Python -- better parsing of fixed-width data

fixedwidthconversion.py

# Preview your column widths easier with regex101.com -- makes it so much easier

from collections import OrderedDict
headers = OrderedDict([
    ("id", 5),
    ("name", 25),
    ("attribute", 8)
    ])

myregex = ""
for item in headers:
    myregex += "(.{" + str(headers[item]) + "})"
print("Test at regex101.com:\r\n\t\t" + myregex)

index = 1
print("How your stuff lines up, with starting position of 1:")
largestitem = 0
for item in headers:
    if len(item) > largestitem:
        largestitem = len(item)
for item in headers:
    print(f"{item}{((largestitem - len(item)) + 3) * ' '}{index}\t{headers[item] - 1 + index}")
    index += headers[item]


for row in mydata:
    line = OrderedDict()
    counter = 0
    for item in headers:
        line[item] = row[counter:headers[item]+counter].strip()
        counter += headers[item]
    DoSomething(line)

sample.py

# Sample conversion to CSV
# Untested. Probably a typo or three.
# For Python 3; if using Python 2, nix the newline stuff
from collections import OrderedDict
import csv

sourcefile = "source.txt"
targetfile = "target.csv"

# Define your fields and column widths here
headers = OrderedDict([
    ("id", 5),
    ("name", 25),
    ("attribute", 8)
    ])

myregex = ""
for item in headers:
    myregex += "(.{" + str(headers[item]) + "})"
print("Test at regex101.com:\r\n\t\t" + myregex)

index = 1
print("How your stuff lines up, with starting position of 1:")
largestitem = 0
for item in headers:
    if len(item) > largestitem:
        largestitem = len(item)
for item in headers:
    print(f"{item}{((largestitem - len(item)) + 3) * ' '}{index}\t{headers[item] - 1 + index}")
    index += headers[item]

with open(targetfile, "w", newline="") as targetfilehandle:
  writer = csv.writer(targetfilehandle)
  writer.writerow(list(headers.keys()))   # Write the CSV header
  with open(sourcefile, "r") as sourcefilehandle:
    inputdata = sourcefilehandle.readlines()
    for row in inputdata:
      line = OrderedDict()
      counter = 0
      for item in headers:
          line[item] = row[counter:headers[item]+counter].strip()
          counter += headers[item]
      writer.writerow(list(row.values())))