f string used to make symbols

[Zohreh Karimzadeh]

Dear sympy

Thank millions  for sharing and caring!

I need to create and use symbols in dynamic way. Infact symbols are made based on user input and used in code so I must use f'strings to make them.

As shown below:

L=[ 'xCl', 'xNa']
for j in range(len(L)):
     locals()[L[j]] = sympy.symbols(L[j])
ss = xCl * xNaCl
i = 'Na'

j='Cl'

gg =f"x{i}"* f"x{j}"
print('gg', gg)

But I got the following error

TypeError: can't multiply sequence by non-int of type 'Symbol'

Could possibly let me know how can do it??

[Chris Smith]

`var` will inject the symbols into the namespace. But you could also use parse_expr and pass it a dictionary with the symbols to use:

from sympy import parse_expr, symbols, var

v =  ['xa', 'xb']

xa, xb = map(var, v)

ok = xa*xb

better = parse_expr('xa*xb', local_dict=dict(zip(v, symbols(v))))

Chris

/c

[Aaron Meurer]

You are trying to multiply the strings but you really want to multiply symbols. Something like Symbol(f"x{i}")*Symbol(f"x{i}").

There should be no need to inject symbols into locals to do what you are trying to do. Symbols do not have to be assigned to a variable to work. 

Aaron Meurer 

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[Jeremy Monat]

While outside the scope of your question, it seems like you're doing chemistry and that's my background so I can't help but mention that ChemPy, which is based on SymPy, can parse chemical formulas into substances and then return properties such as mass (molecular weight):

>>> from chempy import Substance
>>> for chemical in ['Na','Cl','NaCl']:
     substance = Substance.from_formula(chemical)
    mass = substance.mass
     print(mass)
22.98976928
35.45
58.43976928000001

Jeremy Monat

P.S. I actually used that ChemPy functionality, along with SymPy, to develop a cheminformatics app.

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[Zohreh Karimzadeh]

Thank you all;)

Zohreh Karimzadeh

https://www.researchgate.net/profile/Zohreh-Karimzadeh

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