Series: Removed incosistency in order in case of Zero

[iamabhishek0]

Added tests

References to other Issues or PRs

Fixes #13715, #18606

Brief description of what is fixed or changed

Order(0) now returns an object of Order Type

>>> type(Order(0))
<class 'sympy.series.order.Order'>
>>> Order(0)
O(0)

Other comments

Release Notes

• series
  • Order(0) now returns an object of Order Type

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• series
  • Order(0) now returns an object of Order Type (#18629 by @iamabhishek0)

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Added tests

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#### References to other Issues or PRs
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Fixes #13715, #18606 

#### Brief description of what is fixed or changed
Order(0) now returns an object of Order Type
```
>>> type(Order(0))
<class 'sympy.series.order.Order'>
>>> Order(0)
O(0)
```

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
* series
  * Order(0) now returns an object of Order Type
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Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18629 into master will decrease coverage by 11.238%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##            master    #18629        +/-   ##
==============================================
- Coverage   75.586%   64.348%   -11.239%     
==============================================
  Files          644       644                
  Lines       167464    167515        +51     
  Branches     39466     39485        +19     
==============================================
- Hits        126581    107794     -18787     
- Misses       35369     53552     +18183     
- Partials      5514      6169       +655

[asmeurer]

You need to use the unchanged function here. Otherwise this test will always pass, even the way it was before.

[iamabhishek0]

Done.

[asmeurer]

I'm glad to see this doesn't require many changes. I have just the two comments and it looks fine otherwise.

[asmeurer]

You shouldn't modify a list while looping through it in Python.

Can this loop be merged with the one above?

[iamabhishek0]

No, it can't be merged with the loop above because if newseq is empty it doesn't iterate through the loop.
Btw thanks for pointing out the fact one shouldn't modify a list while looping through it.

[jksuom]

I would have tried to handle this in another way: not adding O(0) to order_factors at all. So the first thing after this line

sympy/sympy/core/add.py

Line 125 in 0084405

if o.is_Order:

could be

if o.expr.is_zero:
    continue

[iamabhishek0]

This is a better way of doing it. Thanks @jksuom

[jksuom]

Thanks, this looks good.