Olivier - 2 months ago 14
Python Question

# Convert string expression to mathematical expression?

I'm looking for something that would take a mathematical expression string as an input and would change it by adding parentheses to it.

For examples :

Input 1 :

`"2*4^7"`

Output 1 :
`"(2*(4^7))"`

Input 2 :
`"sin(x)*3-8^9"`

Output 2 :
`"((sin(x)*3)-(8^9))"`

Input 3 :
`"(2-8)/6"`

Output 3 :
`"((2-8)/6)"`

I'm currently trying to code it, but it's taking too much time and I would rather focus on other things, so if you guys know some module that could do that, that would be great.

I'm working with python 3

Thank you!

Using the `ast` module, you can parse your string and then traverse it to build a string representation you require:

``````import ast

def represent(tree):
t = type(tree)
if t == ast.Module:
return represent(tree.body)
elif t == list:
# arithmetic expression should only be one thing
return ", ".join(represent(part) for part in tree)
elif t == ast.Expr:
return represent(tree.value)
elif t == ast.BinOp:
o = type(tree.op)
if o == ast.BitXor:
op = "^"
elif o == ast.Mult:
op = "*"
op = "+"
...
return "({}{}{})".format(
represent(tree.left),
op,
represent(tree.right),
)
elif t == ast.Num:
return str(tree.n)
elif t == ast.Name:
return tree.id
elif t == ast.Call:
return "{}({})".format(
represent(tree.func),
represent(tree.args),
)
...

# prints (((sin(x)*2)*x)+4)
print(represent(ast.parse("sin(x)*2*x+4")))
``````

As @user2357112 noted in the comments, this will only work if you limit yourself to use Python's expression syntax, most importantly by using `**` instead of `^` for exponentiation.