Parse a string and verify it is an integer. In the above diagram:

Note: The starting and ending spaces have been removed from the string.

Python Code - Parse an Integer

# ==========================================================
# Test String to Integer Conversion
# ----------------------------------------------------------
# This is not the most efficient code. I designed it as
# a demonstration. It could be more efficient.
# ==========================================================

# ----------------------------------------------------------
# convert string to integer
# ----------------------------------------------------------

def StringToInteger(str):
    
    # --- initialize local variables
        
    i  = 0                       # string character index
    l  = len(str)                # string length
    vp = True                    # value is positive
    v  = 0                       # string's integer value

    # --- test string length

    if l == 0:
        return False,0

    # --- process string

    sign = False                 # found a [+-] character

    while i < l:

        c = str[i]               # get a string character
        print('c={},v={},i={}'.format(c,v,i))
        i = i + 1                # increment string index

        if c == "+":             # character "+"
            if sign:             # found more that one sign?
                return False,0   # error
            sign = True          # found + sign

        elif c ==  "-":          # character "-"
            if sign:             # found more that one sign?
                return False,0   # error
            sign = True          # found - sign
            vp = False           # value is negative

        elif c ==  "0":          # character "0"
            v = v*10
        elif c ==  "1":          # character "1"
            v = (v*10) + 1
        elif c ==  "2":          # character "2"
            v = (v*10) + 2
        elif c ==  "3":          # character "3"
            v = (v*10) + 3
        elif c ==  "4":          # character "4"
            v = (v*10) + 4
        elif c ==  "5":          # character "5"
            v = (v*10) + 5
        elif c ==  "6":          # character "6"
            v = (v*10) + 6
        elif c ==  "7":          # character "7"
            v = (v*10) + 7
        elif c ==  "8":          # character "8"
            v = (v*10) + 8
        elif c ==  "9":          # character "9"
            v = (v*10) + 9
        else:                    # illegal character
            return False,0

    if not vp:                   # negative value?
        v = -v

    return True,v                # end of string

# ---------------------------------------------------------
# main (test code)
# ---------------------------------------------------------

if __name__ == '__main__':

    # --- python version function -------------------------

    import sys

    def RunningPython3():
        ##print(sys.version_info)
        if sys.version_info[0] == 3:
            return True
        return False

    # --- get use input function --------------------------

    def GetUserInput(prompt,py3):
        if py3:
            return input(prompt)
        else:
            return raw_input(prompt)

    # --- pause the program function ----------------------

    def Pause(py3):
        GetUserInput('\nPress enter to continue ',py3)

    # --- ask the user for input --------------------------

    py3 = RunningPython3()

    while True:

        s = GetUserInput('\nEnter integer number: ',py3)

        s = s.strip()     # remove leading/trailing spaces

        if s == '':       # empty string?
            break

        r,v = StringToInteger(s)
        if r:
            print('Returned Value {}  {}'.format(v,type(v)))
        else:
            print('Oops! an error (returned value={})'.format(v))

        Pause(py3)