How to implement a function with Python(Sympy), realizing the same as '_' and replacement rule in Wolfram Mathematica?

[Xuemei Gu]

hey, everyone.

hello, I recently played with sympy for comparing with wolfram language. I wonder whether there is a simple way to do the _ (blank) as mathematica do.

Here is a link: 

https://stackoverflow.com/questions/53687281/how-to-implement-a-function-with-pythonsympy-realizing-the-same-as-and-re

Can anyone know? Or give any hints? Thanks a lot!

[Ishan Anirudh Joshi]

I think using Wild symbols along with replace could be used for the purpose.

Wild symbols are used for pattern matching.

>>> l1 = Wild('l1')
>>> l2 = Wild('l2')

>>> testexpr = p1[MM]*p2[NN] + p1[XX]*p2[MM] + p1[XX]**2

>>> t = testexpr.replace(p1[l1]*p2[l2], FF1[l1]*FF2[l2])
>>> t =  t.replace(p1[l1]**2, 0)
>>> t

FF1[MM]*FF2[NN] + FF1[XX]*FF2[MM]

Hope that helps!

[Xuemei Gu]

hey,

Thank you very much for the kind reply. I have already solve it as the way you suggested. 

https://stackoverflow.com/questions/53687281/how-to-implement-a-function-with-pythonsympy-realizing-the-same-as-and-re/53770353#53770353

I have another question, let's say:

expression:   a[1]*b[2]*c[3]*d[4]+a[1]**2*b[3]*c[1]*d[4]+a[1]*b[2]**2*c[3]+b[2]*c[3]+...

I only want to take the cases for terms containing b[xxx]*c[xxx]*d[xxx], other cases are all becoming to 0.

Do you have any ideas? I thought about match, but seems wrong.

Thank you very much!

Best regards,

Xuemei Gu

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[Ishan Anirudh Joshi]

For this particular expression I guess, something like this can be done:

>>> w = Wild('w')
>>> x = Wild('x')
>>> y = Wild('y')
>>> z = wild('z')

>>> expr = a[1]*b[2]*c[3]*d[4] + a[1]**2*b[3]*c[1]*d[4] + a[1]*b[2]**2*c[3] + b[2]*c[3]

>>> p = expr.replace(w*b[x]*c[y]*d[z], 0)                                                              # p has the terms which are not needed
>>> sol = expr - p

>>> sol

a[1]**2*b[3]*c[1]*d[4] + a[1]*b[2]*c[3]*d[4]

>>> p