Updating multiple (local) variables by function return value in a loop

Question

Assume I have this (currently return-less) function:

def codepoint_convert(text, offset):
    codepoint = text[offset]

    if codepoint <= 0x01:
        output = "\n"
    elif codepoint >= 0x09 and codepoint <= 0x12: # digits
        output = chr(0x30 + (codepoint-0x09))
    elif codepoint >= 0x13 and codepoint <= 0x2C: # uppercase letters
        output = chr(0x41 + (codepoint-0x13))
    elif codepoint >= 0x2D and codepoint <= 0x46: # lowercase letters
        output = chr(0x61 + (codepoint-0x2D))
    elif codepoint >= 0x47 and codepoint <= 0x62: # multi-byte codepoints
        offset += 1
        codepoint = (codepoint << 8) + text[offset]
        output = cp_dict[codepoint]
    else:
        print("Invalid codepoint at 0x%08X" % offset)

    offset += 1

How would I best update (i.e. increment and append, respectively) both offset and output in a while loop defined like this?:

def main():
    text = "\x0A\x0B\x0C\x01"
    offset = 0
    output = ''
    while offset < len(text):

I have previously used two approaches:

1

def convert_codepoint(text, offset, output):
    # A: see first code snippet
    # B: concatenate to "output" (+=) instead of assigning (=)
    return [offset, output]

def main():
    text = "\x0A\x0B\x0C\x01"
    offset = 0
    output = ''
    while offset < len(text):
        offset, output = convert_codepoint(text, offset, output)

2

offset = 0 # global variable

def convert_codepoint(text, offset):
    global offset
    # A: see first code snippet
    return output

def main():
    text = "\x0A\x0B\x0C\x01"
    output = ''
    while offset < len(text):
        output += convert_codepoint(text, offset)

To me, the first approach is confusing, because it looks like it replaces the offset and output variables instead of updating them, since it uses = instead of += (it does not seem like I could somehow use += in a list-assignment in Python 3.4.2 anyway, since it throws a SyntaxError ("illegal expression for augmented assignment")). And the use of a list as a return value does not seem so port-friendly either.

My gripe with the second approach is that it uses a global variable. I want it to be possible to call convert_codepoint() (e.g. if the script is imported as a module) without having to have a global variable defined. The offset variable may need to be re-initialized from the main function, too, so that could get messy.

Any other approach I could try, to update the variables locally, in a nice and clear way?

I think your first way is the right way. The fact that the variables are being added to can be made clear by renaming the function to something like update_codepoint or get_updated_output or some such thing. — BrenBarn
– BrenBarn, Commented Jan 2, 2015 at 3:29

user1245262 · Accepted Answer · 2015-01-02 03:54:06Z

1

Why not have a function that returns the next output and offset, then append the next output element to your output list:

def get_next_output_offset_pair(text, offset):
  #A: Adapt first code snippet
  return [next_offset, next_output]

def main():
   text = "\x0A\x0B\x0C\x01"
   offset = 0
   output = ''
   while offset < len(text):
     offset, next_output = get_next_output_offset_pair(text, offset)
     output.append(next_output)

Or, you could even do this:

      next_offset, next_output = get_next_output_offset_pair(text, offset)
      output.append(next_output)
      offset = next_offset

I think your first solution is perfectly clear, but your code should make intuitive sense to you without making the next maintainer's life difficult.

answered Jan 2, 2015 at 3:54

user1245262

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