Example of using the Python standard library defaultdict Module

Example of using the Python standard library defaultdict Module

In the Python standard library, collections performs a lot of expansion operations on the data structure of the set type. These operations will bring a lot of convenience when we use the set.

Defaultdict is one of the methods, that is, to add the default type to the value element of the dictionary. I have seen it before but did not pay attention to how to use it. Today I have taken a special look.

The first example is introduced in various major articles:

The Code is as follows:

Import collections as coll

Def default_factory ():

Return 'default value'

D = coll. defaultdict (default_factory, foo = 'bar ')

Print 'd: ', d

Print 'foo => ', d ['foo']

Print 'foo => ', d ['bar'] # The element whose key is 'bar' does not exist and has a default value.

The output result is as follows:

The Code is as follows:

D: defaultdict ( , {'Foo': 'bar '})

Foo => bar

Foo => default value

Conclusion: we can see that when we take an unused key in the dictionary, a value is automatically generated based on default_factory, similar to d. get ('bar', 'default value ')

A comparison example:

If the value of a map in a dictionary is a set, add two elements to the set consecutively. The original dict

The Code is as follows:

Dict_set1 = {}

# If you do not know whether the key in this field exists, you must first determine

If 'key' not in dict_set1:

Dict_set1 ['key'] = set ()

Dict_set1 ['key']. add ('20140901 ')

Dict_set1 ['key']. add ('000 ')

Print dict_set1

If defaultdict is used

The Code is as follows:

Dict_set = coll. defaultdict (set)

Dict_set ['key']. add ('000 ')

Dict_set ['key']. add ('20140901 ')

Print dict_set

The advantage is that you do not need to perform the set initialization judgment.

Two small cases

The Code is as follows:

Ss = '000000'

Dict_int = coll. defaultdict (int)

For s in ss:

Dict_int [s] + = 1

Print dict_int

'''''

The example in the official document shows that this method is simple.

Https://docs.python.org/2/library/collections.html#collections.defaultdict

>>> 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])]

'''

This object is useful when we perform this statistical data operation.