python内建数据结构之dict

作者: lintong | 来源:发表于2015-02-26 21:24 被阅读304次

    Python dictionaries are also known as associative arrays or hash tables. The general syntax of a dictionary is as follows:

    dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}
    

    An empty dictionary without any items is written with just two curly braces, like this: {}.
    Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.

    Accessing Values in Dictionary:

    To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example:

    #!/usr/bin/python
    dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
    print "dict['Name']: ", dict['Name']
    print "dict['Age']: ", dict['Age']
    

    When the above code is executed, it produces the following result:

    dict['Name']: Zara
    dict['Age']: 7
    

    If we attempt to access a data item with a key,which is not part of the dictionary, we get an error as follows:

    #!/usr/bin/python
    dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
    print "dict['Alice']: ", dict['Alice']
    

    When the above code is executed, it produces the following result:

    dict['Zara']:Traceback (most recent call last): File "test.py", line 4, in <module> print "dict['Alice']: ", dict['Alice'];KeyError: 'Alice'
    

    Updating Dictionary:

    You can update a dictionary by adding a new entry or ,modifying an existing entry, or deleting an existing entry as shown below in the simple example:

    #!/usr/bin/python
    dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
    dict['Age'] = 8; # update existing entry
    dict['School'] = "DPS School"; # Add new entry
    print "dict['Age']: ", dict['Age']
    print "dict['School']: ", dict['School'];
    

    You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.
    To explicitly remove an entire dictionary, just use the del statement. Following is a simple example:

    #!/usr/bin/python
    dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
    del dict['Name'] # remove entry with key 'Name'
    dict.clear() # remove all entries in dict
    del dict  # delete entire dictionary
    print "dict['Age']: ", dict['Age']
    print "dict['School']: ", dict['School']
    

    This will produce the following result. Note an exception raised, this is because after del dict dictionary does not exist any more:

    Properties of Dictionary Keys:

    Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.
    There are two important points to remember about dictionary keys:

    • More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. Following is a simple example:
    #!/usr/bin/python
    dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'}
    print "dict['Name']: ", dict['Name']
    

    When the above code is executed, it produces the following result:
    dict['Name']: Manni

    • Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example:
    #!/usr/bin/python
    dict = {['Name']: 'Zara', 'Age': 7}
    print "dict['Name']: ", dict['Name']
    

    Built-in Dictionary Functions & Methods:

    cmp(dict1, dict2)  #Compares elements of both dict.
    len(dict) #Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.
    str(dict) #Produces a printable string representation of a dictionary
    

    Python includes following dictionary methods

    dict.clear() #Removes all elements of dictionary *dict*
    dict.copy() #Returns a shallow copy of dictionary *dict*
    dict.fromkeys() #Create a new dictionary with keys from seq and values *set* to *value*.
    dict.get(key, default=None)] #For *key* key, returns value or default if key not in dictionary
    dict.has_key(key) Returns *true* if key in dictionary *dict*, *false* otherwise
    dict.items() #Returns a list of *dict*'s (key, value) tuple pairs
    dict.keys() #Returns list of dictionary dict's keys
    dict.setdefault(key, default=None) #Similar to get(), but will set dict[key]=default if *key* is not already in dict
    dict.update(dict2) #Adds dictionary *dict2*'s key-values pairs to *dict*
    dict.values() #Returns list of dictionary *dict*'s values
    

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