Convert custom formula to python function

You could use the re module to do what you want via regular-expression pattern matching and relatively straight-forward text substitution.

import re

alias_pattern = re.compile(r'''(?:\[(\w+)\])''')

def mapper(mat):
    func_alias = mat.group(1)
    function = function_alias_mapping.get(func_alias)
    if not function:
        raise NameError(func_alias)
    return function.__name__ + '()'

# must be defined before anything can be mapped to them
def FooFunction(): return 15
def BarFunction(): return 30
def BazFunction(): return 6

function_alias_mapping =  dict(foo=FooFunction, bar=BarFunction, baz=BazFunction)
formula = "(([foo] + [bar]) - ([baz]/2))"  # Custom formula.

converted_formula = re.sub(alias_pattern, mapper, formula)
print('converted_formula = "{}"'.format(converted_formula))

# define contexts and function in which to evalute the formula expression
global_context = dict(FooFunction=FooFunction,
                      BarFunction=BarFunction,
                      BazFunction=BazFunction)
local_context = {'__builtins__': None}

function = lambda: eval(converted_formula, global_context, local_context)
print('answer = {}'.format(function()))  # call function

Output:

converted_formula = "((FooFunction() + BarFunction()) - (BazFunction()/2))"
answer = 42