cf.Query

---

class cf.Query(operator, value, units=None, attr=None, exact=True)

Bases: object

Encapsulate a condition.

A condition that may be applied to any object may be stored in a Query object. A Query object encapulates a condition, such as “strictly less than 3”. When applied to an object, via its evaluate method or the Python == operator, the condition is evaulated in the context of that object.

>>> c = cf.Query('lt', 3)
>>> c
<CF Query: (lt 3)>
>>> c.evaluate(2)
True
>>> c == 2
True
>>> c != 2
False
>>> c.evaluate(3)
False
>>> c == cf.Data([1, 2, 3])
<CF Data(3): [True, True, False]>
>>> c == numpy.array([1, 2, 3])
array([True, True, False])

The following operators are supported when constructing Query instances:

Operator	Description
'lt'	A “strictly less than” condition
'le'	A “less than or equal” condition
'gt'	A “strictly greater than” condition
'ge'	A “greater than or equal” condition
'eq'	An “equal” condition
'ne'	A “not equal” condition
'wi'	A “within a range” condition
'wo'	A “without a range” condition
'set'	A “member of set” condition

Complex conditions

Multiple conditions may be combined with the Python bitwise “and” (&) and “or” (|) operators to form a new Query object.

>>> ge3 = cf.Query('ge', 3)
>>> lt5 = cf.Query('lt', 5)
>>> c = ge3 & lt5
>>> c
>>> <CF Query: [(ge 3) & (lt 5)]>
>>> c == 2
False
>>> c != 2
True
>>> c = ge3 | lt5
>>> c
<CF Query: [(ge 3) | (lt 5)]>
>>> c == 2
True
>>> c &= cf.Query('set', [1, 3, 5])
>>> c
<CF Query: [[(ge 3) | (lt 5)] & (set [1, 3, 5])]>
>>> c == 2
False
>>> c == 3
True

A condition can be applied to an attribute of an object.

>>> upper_bounds_ge_minus4 = cf.Query('ge', -4, attr='upper_bounds')
>>> x
<CF DimensionCoordinate: grid_longitude(9) degrees>
>>> print(x.bounds.array)
[[-4.92 -4.48]
 [-4.48 -4.04]
 [-4.04 -3.6 ]
 [-3.6  -3.16]
 [-3.16 -2.72]
 [-2.72 -2.28]
 [-2.28 -1.84]
 [-1.84 -1.4 ]
 [-1.4  -0.96]]
>>> print((upper_bounds_ge_minus4 == x).array)
[False False  True  True  True  True  True  True  True]
>>> upper_bounds_ge_minus4 = cf.Query('ge', -4, attr='upper_bounds')

A condition can also be applied to attributes of attributes of an object.

>>> t
<CF DimensionCoordinate: time(4) >
>>> t.lower_bounds.month.array
array([12,  3,  6,  9])
>>> c = cf.Query('ge', 8, attr='lower_bounds.month')
>>> c == t
<CF Data(4): [True, ..., True]>
>>> (c == t).array
array([ True,  False, False, True])

The query interface

In general, the query operator must be permitted between the value of the condition and the operand for which it is being evaulated. For example, when the value is an int, the query works if the operand is also an int, but fails if it is a list:

>>> c = cf.Query('lt', 2)
>>> c == 1
True
>>> c == [1, 2, 3]
TypeError: '<' not supported between instances of 'list' and 'int'

This behaviour is overridden if the operand has an appropriate “query interface” method. When such a method exists, it is used instead of the equivalent built-in Python operator.

Query interface method	Description
__query_lt__	Called when an 'lt' condition is evaulated
__query_le__	Called when an 'le' condition is evaulated
__query_gt__	Called when a 'gt' condition is evaulated
__query_ge__	Called when a 'ge' condition is evaulated
__query_eq__	Called when an 'eq' condition is evaulated
__query_ne__	Called when a 'ne' condition is evaulated
__query_wi__	Called when a 'wi' condition is evaulated
__query_wo__	Called when a 'wo' condition is evaulated
__query_set__	Called when a 'set' condition is evaulated

In all cases the query value is the only, mandatory argument of the method.

>>> class myList(list):
...     pass
...
>>> class myList_with_override(list):
...     def __query_lt__(self, value):
...         """Apply the < operator element-wise"""
...         return type(self)([x < value for x in self])
...
>>> c == myList([1, 2, 3])
TypeError: '<' not supported between instances of 'myList' and 'int'
>>> c == myList_with_override([1, 2, 3])
[True, False, False]

When the condition is on an attribute, or nested attributes, of the operand, the query interface method is looked for on the attribute object, rather than the parent object.

If the value has units then the argument passed to query interface method is automatically a Data object that attaches the units to the value.

Initialization

Parameters

operator: str

The query operator.

value:

The value of the condition.

units: str or Units, optional

The units of value. By default, the same units as the operand being tested are assumed, if applicable. If units is specified and value already has units (such as those attached to a Data object), then the pair of units must be equivalent.

attr: str, optional

Apply the condition to the attribute, or nested attributes, of the operand, rather than the operand itself. Nested attributes are specified by separating them with a .. For example, the “month” attribute of the “bounds” attribute is specified as 'bounds.month'. See also the addattr method.

exact: deprecated at version 3.0.0.

Use re.compile objects in value instead.

Attributes

attr	TODO
operator	TODO
value	TODO
isquery

Methods

addattr	Return a Query object with a new left hand side operand attribute to be used during evaluation.
copy	Return a deep copy.
dump	Return a string containing a full description of the instance.
equals	TODO
equivalent	Deprecated at version 3.0.0.
evaluate	Evaluate the query operation for a given left hand side operand.
inspect	Inspect the object for debugging.