Meta-Group to Dictionary

Tuples: 1. Use brackets (()) to wrap, can not be changed (although their content can), tuples can be viewed as a read-only list

A. Dict.fromkeys (S)

S is a list or tuple ...

The key,value of the elements in S as the dictionary defaults to none, or you can specify an initial value, as shown in the following code example:

Mydict = Dict.fromkeys (' Hello ', True) for K in Mydict.keys ():    print (k, mydict[k])

The output is as follows:

H True

E True

L True

o True

B. collections.defaultdict ([default_factory[,...]])

Default_factory the value type of the specified dictionary

>>> s = [(' Yellow ', 1), (' Blue ', 2), (' Yellow ', 3), (' Blue ', 4), (' Red ', 1)]>>> d = defaultdict (list) ;>> for K, V in S: ...     D[k].append (v) ...>>> d.items ('   blue ', [2, 4]), (' Red ', [1]), (' Yellow ', [1, 3])]

The above code efficiency is higher than the following equivalent code:

>>> d = {}>>> for K, V in S: ...     D.setdefault (k, []). Append (v) ...>>> D.items ()

[(' Blue ', [2, 4]), (' Red ', [1]), (' Yellow ', [1, 3])]

If an int is passed to default_dict, it can be used to count:

>>> s = ' Mississippi ' >>> d = defaultdict (int) >>> for k in s: ...     D[k] + = 1...>>> D.items ()

[(' I ', 4), (' P ', 2), (' s ', 4), (' m ', 1)]

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