custom eval function in python for a nonempty sting that may contain only '0'..'9', '+', '*'

custom_eval_string_numeric_expression.py

ORD_OF_ZERO = ord('0')
def _eval(str):
	''' Returns an integer, a result of evaluation of a nonempty str
	that may contain only '0'..'9', '+', '*'.

	Summary of the algorithm:
	* .split('+') the string into products (strings),
	* then .split('*') each product string into numbers (strings),
	* evaluate each number (string) into an integer,
	* multiply all numbers within each product to get the numerical product,
	* then just add up all numerical products to get The Sum, the result.'''

	thesum = 0 # The final sum to return in the end.
	products = str.split('+')
	for product in products: # Product is a string like '0' or '123' or '5*67*0'
		theproduct = 1
		numbers = product.split('*')
		for number in numbers: # Number is a string such as '59' or '123'
			thenumber = 0
			for digit in number: # Digit is a character such as '5' or '9'
				digit_int = ord(digit)-ORD_OF_ZERO # Simply converting a '5' into a 5
				thenumber = thenumber*10 + digit_int
			theproduct *= thenumber
		thesum += theproduct
	return thesum