Python implementation of atoi

Question

I made an implementation of atoi (ascii to integer) in Python a while ago for fun, and I'd like to know what I could do to improve it.

---

class CannotConvertToInteger(Exception):
  """A non-numeric character was present in the string passed to atoi"""
  pass

def atoi(string : str) -> int:
  sign = multiplier = 1
  val = 0
  if string[0] == '-':
    string = string[1:]
    sign = -1
  elif string[0] == '+':
    string = string[1:]
  for i in string[::-1]:
    code = ord(i)
    try:
      if ((code > 57) or (code < 48)):
        raise CannotConvertToInteger
      else:
        val += (code - 48) * multiplier
        multiplier *= 10
    except CannotConvertToInteger:
      return print('Cannot convert string to an integer!')
  return (val * sign)

test_string = input('Enter an optionally signed integer: ')
result = atoi(test_string)
if result:
  print('It was a valid int! atoi() returned:', result)
else:
  print('It was an invalid int! atoi() returned:', result)

input()

---

One specific question I have is if it's bad practice to return a print call, as a way to print an error and simultaneously return from the function? I did that so that I could print the error and return None on the same line.

Answer 1

• Python already has an exception that denotes that the value you passed is inappropriate somehow. It is ValueError, which is what the built-in int also raises if a wrong string is passed.

  In addition, defining a nice readable error which you can raise, only to catch it directly within the function and to return None (the output of print) and print to the terminal is not ideal. Just let the exception rise to the caller of the function, it should be their problem if the function is used in a bad way (which you are telling them all about with that exception).

• You should avoid magic constants. What are 57 and 48? Either give them names and use compound comparisons:

  zero = ord("0")
  nine = ord("9")
  
  if not zero <= code <= nine:
      ...
  

  Or, maybe even better, write a isdigit function:

  def isdigit(s):
      return s in set("0123456789")
  

  Which can be slightly sped up by using the standard library string module:

  from string import digits
  
  DIGITS = set(digits)
  
  def isdigit(s):
      return s in DIGITS
  

  Incidentally, don't shadow the standard library string module, just call the input s or x, as int does.

  Note that there are also str.isdigit, but this unfortunately also returns true for unicode digits, such as all of ¹²³⁴⁵⁶⁷⁸⁹⁰. Only with a whitelist can you fully control what counts as a digit in your case.

• Instead of iterating over the string and directly calling ord, you can use map (and reversed):

  for code in map(ord, reversed(string)):
      ...
  

  You could also iterate over multiplier (or rather its exponent) at the same time using enumerate:

  for exponent, code in enumerate(map(ord, reversed(string))):
      ...
      value += (code - zero) * 10 ** exponent
  

• Actually directly manipulating ASCII values is not the most robust (although it works). Instead you could just make a dictionary that maps strings to integer values:

  VALUES = {c: d for d, c in enumerate(DIGITS)}
  

• Using string[0] to check for a sign character can fail if the empty string is passed. Instead you can use str.startswith("+") or even str.startswith(("+", "-")). This will just return False for an empty string.

• With all this done, your function can actually easily be extended to arbitrary bases (but let's stick to maximum base 36, like int, i.e. all digits and lowercase letters):

  DIGITS = string.digits + string.ascii_lowercase
  VALUES = {c: d for d, c in enumerate(DIGITS)}
  
  def isdigit(s, base=10):
      return s in DIGITS[:base]
  
  def atoi(x : str, base : int = 10):
      if not 2 <= base <= 36:
          raise ValueError("Only 2 <= base <= 36 currently supported")
      sign = 1
      if x.startswith(("+", "-")):
          if x[0] == "-":
              sign = -1
          x = x[1:]
      value = 0
      for exp, c in enumerate(reversed(x)):
          if c not in VALUES or VALUES[c] >= base:
              raise ValueError(f"{c} is not a valid digit in base {base}")
          value += VALUES[c] * base ** exp
      return sign * value
  

  This works, as demonstrated below:

  atoi("12345")
  # 12345
  atoi("+12345")
  # 12345
  atoi("-12345")
  # -12345
  atoi("12345", base=6)
  # 1865
  atoi("12345", base=5)
  # ValueError: 5 is not a valid digit in base 5
  atoi("101010", base=2)
  # 42
  atoi("1234567890abcdef", base=16)
  # 1311768467294899695
  atoi("1234567890abcdefghijklmnopqrstuvwxyz", base=36)
  # 3126485650002806059265235559620383787531710118313327355
  atoi("")
  # 0
  atoi("111", base=1)
  # ValueError: Only 2 <= base <= 36 currently supported
  atoi("Az", base=62)
  # ValueError: Only 2 <= base <= 36 currently supported
  

• You should surround your calling code with a if __name__ == "__main__": guard to allow importing from this module from another script without the user input/output being run:

  if __name__ == "__main__":
      x = input('Enter an optionally signed integer: ')
      try:
          print('It was a valid int! atoi() returned:', atoi(x))
      except ValueError:
          print('It was an invalid int!)
  

Answer 2

Error handling

One specific question I have is if it's bad practice to return a print call, as a way to print an error and simultaneously return from the function? I did that so that I could print the error and return None on the same line.

That is unconventional to say the least. By catching the exception internally and print it to the console you take away the ability to handle exceptions in calling code. You should raise an exception if an error occured that cannot be handled by the function itself to give the caller the possibility to decide how to handle this. Also, ask yourself: What is the advantage of your chosen approach? Is

result = atoi(test_string)
if result:
  print('It was a valid int! atoi() returned:', result)
else:
  print('It was an invalid int! atoi() returned:', result)

really any better than, for example:

try:
    result = atoi(test_string)
    print('It was a valid int! atoi() returned:', result)
except CannotConvertToInteger:
    print('It was an invalid int!)

The code itself

• The function would profit from some blank lines to seperate logical blocks.
• string[::-1] actually creates a copy, since strings are immutable. You can avoid that by using reversed(string), which is perfectly fine for your use-case since you only want the single digits, not the whole thing reversed.

• This convoluted structure

  try:
      if ((code > 57) or (code < 48)):
          raise CannotConvertToInteger
      else:
          val += (code - 48) * multiplier
          multiplier *= 10
  except CannotConvertToInteger:
      return print('Cannot convert string to an integer!')
  

  is the price you pay for the way you chose to handle your error cases. As said above, removing the try: ... catch ...: is the favorable approach here.

• @Graipher's answer has more good points about using built-in exceptions and avoiding magical numbers, that I almost immediately thought about when writing this up, but then forgot in the process.

• Almost as a side note: you don't need parens around conditions in Python. Most people only ever use them if the conditions get very long and need to spread over multiple lines. The same goes for the return value. Here the parens are even more unnecessary.

• You should have a look at the official Style Guide for Python Code (often just called PEP8). The recommendations most relevant to your code would be to use 4 spaces per indentation level and avoid multiple initializations per source line. The meta-site for Code Review also has a nice list of tools that can help you check this automatically.