Fix typos and formatting in map_reduce

[slelievre]

In src/sage/parallel/map_reduce.py,

• fix typos and formatting
• minor rewording
• use polynomial variables instead of symbolic variables

[slelievre]

In preparation for other tasks around map_reduce:

• ​https://trac.sagemath.org/query?order=id&desc=1&summary=~map_reduce&or&summary=~mapreduce

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New commits:

​c6a99cc

Fix typos and formatting in map_reduce.py

[kdilks]

Checked the built documentation and tested, here are my comments:

• There are some lambda functions early on that don't have the semicolon in the right spot.

• In "For this, take a Map function that associates..." - why is "Map" capitalized?

• I think most conventions for sigma notation leave out the "i=" in the superscript. Only comes up in one changed line, but appears a few other places.

• The "x-factorial" isn't standard terminology, it's always "q-factorial". I'd suggest either changing the variable to q, or saying that this is the q-factorial (in the variable x). You can even link to sage.combinat.q_analogues.q_factorial.

• The associated test also fails. Since x is previously defined as the generator of a polynomial ring, Sage will take an expression like (1-x^i)/(1-x) and create it in an associated fraction field. Here, the method you would use is reduce(). Except this is a method that just updates the internal representation of the fraction field element and won't return anything, so you'd need to do

sage: A = (prod((1-x**i)/(1-x) for i in range(1, 6))).reduce(); A

The alternative is to do something to clear x and have it return to being a formal parameter, and then (1-x^i)/(1-x) will be treated as a more generic rational expression where simplify_rational() will work.

• Broken doclinks: profile.profile, sage.parallel.map_reduce.logger, a bunch of things that look similar to :meth:`master._shutdown`

• I don't think "Active Tasks" should be capitalized in the ActiveTaskCounterDarwin class docstring.

• The last part of this line

during normal usage. Most of the time one should leave it to ``True``,

I think should be leave it as ``True``, or a little more awkwardly, leave it set to ``True''.

• Pretty sure that unpaired right parenthesis should be a comma in

During normal time, that is when all workers are active) a worker ``W`` is

• post_process and map_function should have their input as single element list (or something matching the rest of the conventions chosen).

• random_worker docstring has a period at the end.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​6042f07

27789: address reviewer comments

[slelievre]

Thanks for the thorough review. I tried to address all your comments, please check.

[slelievre]

Please also add your full name to the "Reviewers" field.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​203062c

27789: address reviewer comments

[slelievre]

Sorry, I had left out one of the reviewer comments, I did a force push, hope that's okay.