You can't use exec() to do a return statement for a function that is
not also part of the string. What you want is eval(), which evaluates
an expression and returns it, like
return eval(string)
However, you should consider just using sympify() or parse_expr() if
you are evaluating strings to SymPy expressions, as they are designed
for that and will give you things like automatically defining the
symbols.
Actually, for what you've shown, I don't see the need to use exec() or
eval() at all. You can just create the symbols from the strings, like
Symbol('a' + str(j)). Symbols are completely defined by name, so if
two symbols have the same name, they will be equal.
Aaron Meurer
On Wed, Nov 4, 2020 at 2:24 AM Thomas Ligon <thomassli...@gmail.com> wrote:
>
> Now I see a solution: forget the function and create the expression exp using
> string manipulation and exec, something like
> strExp = 'complex string'
> strSym = 'exp = ' + strExp
> exec(strSym)
>
> On Wednesday, November 4, 2020 at 9:51:35 AM UTC+1 Thomas Ligon wrote:
>>
>> Thanks!
>> The automatic subscript is just a minor irritation, and your response is
>> helpful.
>> I have included a new test program that hopefully no longer oversimplifies
>> the situation. Currently, I have code (2000 lines) for investigating a very
>> complex series, and it runs to a maximum of 3 terms of the series, requiring
>> index values up to 6. I showed the results to the mathematics professor who
>> has mentored my work and he immediately says that it is valuable and I
>> should publish a version that can go up to a very large number of terms. In
>> order to do that, I need to produce a fully automated version with a
>> variable number of terms and indices.
>> So, for variable j, I want to use a_j in a formula.
>> The expression exp on line 20 does that, where I use get_a(1) to represent
>> aj for variable j, where j happens to be 1.
>> In line 5, the return is part of a function. In line 8, I expect return in
>> exec ('return...') to be a part of a function.
>>
>> maxIndex = 2
>> import sympy
>> from sympy import symbols, latex
>> def get_a1():
>> return a1
>> def get_a2():
>> strSym = 'return a2'
>> exec(strSym)
>> def get_a (j):
>> strSym = 'return a' + str(j)
>> exec(strSym)
>> lam = symbols('\\lambda')
>> for ind in range(1,maxIndex+1):
>> strSym = 'a' + str(ind) + ' = symbols(\'a_' + str(ind) + '\')'
>> exec(strSym)
>> #a1 = symbols('a_1')
>> #a2 = symbols('a_2')
>> print(latex(get_a1()))
>> #print(latex(get_a2())) # causes 'return' outside function (<string>, line 1)
>> exp = 3*get_a(1) + 5*get_a(2)*lam**2 # causes 'return' outside function
>> (<string>, line 1)
>> print(latex(exp))
>> print('end testSym')
>>
>> On Tuesday, November 3, 2020 at 11:29:00 PM UTC+1 asme...@gmail.com wrote:
>>>
>>> I'm unclear what you're trying to achieve with the functions. exec()
>>> takes a string of Python code and executes it. The entire string must
>>> be valid Python code by itself, so exec('return x') fails because a
>>> bare "return x" is not valid Python. "return" must be inside a
>>> function definition to be valid. But even defSym1 doesn't do anything
>>> useful beyond just returning Sym1, so there's no point to having it
>>> instead of just "Sym1" directly.
>>>
>>> The LaTeX version of Sym1 contains _ because SymPy automatically
>>> assumes that symbol names ending in numbers are subscripted, so it
>>> renders it as a Sym_1 in LaTeX. If you don't want the 1 to be
>>> subscripted, you can use something like Symbol("{Sym1}").
>>>
>>> Aaron Meurer
>>>
>>> On Tue, Nov 3, 2020 at 9:42 AM Thomas Ligon <thomas...@gmail.com> wrote:
>>> >
>>> > The following code is not working as I expected.
>>> > Why does Sym1 contain an underline (_{1})?
>>> > Why does return fail (unhandled exception) in line 7?
>>> >
>>> > Code:
>>> > import sympy
>>> > from sympy import symbols, latex
>>> > def defSym1():
>>> > return Sym1
>>> > def defSym2():
>>> > strSym = 'return Sym2'
>>> > exec(strSym)
>>> > Sym1 = symbols('Sym1')
>>> > Sym2 = symbols('Sym2')
>>> > print(latex(defSym1()))
>>> > print(latex(defSym2()))
>>> > print('end testSym')
>>> >
>>> > Output:
>>> > Sym_{1}
>>> > 'return' outside function (<string>, line 1)
>>> >
>>> >
>>> >
>>> >
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>
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