terça-feira, 6 de dezembro de 2016

Republicado de www.tutorialspoint.com
Cada chave é separada do seu valor por um dois-pontos (:), os itens são separados por vírgulas, ea coisa toda é fechada em chaves. Um dicionário vazio sem quaisquer itens é escrito com apenas duas chaves, como este: {}.
As chaves são únicas dentro de um dicionário, enquanto os valores podem não ser. Os valores de um dicionário podem ser de qualquer tipo, mas as chaves devem ser de um tipo de dados imutável, como seqüências de caracteres, números ou tuplas.

Acessando Valores no Dicionário:

Para acessar elementos de dicionário, você pode usar os colchetes familiares junto com a chave para obter seu valor. A seguir é um exemplo simples -
#!/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['Alice']:
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 a key-value pair, 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']

When the above code is executed, it produces the following result −
dict['Age']:  8
dict['School']:  DPS School

Delete Dictionary Elements

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 produces the following result. Note that an exception is raised because after del dict dictionary does not exist any more −
dict['Age']:
Traceback (most recent call last):
  File "test.py", line 8, in <module>
    print "dict['Age']: ", dict['Age'];
TypeError: 'type' object is unsubscriptable
Note: del() method is discussed in subsequent section.

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 −

(a) 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. For 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

(b) 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']

When the above code is executed, it produces the following result −
Traceback (most recent call last):
   File "test.py", line 3, in <module>
      dict = {['Name']: 'Zara', 'Age': 7};
TypeError: list objects are unhashable

Built-in Dictionary Functions & Methods −

Python includes the following dictionary functions −
SNFunction with Description
1 cmp(dict1, dict2)
Compares elements of both dict.
2 len(dict)
Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.
3 str(dict)
Produces a printable string representation of a dictionary
4 type(variable)
Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.
Python includes following dictionary methods −
SNMethods with Description
1 dict.clear()
Removes all elements of dictionary dict
2 dict.copy()
Returns a shallow copy of dictionary dict
3 dict.fromkeys()
Create a new dictionary with keys from seq and values set to value .
4 dict.get(key, default=None)
For key key, returns value or default if key not in dictionary
5 dict.has_key(key)
Returns true if key in dictionary dict , false otherwise
6 dict.items()
Returns a list of dict 's (key, value) tuple pairs
7 dict.keys()
Returns list of dictionary dict's keys
8 dict.setdefault(key, default=None)
Similar to get(), but will set dict[key]=default if key is not already in dict
9 dict.update(dict2)
Adds dictionary dict2 's key-values pairs to dict
10 dict.values()
Returns list of dictionary dict 's values

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