `date` on or after which a matching `CloseApproach` occurs.
:param end_date: A `date` on or before which a matching `CloseApproach` occurs.
:param distance_min: A minimum nominal approach distance for a matching `CloseApproach`.
:param distance_max: A maximum nominal approach distance for a matching `CloseApproach`.
:param velocity_min: A minimum relative approach velocity for a matching `CloseApproach`.
:param velocity_max: A maximum relative approach velocity for a matching `CloseApproach`.
:param diameter_min: A minimum diameter of the NEO of a matching `CloseApproach`.
:param diameter_max: A maximum diameter of the NEO of a matching `CloseApproach`.
:param hazardous: Whether the NEO of a matching `CloseApproach` is potentially hazardous.
:return: A collection of filters for use with `query`.
"""
filters = []
if date is not None:
filters.append(DateFilter(operator.eq, date))
if start_date is not None:
filters.append(DateFilter(operator.ge, start_date))
if end_date is not None:
filters.append(DateFilter(operator.le, end_date))
if distance_min is not None:
filters.append(DistanceFilter(operator.ge, distance_min))
if distance_max is not None:
filters.append(DistanceFilter(operator.le, distance_max))
if velocity_min is not None:
filters.append(VelocityFilter(operator.ge, velocity_min))
if velocity_max is not None:
filters.append(VelocityFilter(operator.le, velocity_max))
if diameter_min is not None:
filters.append(DiameterFilter(operator.ge, diameter_min))
if diameter_max is not None:
filters.append(DiameterFilter(operator.le, diameter_max))
if hazardous is not None:
filters.append(HazardousFilter(operator.eq, hazardous))
return tuple(filters)
def limit(iterator, n=None):
"""Produce a limited stream of values from an iterator.
If `n` is 0 or None, don't limit the iterator at all.
:param iterator: An iterator of values.
:param n: The maximum number of values to produce.
:yield: The first (at most) `n` values from the iterator.
"""
if n == 0 or n is None:
return iterator
else:
return tuple(iterator)[0:n]