13.09.13 21:40, Antoine Pitrou написав(ла):
Both are instances of a more general pattern, where a given
transformation function is applied to keys when looking them up: that
function being ``str.lower`` in the former example and the built-in
``id`` function in the latter.
Please use str.casefold in examples.
>>> d = TransformDict(str.lower, [('Foo': 1)], Bar=2)
{'Foo': 1} or [('Foo', 1)].
Providing a specialized container, not generic
----------------------------------------------
It was asked why we would provide the generic TransformDict construct
rather than a specialized case-insensitive dict variant. The answer
is that it's nearly as cheap (code-wise and performance-wise) to provide
the generic construct, and it can fill more use cases.
Except lightweight IdentityDict which can be implemented more efficient
than TransformDict(id). It doesn't need in calling the transform
function, computing the hash of transformed key, comparing keys. But
perhaps in many cases TransformDict(id) is enough.
Python's own pickle module uses identity lookups for object
memoization:
http://hg.python.org/cpython/file/0e70bf1f32a3/Lib/pickle.py#l234
Also copy, json, cProfile, doctest and _threading_local.
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