Allow subclasses of sympy objects to override _repr_latex_

[eric-wieser]

The previous behavior was to completely ignore this hook once init_printing had been called, which was surprising to the user.

References to other Issues or PRs

This fixes an issue related to #10062, but only for subclasses of sympy types.

Brief description of what is fixed or changed

init_printing() no longer causes _repr_latex_ methods defined on Printable subclasses to be completely ignored. The rules in IPython for looking up magic methods are currently (ipython/ipython#8938):

• Walk the mro for explicitly-registered printers
• Walk the mro for method-registered printers

But would make more sense as:

• Walk the mro for:
  • explicitly-registered printers
  • method-registered printers

Since this probably won't change any time soon, the workaround is to register printers for Printable via methods not explicit hooks - this allows users to override them if they so wish.

Other comments

Release Notes

• interactive
  • The _repr_latex_, _repr_svg_, and _repr_png_ methods of subclasses of sympy objects are no longer discarded when init_printing() is called.

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• interactive
  • The _repr_latex_, _repr_svg_, and _repr_png_ methods of subclasses of sympy objects are no longer discarded when init_printing() is called. (#19944 by @eric-wieser)

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The previous behavior was to completely ignore this hook once `init_printing` had been called, which was surprising to the user.

<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
format, e.g. "Fixes #1234" (see
https://tinyurl.com/auto-closing for more information). Also, please
write a comment on that issue linking back to this pull request once it is
open. -->
This fixes an issue related to #10062, but only for subclasses of sympy types.

#### Brief description of what is fixed or changed
`init_printing()` no longer causes `_repr_latex_` methods defined on `Printable` subclasses to be completely ignored. The rules in IPython for looking up magic methods are currently (https://github.com/ipython/ipython/issues/8938):
* Walk the mro for explicitly-registered printers
* Walk the mro for method-registered printers

But would make more sense as:
* Walk the mro for:
   * explicitly-registered printers
   * method-registered printers

Since this probably won't change any time soon, the workaround is to register printers for `Printable` via methods not explicit hooks - this allows users to override them if they so wish.

#### Other comments


#### Release Notes

<!-- Write the release notes for this release below. See
https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
automatically to see if they are formatted correctly. -->

<!-- BEGIN RELEASE NOTES -->
* interactive
   * The `_repr_latex_`, `_repr_svg_`, and `_repr_png_` methods of subclasses of sympy objects are no longer discarded when `init_printing()` is called.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19944 into master will increase coverage by 0.024%.
The diff coverage is 23.076%.

@@              Coverage Diff              @@
##            master    #19944       +/-   ##
=============================================
+ Coverage   75.778%   75.803%   +0.024%     
=============================================
  Files          668       668               
  Lines       173216    173193       -23     
  Branches     40837     40842        +5     
=============================================
+ Hits        131261    131286       +25     
+ Misses       36221     36173       -48     
  Partials      5734      5734               

[oscarbenjamin]

Looks good to me

[eric-wieser]

Might come back and add an xref to the upstream ipython issue tomorrow.

[eric-wieser]

Changed the implementation to be a bit more verbose but hopefully easier to understand - probably needs re-reviewing

[oscarbenjamin]

Looks good