I added another note to that wiki page. People often forget that Symbol
names can contain any characters, but anything that uses sympify() will
only parse valid Python variable names into Symbols. So for example
>>> x = Symbol('2x')>>> solve(x - 1, x)[1]>>> solve(x - 1, '2x')Traceback (most
>>> recent call last):
File "<ipython-input-12-71e92ee34319>", line 1, in <module>
solve(Symbol('2d') - 1, '2d ')
File
"/Users/aaronmeurer/Documents/Python/sympy/sympy/sympy/solvers/solvers.py",
line 672, in solve
f, symbols = (_sympified_list(w) for w in [f, symbols])
File
"/Users/aaronmeurer/Documents/Python/sympy/sympy/sympy/solvers/solvers.py",
line 672, in <genexpr>
f, symbols = (_sympified_list(w) for w in [f, symbols])
File
"/Users/aaronmeurer/Documents/Python/sympy/sympy/sympy/solvers/solvers.py",
line 663, in _sympified_list
return map(sympify, w if iterable(w) else [w])
File "/Users/aaronmeurer/Documents/Python/sympy/sympy/sympy/core/sympify.py",
line 297, in sympify
raise SympifyError('could not parse %r' % a, exc)SympifyError:
Sympify of expression 'could not parse u'2d'' failed, because of
exception being raised:SyntaxError: invalid syntax (<string>, line 1)
Aaron Meurer
On Wed, Jul 10, 2013 at 9:29 AM, Amit Saha <[email protected]> wrote:
> On Thu, Jul 11, 2013 at 12:26 AM, Aaron Meurer <[email protected]> wrote:
> > On Jul 10, 2013, at 9:17 AM, Amit Saha <[email protected]> wrote:
> >
> >> On Wed, Jul 10, 2013 at 1:24 AM, Aaron Meurer <[email protected]>
> wrote:
> >>> Yep, strings often (but won't always!) work as input. See
> >>>
> https://github.com/sympy/sympy/wiki/Idioms-and-Antipatterns#strings-as-input
> .
> >>
> >> Thanks. Yeah, I have learned that from one of my earlier posts to the
> >> forum. In this case it should be fine, I guess.
> >> In fact, I think I made my original query ambiguous: I was really
> >> asking about passing the symbol to be solved for as a string,
> >> *not* the expression.
> >
> > Even then, you should avoid string. Many arguments to many SymPy
> > functions accept strings like this, but it's bad practice to use them.
>
> I agree.
>
> >
> >>
> >> Is this a better approach:
> >>
> >>>>> expr = 'x*2 + 5 + y'
> >>>>> expr=sympify(expr)
> >>>>> for symbol in expr.atoms(Symbol):
> >> ... if str(symbol)=='x':
> >> ... solve(expr, symbol)
> >> ...
> >> [-y/2 - 5/2]
> >>
> >> So, I look for the symbol I want to solve for and then invoke solve
> >> using the symbol itself, this having to avoid passing a string.
> >
> > It's better to look at symbol.name. Two symbols can have the same
> > string representation but be unequal.
>
> Indeed, thanks for the pointer to symbol.name
>
>
>
> >
> > Aaron Meurer
> >
> >>
> >> --
> >> http://echorand.me
> >>
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