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Python lib浏览笔记 - 类型

27 Jun 2015

Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below:

if concatenating str objects, you can build a list and use str.join() at the end or else write to a io.StringIO instance and retrieve its value when complete. if concatenating bytes objects, you can similarly use bytes.join() or io.BytesIO, or you can do in-place concatenation with a bytearray object. bytearray objects are mutable and have an efficient overallocation mechanism. if concatenating tuple objects, extend a list instead for other types, investigate the relevant class documentation.
The only operation that immutable sequence types generally implement that is not also implemented by mutable sequence types is support for the hash() built-in.
list.sort() modifies the sequence in place for economy of space when sorting a large sequence.use sorted() to explicitly request a new sorted list instance.
it is actually the comma which makes a tuple, not the parentheses. The parentheses are optional, except in the empty tuple case, or when they are needed to avoid syntactic ambiguity. e.g.

>>> a = 1,2,3 >>> a (1,2,3)

Ranges implement all of the common sequence operations except concatenation and repetition. range 类实现了collections.abc.Sequence 定义的方法里除连接(concat)和重复(repetition)之外所有操作。
print(): '%r' use repr() , '%s' use str() , '%a' use ascii()
set is muable, frozenset is immutable.