Converting a python numeric expression to LaTeX

Question

I need to convert strings with valid python syntax such as:

'1+2**(x+y)'

and get the equivalent LaTeX:

$1+2^{x+y}$

I have tried sympy's latex function but it processes actual expression, rather than the string form of it:

>>> latex(1+2**(x+y))
'$1 + 2^{x + y}$'
>>> latex('1+2**(x+y)')
'$1+2**(x+y)$'

but to even do this, it requires x and y to be declared as type "symbols".

I want something more straight forward, preferably doable with the parser from the compiler module.

>>> compiler.parse('1+2**(x+y)')
Module(None, Stmt([Discard(Add((Const(1), Power((Const(2), Add((Name('x'), Name('y'))))))))]))

Last but not least, the why: I need to generate those latex snipptes so that I can show them in a webpage with mathjax.

Answer 1

Here's a rather long but still incomplete method that doesn't involve sympy in any way. It's enough to cover the example of (-b-sqrt(b**2-4*a*c))/(2*a) which gets translated to \frac{- b - \sqrt{b^{2} - 4 \; a \; c}}{2 \; a} and renders as

It basically creates the AST and walks it producing the latex math the corresponds to the AST nodes. What's there should give enough of an idea how to extend it in the places it's lacking.

 import ast  class LatexVisitor(ast.NodeVisitor):      def prec(self, n):         return getattr(self, 'prec_'+n.__class__.__name__, getattr(self, 'generic_prec'))(n)      def visit_Call(self, n):         func = self.visit(n.func)         args = ', '.join(map(self.visit, n.args))         if func == 'sqrt':             return '\sqrt{%s}' % args         else:             return r'\operatorname{%s}\left(%s\right)' % (func, args)      def prec_Call(self, n):         return 1000      def visit_Name(self, n):         return n.id      def prec_Name(self, n):         return 1000      def visit_UnaryOp(self, n):         if self.prec(n.op) > self.prec(n.operand):             return r'%s \left(%s\right)' % (self.visit(n.op), self.visit(n.operand))         else:             return r'%s %s' % (self.visit(n.op), self.visit(n.operand))      def prec_UnaryOp(self, n):         return self.prec(n.op)      def visit_BinOp(self, n):         if self.prec(n.op) > self.prec(n.left):             left = r'\left(%s\right)' % self.visit(n.left)         else:             left = self.visit(n.left)         if self.prec(n.op) > self.prec(n.right):             right = r'\left(%s\right)' % self.visit(n.right)         else:             right = self.visit(n.right)         if isinstance(n.op, ast.Div):             return r'\frac{%s}{%s}' % (self.visit(n.left), self.visit(n.right))         elif isinstance(n.op, ast.FloorDiv):             return r'\left\lfloor\frac{%s}{%s}\right\rfloor' % (self.visit(n.left), self.visit(n.right))         elif isinstance(n.op, ast.Pow):             return r'%s^{%s}' % (left, self.visit(n.right))         else:             return r'%s %s %s' % (left, self.visit(n.op), right)      def prec_BinOp(self, n):         return self.prec(n.op)      def visit_Sub(self, n):         return '-'      def prec_Sub(self, n):         return 300      def visit_Add(self, n):         return '+'      def prec_Add(self, n):         return 300      def visit_Mult(self, n):         return '\\;'      def prec_Mult(self, n):         return 400      def visit_Mod(self, n):         return '\\bmod'      def prec_Mod(self, n):         return 500      def prec_Pow(self, n):         return 700      def prec_Div(self, n):         return 400      def prec_FloorDiv(self, n):         return 400      def visit_LShift(self, n):         return '\\operatorname{shiftLeft}'      def visit_RShift(self, n):         return '\\operatorname{shiftRight}'      def visit_BitOr(self, n):         return '\\operatorname{or}'      def visit_BitXor(self, n):         return '\\operatorname{xor}'      def visit_BitAnd(self, n):         return '\\operatorname{and}'      def visit_Invert(self, n):         return '\\operatorname{invert}'      def prec_Invert(self, n):         return 800      def visit_Not(self, n):         return '\\neg'      def prec_Not(self, n):         return 800      def visit_UAdd(self, n):         return '+'      def prec_UAdd(self, n):         return 800      def visit_USub(self, n):         return '-'      def prec_USub(self, n):         return 800     def visit_Num(self, n):         return str(n.n)      def prec_Num(self, n):         return 1000      def generic_visit(self, n):         if isinstance(n, ast.AST):             return r'' % (n.__class__.__name__, ', '.join(map(self.visit, [getattr(n, f) for f in n._fields])))         else:             return str(n)      def generic_prec(self, n):         return 0  def py2tex(expr):     pt = ast.parse(expr)     return LatexVisitor().visit(pt.body[0].value)  

Answer 2

You can use sympy.latex with eval:

s = "1+2**(x+y)" sympy.latex(eval(s))   # prints '$1 + {2}^{x + y}$' 

You still have to declare the variables as symbols, but if this is really a problem, it's much easier to write a parser to do this than to parse everything and generate the latex from scratch.

Answer 3

You can use SymPy. Just pass the string to the sympify() function first, which will convert it to a valid SymPy expression (i.e., create the Symbols for you, etc.). So you could do

>>> latex(sympify('1+2**(x+y)'))
1 + 2^{x + y}

S() is also a shortcut to sympify(), i.e., latex(S('1+2**(x+y)')) also works.

Answer 4

Just a little fix to Geoff Reedy excellent answer:

class GenerateSymbols(defaultdict):
    def __missing__(self, key):
        self[key] = sympy.Symbol(key)
        return self[key]

Before it would not add the new item to the dict. Now you can use this with your expression:

d= GenerateSymbols()    
eq = '(-b-sqrt(b**2-4*a*c))/(2*a)'

and you can further simplify it before converting it to LaTeX:

sympy.latex(sympy.simplify(eval(eq,d)))

and you get this:

'$- \\frac{b + \\operatorname{sqrt}\\left(- 4 a c + b^{2}\\right)}{2 a}$'