Python: determine if a string contains math?

Question

Given these strings:

\"1 + 2\"
\"apple,pear\"

How can I use Python 3(.5) to determine that the first string contains a math problem and

Answer 1

You can use a parsing library such as pyPEG, although there is room for improvment do more than this you could define a grammar like this:

from pypeg2 import optional, List, Namespace
import re

number = re.compile(r'\d+')
binop = re.compile(r'\+|\*') # Exercise: Extend to other binary operators


class BinOp(Namespace):
    grammar = binop


class Number(Namespace):
    grammar = number, optional("."), optional(number)


class Expression(Namespace):
    grammar = Number, optional(BinOp, Number)


class Equation(List):
    grammar = Expression, optional("="), optional(Expression)

You can handle the error when an invalid expression is passed through and use the parse function to validate expressions:

>>> import pypeg2
>>> f = pypeg2.parse("3=3", Equation)
>>> f = pypeg2.parse("3 = 3", Equation)
>>> f = pypeg2.parse("3 + 3 = 3", Equation)
>>> f = pypeg2.parse("3 * 3 = 3", Equation)
>>> f = pypeg2.parse("3hi", Equation)
Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/local/lib/python3.5/site-packages/pypeg2/__init__.py", line 669, in parse
    raise parser.last_error
  File "", line 1
    3hi
     ^
SyntaxError: expecting match on \d+