67 lines
1.5 KiB
Python
67 lines
1.5 KiB
Python
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"""
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Replacement rules.
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"""
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class Transform:
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"""
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Immutable mapping that can be used as a generic transformation rule.
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Parameters
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==========
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transform : callable
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Computes the value corresponding to any key.
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filter : callable, optional
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If supplied, specifies which objects are in the mapping.
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Examples
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========
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>>> from sympy.core.rules import Transform
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>>> from sympy.abc import x
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This Transform will return, as a value, one more than the key:
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>>> add1 = Transform(lambda x: x + 1)
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>>> add1[1]
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2
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>>> add1[x]
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x + 1
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By default, all values are considered to be in the dictionary. If a filter
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is supplied, only the objects for which it returns True are considered as
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being in the dictionary:
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>>> add1_odd = Transform(lambda x: x + 1, lambda x: x%2 == 1)
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>>> 2 in add1_odd
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False
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>>> add1_odd.get(2, 0)
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0
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>>> 3 in add1_odd
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True
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>>> add1_odd[3]
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4
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>>> add1_odd.get(3, 0)
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4
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"""
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def __init__(self, transform, filter=lambda x: True):
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self._transform = transform
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self._filter = filter
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def __contains__(self, item):
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return self._filter(item)
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def __getitem__(self, key):
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if self._filter(key):
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return self._transform(key)
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else:
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raise KeyError(key)
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def get(self, item, default=None):
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if item in self:
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return self[item]
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else:
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return default
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