Abstract

This PEP suggests a “sort by value” operation for dictionaries. The primary benefit would be in terms of “batteries included” support for a common Python idiom which, in its current form, is both difficult for beginners to understand and cumbersome for all to implement.

BDFL Pronouncement

This PEP is rejected because the need for it has been largely fulfilled by Py2.4’s sorted() builtin function:

>>> sorted(d.iteritems(), key=itemgetter(1), reverse=True)
[('b', 23), ('d', 17), ('c', 5), ('a', 2), ('e', 1)]

or for just the keys:

sorted(d, key=d.__getitem__, reverse=True)
['b', 'd', 'c', 'a', 'e']

Also, Python 2.5’s heapq.nlargest() function addresses the common use case of finding only a few of the highest valued items:

>>> nlargest(2, d.iteritems(), itemgetter(1))
[('b', 23), ('d', 17)]

Motivation

A common use of dictionaries is to count occurrences by setting the value of d[key] to 1 on its first occurrence, then increment the value on each subsequent occurrence. This can be done several different ways, but the get() method is the most succinct:

d[key] = d.get(key, 0) + 1

Once all occurrences have been counted, a common use of the resulting dictionary is to print the occurrences in occurrence-sorted order, often with the largest value first.

This leads to a need to sort a dictionary’s items by value. The canonical method of doing so in Python is to first use d.items() to get a list of the dictionary’s items, then invert the ordering of each item’s tuple from (key, value) into (value, key), then sort the list; since Python sorts the list based on the first item of the tuple, the list of (inverted) items is therefore sorted by value. If desired, the list can then be reversed, and the tuples can be re-inverted back to (key, value). (However, in my experience, the inverted tuple ordering is fine for most purposes, e.g. printing out the list.)

For example, given an occurrence count of:

>>> d = {'a':2, 'b':23, 'c':5, 'd':17, 'e':1}

we might do:

>>> items = [(v, k) for k, v in d.items()]
>>> items.sort()
>>> items.reverse()             # so largest is first
>>> items = [(k, v) for v, k in items]

resulting in:

>>> items
[('b', 23), ('d', 17), ('c', 5), ('a', 2), ('e', 1)]

which shows the list in by-value order, largest first. (In this case, 'b' was found to have the most occurrences.)

This works fine, but is “hard to use” in two aspects. First, although this idiom is known to veteran Pythoneers, it is not at all obvious to newbies – either in terms of its algorithm (inverting the ordering of item tuples) or its implementation (using list comprehensions – which are an advanced Python feature.) Second, it requires having to repeatedly type a lot of “grunge”, resulting in both tedium and mistakes.

We therefore would rather Python provide a method of sorting dictionaries by value which would be both easy for newbies to understand (or, better yet, not to have to understand) and easier for all to use.

Rationale

As Tim Peters has pointed out, this sort of thing brings on the problem of trying to be all things to all people. Therefore, we will limit its scope to try to hit “the sweet spot”. Unusual cases (e.g. sorting via a custom comparison function) can, of course, be handled “manually” using present methods.

Here are some simple possibilities:

The items() method of dictionaries can be augmented with new parameters having default values that provide for full backwards-compatibility:

(1) items(sort_by_values=0, reversed=0)

or maybe just:

(2) items(sort_by_values=0)

since reversing a list is easy enough.

Alternatively, items() could simply let us control the (key, value) order:

(3) items(values_first=0)

Again, this is fully backwards-compatible. It does less work than the others, but it at least eases the most complicated/tricky part of the sort-by-value problem: inverting the order of item tuples. Using this is very simple:

items = d.items(1)
items.sort()
items.reverse()         # (if desired)

The primary drawback of the preceding three approaches is the additional overhead for the parameter-less items() case, due to having to process default parameters. (However, if one assumes that items() gets used primarily for creating sort-by-value lists, this is not really a drawback in practice.)

Alternatively, we might add a new dictionary method which somehow embodies “sorting”. This approach offers two advantages. First, it avoids adding overhead to the items() method. Second, it is perhaps more accessible to newbies: when they go looking for a method for sorting dictionaries, they hopefully run into this one, and they will not have to understand the finer points of tuple inversion and list sorting to achieve sort-by-value.

To allow the four basic possibilities of sorting by key/value and in forward/reverse order, we could add this method:

(4) sorted_items(by_value=0, reversed=0)

I believe the most common case would actually be by_value=1, reversed=1, but the defaults values given here might lead to fewer surprises by users: sorted_items() would be the same as items() followed by sort().

Finally (as a last resort), we could use:

(5) items_sorted_by_value(reversed=0)

Implementation

The proposed dictionary methods would necessarily be implemented in C. Presumably, the implementation would be fairly simple since it involves just adding a few calls to Python’s existing machinery.

Concerns

Aside from the run-time overhead already addressed in possibilities 1 through 3, concerns with this proposal probably will fall into the categories of “feature bloat” and/or “code bloat”. However, I believe that several of the suggestions made here will result in quite minimal bloat, resulting in a good tradeoff between bloat and “value added”.

Tim Peters has noted that implementing this in C might not be significantly faster than implementing it in Python today. However, the major benefits intended here are “accessibility” and “ease of use”, not “speed”. Therefore, as long as it is not noticeably slower (in the case of plain items(), speed need not be a consideration.

References

A related thread called “counting occurrences” appeared on comp.lang.python in August, 2001. This included examples of approaches to systematizing the sort-by-value problem by implementing it as reusable Python functions and classes.

Copyright

This document has been placed in the public domain.