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CHAPTER 4 DICTIONARIES: WHEN INDICES WON T DO
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An example run of this program follows. Notice how the added flexibility of get allows the program to give a useful response, even though the user enters values we weren t prepared for: Name: Gumby Phone number (p) or address (a) batting average Gumby's batting average is not available.
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has_key
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The has_key method checks whether a dictionary has a given key. The expression d.has_key(k) is equivalent to k in d. The choice of which to use is largely a matter of taste, although has_key is on its way out of the language (it will be gone in Python 3.0). Here is an example of how you might use has_key: >>> d = {} >>> d.has_key('name') False >>> d['name'] = 'Eric' >>> d.has_key('name') True
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items and iteritems
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The items method returns all the items of the dictionary as a list of items in which each item is of the form (key, value). The items are not returned in any particular order: >>> d = {'title': 'Python Web Site', 'url': 'http://www.python.org', 'spam': 0} >>> d.items() [('url', 'http://www.python.org'), ('spam', 0), ('title', 'Python Web Site')] The iteritems method works in much the same way, but returns an iterator instead of a list: >>> it = d.iteritems() >>> it <dictionary-iterator object at 169050> >>> list(it) # Convert the iterator to a list [('url', 'http://www.python.org'), ('spam', 0), ('title', 'Python Web Site')] Using iteritems may be more efficient in many cases (especially if you want to iterate over the result). For more information on iterators, see 9.
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keys and iterkeys
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The keys method returns a list of the keys in the dictionary, while iterkeys returns an iterator over the keys.
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CHAPTER 4 DICTIONARIES: WHEN INDICES WON T DO
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The pop method can be used to get the value corresponding to a given key, and then remove the key-value pair from the dictionary: >>> d = {'x': 1, 'y': 2} >>> d.pop('x') 1 >>> d {'y': 2}
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popitem
The popitem method is similar to list.pop, which pops off the last element of a list. Unlike list.pop, however, popitem pops off an arbitrary item because dictionaries don t have a last element or any order whatsoever. This may be very useful if you want to remove and process the items one by one in an efficient way (without retrieving a list of the keys first): >>> d {'url': 'http://www.python.org', 'spam': 0, 'title': 'Python Web Site'} >>> d.popitem() ('url', 'http://www.python.org') >>> d {'spam': 0, 'title': 'Python Web Site'} Although popitem is similar to the list method pop, there is no dictionary equivalent of append (which adds an element to the end of a list). Because dictionaries have no order, such a method wouldn t make any sense.
setdefault
The setdefault method is somewhat similar to get, in that it retrieves a value associated with a given key. In addition to the get functionality, setdefault sets the value corresponding to the given key if it is not already in the 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'}
CHAPTER 4 DICTIONARIES: WHEN INDICES WON T DO
As you can see, when the key is missing, setdefault returns the default and updates the dictionary accordingly. If the key is present, its value is returned and the dictionary is left unchanged. The default is optional, as with get; if it is left out, None is used: >>> d = {} >>> print d.setdefault('name') None >>> d {'name': None}
update
The update method updates one dictionary with the items of another: >>> d = { 'title': 'Python Web Site', 'url': 'http://www.python.org', 'changed': 'Mar 14 22:09:15 MET 2008' } >>> x = {'title': 'Python Language Website'} >>> d.update(x) >>> d {'url': 'http://www.python.org', 'changed': 'Mar 14 22:09:15 MET 2008', 'title': 'Python Language Website'} The items in the supplied dictionary are added to the old one, supplanting any items there with the same keys. The update method can be called in the same way as the dict function (or type constructor), as discussed earlier in this chapter. This means that update can be called with a mapping, a sequence (or other iterable object) of (key, value) pairs, or keyword arguments.
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