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Comment: sorting by converting to (key,dict), then sorting, then converting back to dict
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| Deletions are marked like this. | Additions are marked like this. |
| Line 11: | Line 11: |
| a = { "key1":5, "key2":8, "key3":2 } b = { "key1":7, "key2":4, "key3":9 } c = { "key1":6, "key2":1, "key3":1 } l = [ a,b,c ] # how do you sort this list? |
a = {"key1": 5 , "key2": 8, "key3": 2} b = {"key1": 7 , "key2": 4, "key3": 9} c = {"key1": 6 , "key2": 1, "key3": 1} undecorated = [a, b, c] # how do you sort this list? |
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| There are many ways to do this; Here's one way I've been doing it: | There are many ways to do this. Here's the fastest way to do it, as it avoids using a custom comparison function, instead using builtin comparisons. This is the ''decorate-sort-undecorate'' pattern, or the ''Schwartzian transform'' if you're coming from Perl. |
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| sort_key = "key2" l = [(d[sort_key],d) for d in l] l.sort() l = [d for (k,d) in l] |
sort_on = "key2" decorated = [(dict_[sort_on], dict_) for dict_ in undecorated] decorated.sort() result = [dict_ for (key, dict_) in decorated] |
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(The variable was named {{{dict_}}} because {{{dict}}} is already a builtin.) Starting with Py2.4 the {{{list.sort()}}} method provides a {{{key=}}} argument for doing the transform in a single step. The new {{{sorted()}}} built-in function goes a step further and encapsulates making a new sorted list while leaving the original intact. Also, the new {{{operator.itemgetter()}}} function helps by constructing functions for key access: {{{ >>> from operator import itemgetter >>> result = sorted(undecorated, key=itemgetter('key2')) }}} == See Also == * [[HowTo/Sorting]] |
Sorting Lists of Dictionaries
Frequently you want to sort a list of dictionaries, based on some particular key.
For example:
1 a = {"key1": 5 , "key2": 8, "key3": 2}
2 b = {"key1": 7 , "key2": 4, "key3": 9}
3 c = {"key1": 6 , "key2": 1, "key3": 1}
4 undecorated = [a, b, c] # how do you sort this list?
There are many ways to do this. Here's the fastest way to do it, as it avoids using a custom comparison function, instead using builtin comparisons. This is the decorate-sort-undecorate pattern, or the Schwartzian transform if you're coming from Perl.
1 sort_on = "key2"
2 decorated = [(dict_[sort_on], dict_) for dict_ in undecorated]
3 decorated.sort()
4 result = [dict_ for (key, dict_) in decorated]
(The variable was named dict_ because dict is already a builtin.)
Starting with Py2.4 the list.sort() method provides a key= argument for doing the transform in a single step. The new sorted() built-in function goes a step further and encapsulates making a new sorted list while leaving the original intact. Also, the new operator.itemgetter() function helps by constructing functions for key access:
>>> from operator import itemgetter
>>> result = sorted(undecorated, key=itemgetter('key2'))