On Sun, 5 Jun 2016 01:29 pm, Lawrence D’Oliveiro wrote: > On Saturday, June 4, 2016 at 11:37:18 PM UTC+12, Ned Batchelder wrote: >> On Friday, June 3, 2016 at 11:43:33 PM UTC-4, Lawrence D’Oliveiro wrote: >> > On Saturday, June 4, 2016 at 3:00:36 PM UTC+12, Steven D'Aprano wrote: >> > > You can exit a loop because you have run out of items to process, or >> > > you can exit the loop because a certain condition has been met. >> > >> > But why should they be expressed differently? >> > >> > item_iter = iter(items) >> > while True : >> > item = next(item_iter, None) >> > if item == None : >> > break >> > if is_what_i_want(item) : >> > break >> > #end while >> >> Do you actually write loops like this? > > Is that a non-trolling question? Yes. All the time.
Really? Well, you'd fail my code review, because that code is broken. If items contains None, your loop will silently end early. That's a bug. >> If this appeared in a code review, first we'd have a conversation about >> what this code was meant to do ... > > I would hope not. Clearly. Nevertheless, its a conversation that needs to be had. >> ...and then I would ask, "Why aren't you using a for loop?" > > ... and then I would ask, “Didn’t you read my previous postings where I > pointed out the issues with them?” I don't think that very many people would agree with you or consider them problems at all. They're more like features than problems. Your objections to for-loops feel kind of like "I don't like bread knives because they make it too easy to slice bread". Okay, you don't like for-loops, because they make looping a fixed number of times with an optional early exit too much of a "cognitive burden" for you. You have my sympathy, but nobody else I've come across in nearly two decades of Python programming finds them a cognitive burden. > Here <https://en.wikibooks.org/wiki/Python_Programming/Databases> is > another example: see the section “Looping on Field Breaks”. That section was written by you and is not independent confirmation that others agree with your issues with for-loops. > A while-True scales gracefully to complex situations like that. Graceful like a hippopotamus. I don't know that the situation is complex, your description is pretty clear and to the point: Consider the following scenario: your sales company database has a table of employees, and also a table of sales made by each employee. You want to loop over these sale entries, and produce some per-employee statistics. but the while loop you have certainly is complex. If I understand your intent correctly, then I think this is both more elegant and likely faster than the while loop you use: # Beware of bugs in the following code: # I have only proven it is correct, I haven't tested it. rows = db_iter( db = db, cmd = "select employees.name, sales.amount, sales.date from" " employees left join sales on employees.id = sales.employee_id" " order by employees.name, sales.date" ) default = {'total sales': 0.0, 'number of sales': 0, 'earliest date': None, 'latest date': None} prev_employee_name = None stats = {} for (employee_name, amount, date) in rows: if (employee_name != prev_employee_name and prev_employee_name is not None): # Print the previous employee's stats report(prev_employee_name, stats) # and prepare for the next employee. previous_employee_name = employee_name stats = default.copy() stats['total sales'] += amount stats['number of sales'] += 1 if stats['earliest date'] is None: stats['earliest date'] = date stats['latest date'] = date if prev_employee_name is not None: report(prev_employee_name, stats) No breaks needed at all, which makes it much more understandable: you know instantly from looking at the code that it processes every record exactly once, then exits. But it is a *tiny* bit ugly, due to the need to print the last employee's statistics after the loop is completed. We can fix that in two ways: (1) Give up the requirement to print each employee's stats as they are completed, and print them all at the end; or (2) Put a sentinel at the end of rows. The first may not be suitable for extremely large data sets, but it is especially elegant: rows = db_iter( ... # as above ) default = {'total sales': 0.0, 'number of sales': 0, 'earliest date': None, 'latest date': None} stats = {} for (employee_name, amount, date) in rows: record = stats.setdefault(employee_name, default.copy()) stats['total sales'] += amount stats['number of sales'] += 1 if stats['earliest date'] is None: stats['earliest date'] = date stats['latest date'] = date for employee_name in stats: report(employee_name, stats[employee_name]) As you now have all the statistics available, you can look for under-performing or over-performing sales people, run comparisons between staff, etc. Solution (2) using a sentinel gets rid of the need to print anything outside of the loop by simply ensuring that the very last record is a meaningless sentinel that can be ignored: from itertools import chain rows = db_iter( ... # as above ) default = {'total sales': 0.0, 'number of sales': 0, 'earliest date': None, 'latest date': None} prev_employee_name = None stats = {} for (employee_name, amount, date) in chain(rows, ('', 0, None)): if (employee_name != prev_employee_name and prev_employee_name is not None): # Print the previous employee's stats report(prev_employee_name, stats) # and prepare for the next employee. previous_employee_name = employee_name stats = default.copy() stats['total sales'] += amount stats['number of sales'] += 1 if stats['earliest date'] is None: stats['earliest date'] = date stats['latest date'] = date Again, there are no breaks needed, so you know that every record is processed exactly once, and all but the last (the sentinel) is printed. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
