replace exact flag only applies when symbols > 1

[smichr]

References to other Issues or PRs

close #16725

Brief description of what is fixed or changed

The exact flag prevents an expression with a single Wild from matching a zero but the docstring says that it should only do that if there is more than one Wild symbol in the expression.

The doc string had been updated to indicate that setting the flag explicitly will override the default behavior.

Other comments

Very interesting that the manual_subs test failed in 3.7 but not in 3.6 when exact was set to false. The docstring shows the rather convoluted expression that will be obtained from non-atomic wild expressions.

Release Notes

• core
  • replace docstring updated to clarify use of exact flag

[sympy-bot]

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• core
  • replace docstring updated to clarify use of exact flag (#16785 by @smichr)

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#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
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close #16725 

#### Brief description of what is fixed or changed

The exact flag prevents an expression with a single Wild from matching a zero but the docstring says that it should only do that *if there is more than one Wild symbol in the expression.*

The doc string had been updated to indicate that setting the flag explicitly will override the default behavior.

#### Other comments

Very interesting that the `manual_subs` test failed in 3.7 but not in 3.6 when exact was set to false. The docstring shows the rather convoluted expression that will be obtained from non-atomic wild expressions.

#### Release Notes

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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 -->
- core
    - replace docstring updated to clarify use of exact flag
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #16785 into master will decrease coverage by 0.003%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #16785       +/-   ##
=============================================
- Coverage   73.821%   73.818%   -0.004%     
=============================================
  Files          619       619               
  Lines       159559    159560        +1     
  Branches     37455     37457        +2     
=============================================
- Hits        117789    117784        -5     
- Misses       36304     36308        +4     
- Partials      5466      5468        +2

[oscarbenjamin]

Looks good