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As a Python developer, I recently hit an odd issue where loops in my code weren't iterating as I expected. I had a function that was iterating over a list of date ranges to check if a timestamp fell within any of the ranges.

My code looked something like this:

def check_timestamp(timestamp):

  date_ranges = generate_ranges() 

  for range in date_ranges:
    print("First loop")

  for range in date_ranges:  
    print("Second loop")

def generate_ranges():
  return (
    {'start': date1, 'end': date2} 
    for date1, date2 in zip(start_dates, end_dates)
  )

• no "Second loop" prints.

After some debugging, I noticed the issue disappeared when I changed generate_ranges() to return a list instead of a generator expression:

def generate_ranges():
  return [
    {'start': date1, 'end': date2}
    for date1, date2 in zip(start_dates, end_dates) 
  ]

It seems the generator expression was introducing some unexpected behavior between the loops. By forcing evaluation into a list, it resolved the quirkiness.

So if you ever see loops in your Python code not iterating as expected, check for any generator expressions that could be introducing subtle bugs. Converting them to standard lists/tuples is an easy way to rule out iterator issues.

Let me know if you have any other questions!