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We begin by creating an empty dictionary Then we iterate over each le listed on the command line and each line within each le We must account for the fact that each line may refer to any number of Web sites, which is why we keep calling strfind() until it fails If we nd the string http:// , we increment i (our starting index position) by the length of http:// , and then we look at each succeeding character until we reach one that isn t valid for a Web site s name If we nd a site (and as a simply sanity check, only if it contains a period), we add it to the dictionary We cannot use the syntax sites[site]add(filename) because this will raise a KeyError exception the rst time a new site is encountered after all, we can t add to a set that is the value of an item that does not yet exist in the dictionary So we must use a different approach The dictsetdefault() method returns an object reference to the item in the dictionary that has the given key (the rst argument) If there is no such item, the method creates a new item with the key and sets its value either to None, or to the given default value (the second argument) In this case we pass a default value of set(), that is, an empty set So the call to dictsetdefault() always returns an object reference to a value, either one that existed before or a new one (Of course, if the given key is not hashable a TypeError exception will be raised) In this example, the returned object reference always refers to a set (an empty set the rst time any particular key, that is, site, is encountered), and we then add the lename that refers to the site to the site s set of lenames By using a set we ensure that even if a le refers to a site repeatedly, we record the lename only once for the site To make the dictsetdefault() method s functionality clear, here are two equivalent code snippets:
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if site not in sites: sites[site] = set() sites[site]add(fname)
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sitessetdefault(site, set())add(fname)
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For the sake of completeness, here is the rest of the program:
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for site in sorted(sites): print("{0} is referred to in:"format(site)) for filename in sorted(sites[site], key=strlower): print(" {0}"format(filename))
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Each Web site is printed with the les that refer to it printed indented underneath The sorted() call in the outer for in loop sorts all the dictionary s keys whenever a dictionary is used in a context that requires an iterable it is the keys that are used If we want the iterable to be the (key, value) items or the values, we can use dictitems() or dictvalues() The inner for in loop iterates over the sorted lenames from the current site s set of lenames
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sorted()
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Although a dictionary of web sites is likely to contain a lot of items, many other dictionaries have only a few items For small dictionaries, we can print their contents using their keys as eld names and using mapping unpacking to convert the dictionary s key value items into key value arguments for the strformat() method
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>>> greens = dict(green="#0080000", olive="#808000", lime="#00FF00") >>> print("{green} {olive} {lime}"format(**greens)) #0080000 #808000 #00FF00
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Here, using mapping unpacking (**) has exactly the same effect as writing format(green=greensgreen, olive=greensolive, lime=greenslime), but is easier to write and arguably clearer Note that it doesn t matter if the dictionary has more keys than we need, since only those keys whose names appear in the format string are used
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