Add code printer for KroneckerDelta

[m93a]

References to other Issues or PRs

Maybe related to #17396

Brief description of what is fixed or changed

Added python printer for KroneckerDelta

Other comments

Release Notes

• printing
  • added Python printer for KroneckerDelta

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• printing
  • added Python printer for KroneckerDelta (#18185 by @m93a)

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#### References to other Issues or PRs
Maybe related to #17396


#### Brief description of what is fixed or changed
Added python printer for KroneckerDelta

#### Other comments


#### Release Notes

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* printing
  * added Python printer for KroneckerDelta
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Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18185 into master will increase coverage by 0.388%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #18185       +/-   ##
=============================================
+ Coverage   74.959%   75.347%   +0.388%     
=============================================
  Files          642       640        -2     
  Lines       167079    167177       +98     
  Branches     39312     39426      +114     
=============================================
+ Hits        125241    125964      +723     
+ Misses       36305     35675      -630     
- Partials      5533      5538        +5

[oscarbenjamin]

This looks good but it will need to be tested. Can you add a test in sympy/printing/tests/test_pycode.py?

[asmeurer]

I actually meant the entire expression should be in parentheses. For example, pycode(KroneckerDelta(x, y) + 1) is wrong. For the inner expression it should use self.parenthesize, although I think there won't actually be any cases where it is needed because == has very high precedence in Python.

[m93a]

Ah, I see. I'm not sure I know how to use self.parenthesize… Should it be something like this?

a, b = expr.args
PREC = precedence(expr)

a = self.parenthesize(a, PREC)
b = self.parenthesize(b, PREC)

return '(1 if {a} == {b} else 0)'.format(...)

[asmeurer]

Yes, except I'm not sure precedence(expr) will work without a change to precedence.

You can check with some operators that have higher precedence than == https://docs.python.org/3/reference/expressions.html#operator-precedence. The only relevant one I see that would be relevant are logic operations, which don't actually make sense inside of a KroneckerDelta, but I guess you can put them there to test.

[m93a]

After reading through precedence and parenthesize, I think this code would've worked:

a, b = expr.args
PREC = PRECEDENCE['Relational']

a = self.parenthesize(a, PREC)
b = self.parenthesize(b, PREC)

return '(1 if {a} == {b} else 0)'.format(...)

But the actual implementation of KroneckerDelta relies on the argument being numbers, not logical expressions, and it threw an error for KroneckerDelta(And(...), ...). So I'm just leaving the parenthesize out and going with a simpler version.

[m93a]

Ping! I think this PR is ready to be merged.

[oscarbenjamin]

I'm not very familiar with the code printers. How would I test this manually?

[m93a]

The Python code printer is used in lambdify, so this code is the most straigtforward use case:

from sympy import *
x, y = symbols('x y')
f = KroneckerDelta(x, y)
delta = lambdify((x, y), f)

delta(1, 2)
# 0

delta(2, 2)
# 1

To see the actual code output, you can use the pycode printer:

from sympy.printing.pycode import pycode
pycode(f)
# '(1 if x==y else 0)'

[oscarbenjamin]

This looks good. Thanks!

[maclomhair]

I'd like to add that this approach broke my setup for generating matrices out of matrix elemtens with KroneckerDeltas in it since it doesn't permit np.array inputs.

I found a workaround by replacing the KroneckerDelta before a lambdify with a generic function and then define the np.eye matrix as that generic function.

Maybe there is a cleaner way to account for such a use case, I'd gladly learn about it...

I used a custom function mapping before, like so:

syp.lambdify([Ei, V0, n, m], i, ["scipy", "numpy", {
    'KroneckerDelta': lambda x, y: np.eye(x.shape[0])
}])

[oscarbenjamin]

@maclomhair I think it would be good to open a new issue to discuss that.

If this has broken your code since the 1.6 release it would be good to show an example of the code that was working before and what has changed.