Detailed explanation of several methods of dictionaries in Python

Create dictionary

>>> phonebook={'Alice':'2897','Alan':'0987','Jery':'6754'}

dict function

>>> items=[('name','Gumby'),('age',42)]
>>> d=dict(items)
>>> d
{'age': 42, 'name': 'Gumby'}
>>> d['name']
'Gumby'
>>> d=dict(name='July',age=24)
>>> d
{'age': 24, 'name': 'July'}

Basic dictionary operations (mostly similar to sequences)

len(d) returns the number of items (key-value pairs) in d

d[k] returns the number associated with the key The value on k

d[k]=v associates the value v to k

del d[k] deletes the item with key k

k in d checks if there is an item with key k in d

Format string

>>> phonebook
{'Jery': '6754', 'Alice': '2897', 'Alan': '0987'}
>>> "Alan's phone number is %(Alan)s." % phonebook
"Alan's phone number is 0987."

Methods:

clear Clear all items in the dictionary

>>> d={}
>>> d['name']='Gumby'
>>> d['age']=42
>>> d
{'age': 42, 'name': 'Gumby'}
>>> d.clear()
>>> d
{}

>>> x={}
>>> y=x #x和y对应同一个字典
>>> x['key']='value'
>>> y
{'key': 'value'}
>>> x={} #x关联到新的空字典
>>> y
{'key': 'value'}

>>> x={}
>>> y=x
>>> x['key']='value'
>>> y
{'key': 'value'}
>>> x.clear()
>>> y
{}

copy Return a new dictionary with the same key-value pairs (shallow copy)

>>> x={'name':'admin','machines':['foo','bar','bax']}
>>> y=x.copy()
>>> y['name']='yhk' #替换值，原字典不受影响
>>> y['machines'].remove('bar') #修改了某个值(原地修改不是替换)，原字典会改变
>>> y
{'name': 'yhk', 'machines': ['foo', 'bax']}
>>> x
{'name': 'admin', 'machines': ['foo', 'bax']}

deepcopy Deep copy

>>> from copy import deepcopy
>>> d={}
>>> d['name']=['Aly','Bob']
>>> c=d.copy()
>>> e=deepcopy(d)
>>> d['name'].append('Ageal')
>>> c
{'name': ['Aly', 'Bob', 'Ageal']}
>>> e
{'name': ['Aly', 'Bob']}

fromkeys using the given Create a new dictionary with the key, and the default corresponding value of each key is none

>>> {}.fromkeys(['name','age'])
{'age': None, 'name': None}
>>> dict.fromkeys(['name','age'])
{'age': None, 'name': None}
>>> dict.fromkeys(['name','age'],'(unknown)')
{'age': '(unknown)', 'name': '(unknown)'}

get accesses the dictionary item

>>> d={}
>>> print d['name']

Traceback (most recent call last):
  File "<pyshell#60>", line 1, in <module>
    print d[&#39;name&#39;]
KeyError: &#39;name&#39;
>>> print d.get(&#39;name&#39;)
None
>>> d.get(&#39;name&#39;,&#39;N/A&#39;)
&#39;N/A&#39;
>>> d[&#39;name&#39;]=&#39;Eric&#39;
>>> d.get(&#39;name&#39;)
&#39;Eric&#39;

has_key to check whether there is the given key in the dictionary (python3.0 does not have this function)

>>> d={}
>>> d.has_key('name')
False
>>> d['name']='Eric'
>>> d.has_key('name')
True

items and iteritems

items returns all dictionary items as a list, each item in these list items comes from (key, value)

iteritems returns an iterator object

>>> d={'title':'My Time!','url':'http://www,python.org','spam':0}
>>> d.items()
[('url', 'http://www,python.org'), ('spam', 0), ('title', 'My Time!')]
>>> s=d.iteritems()
>>> s
<dictionary-itemiterator object at 0x0000000003068728>
>>> list(s)
[(&#39;url&#39;, &#39;http://www,python.org&#39;), (&#39;spam&#39;, 0), (&#39;title&#39;, &#39;My Time!&#39;)]

keys and iterkeys keys return the keys in the dictionary Returns as a list iterkeys returns an iterator over the key

pop removes

>>> d={'x':1,'y':2}
>>> d.pop('x')
>>> d
{'y': 2}

popitem removes a random item

>>> d={'x':1,'y':2}
>>> d.popitem()
('y', 2)
>>> d
{'x': 1}

setdefault When the key does not exist, returns the default value and updates the corresponding dictionary

>>> d={}
>>> d.setdefault('name','N/A')
'N/A'
>>> d
{'name': 'N/A'}
>>> d['name']='Gumby'
>>> d.setdefault('name','N/A')
'Gumby'
>>> d
{'name': 'Gumby'}

update utilizes a dictionary item Update another dictionary

>>> d={'x':1,'y':2,'z':3}
>>> f={'y':5}
>>> d.update(f)
>>> d
{'y': 5, 'x': 1, 'z': 3}

values ​​and itervalues ​​alues ​​return the value in the dictionary (itervalues ​​returns the iterator of the value)

>>> d={}
>>> d[1]=1
>>> d[2]=2
>>> d[3]=3
>>> d.values()
[1, 2, 3]

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