> my_list = ['a', 'b', 'c']
> for idx, val in enumerate(my_list):
...  print(idx, val)
...
0 a
1 b
2 c

> my_list = ['a', 'b', 'c']
> for idx, val in enumerate(my_list, 1):
...  print(idx, val)
...
1 a
2 b
3 c

def parse_data(filename):
 with open(filename, 'rt') as f:
  for lineno, line in enumerate(f, 1):
   fields = line.split()
   try:
    count = int(fields[1])
    ...
   except ValueError as e:
    print('Line {}: Parse error: {}'.format(lineno, e))

word_summary = defaultdict(list)

with open('myfile.txt', 'r') as f:
 lines = f.readlines()

for idx, line in enumerate(lines):
 # Create a list of words in current line
 words = [w.strip().lower() for w in line.split()]
 for word in words:
  word_summary[word].append(idx)

lineno = 1
for line in f:
 # Process line
 ...
 lineno += 1

for lineno, line in enumerate(f):
 # Process line
 ...

data = [ (1, 2), (3, 4), (5, 6), (7, 8) ]

# Correct!
for n, (x, y) in enumerate(data):
 ...
# Error!
for n, x, y in enumerate(data):
 ...