Python Doc Problem Example: sort() (reprise)

[Xah Lee]

Python Doc Problem Example: sort()

Xah Lee, 200503
Exhibit: Incompletion & Imprecision

Python doc “3.6.4 Mutable Sequence Types” at
http://python.org/doc/2.4/lib/typesseq-mutable.html

in which contains the documentation of the “sort” method of a list.
Quote:

«
Operation 	Result 	Notes
s.sort([cmp[, key[, reverse]]]) 	sort the items of s in place 	(7),
(8), (9), (10)

    (7) The sort() and reverse() methods modify the list in place for
economy of space when sorting or reversing a large list. To remind you
that they operate by side effect, they don't return the sorted or
reversed list.

    (8) The sort() method takes optional arguments for controlling the
comparisons.

    cmp specifies a custom comparison function of two arguments (list
items) which should return a negative, zero or positive number
depending on whether the first argument is considered smaller than,
equal to, or larger than the second argument: "cmp=lambda x,y:
cmp(x.lower(), y.lower())"

    key specifies a function of one argument that is used to extract a
comparison key from each list element: "cmp=str.lower"

    reverse is a boolean value. If set to True, then the list elements
are sorted as if each comparison were reversed.

    In general, the key and reverse conversion processes are much
faster than specifying an equivalent cmp function. This is because cmp
is called multiple times for each list element while key and reverse
touch each element only once.

    Changed in version 2.3: Support for None as an equivalent to
omitting cmp was added.

    Changed in version 2.4: Support for key and reverse was added.

    (9) Starting with Python 2.3, the sort() method is guaranteed to be
stable. A sort is stable if it guarantees not to change the relative
order of elements that compare equal -- this is helpful for sorting in
multiple passes (for example, sort by department, then by salary
grade).

    (10) While a list is being sorted, the effect of attempting to
mutate, or even inspect, the list is undefined. The C implementation of
Python 2.3 and newer makes the list appear empty for the duration, and
raises ValueError if it can detect that the list has been mutated
during a sort.
»


As a piece of documentation, this is a lousy one.

The question Python doc writers need to ask when evaluating this piece
of doc are these:

• can a experienced programer who is expert at several languages but
new to Python, and also have read the official Python tutorial, can he,
read this doc, and know exactly how to use sort with all the options?

• can this piece of documentation be rewritten fairly easily, so that
the answer to the previous question is a resounding yes?

To me, the answers to the above questions are No and Yes. Here are some
issues with the doc:

• In the paragraph about the “key” parameter, the illustration
given is: “cmp=str.lower”. It should be be “key=str.lower”

• This doc lacks examples. One or two examples will help a lot,
especially to less experienced programers. (which comprises the
majority of readers) In particular, it should give a full example of
using the comparison function and one with the “key” parameter.
Examples are particularly needed here because these parameteres are
functions, often with the “lambda” construct. These are unusual and
advanced constructs among imperative programers.

• This doc fails to mention what happens when the predicate and the
shortcut version conflicts. e.g. “myList.sort(cmp=lambda x,y:
cmp(x[0], y[0]), key=lambda x: str(x[1]) )”

• The syntax notation Python doc have adopted for indicating optional
parameters, does not give a clear view just exactly what combination of
optional parameters can be omitted. The notation: “s.sort([cmp[,
key[, reverse]]])” gives the impression that only trailing arguments
can be omitted, which is not true.

• The doc gives no indication of how to omit a optional arg. Should
it be “nul”, “Null”, 0, or left empty? Since it doesn't give
any examples, doc reader who isn't Python experts is left to guess at
how true/false values are presented in Python.

• On the whole, the way this doc is written does not give a clear
picture of the roles of the supplied options, nor how to use them.

Suggested Quick Remedy: add a example of using the cmp function. And a
example using the “key” function. Add a example of Using one of
them and with reverse. (the examples need not to come with much
explanations. One sentence annotation is better than none.)

Other than that, the way the doc is layed out with a terse table and
run-on footnotes (employed in several places in Python doc) is not
inductive. For a better improvement, there needs to be a overhaul of
the organization and the attitude of the entire doc. The organization
needs to be programing based, as opposed to implementation or computer
science based. (in this regard, one can learn from the Perl folks). As
to attitude, the writing needs to be Python-as-is, as opposed to
computer science framework, as indicated in the early parts of this
critique series.
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 Xah
 xah at xahlee.org
∑ http://xahlee.org/