关于Hetland Python书中interval函数参数与循环的技术疑问

Let's break down your questions one by one to clear up the confusion:

1. The Role of the Two start Variables in the If Statement #

That line start, stop = 0, start uses Python's simultaneous tuple assignment—a concise way to reassign multiple variables in one go. Here's the play-by-play:

• When you call the function with only one argument (like interval(10)), the stop parameter defaults to None, so the if condition triggers.
• The right-hand side (0, start) first evaluates to a tuple containing 0 and the original value you passed as start (in this case, 10).
• We then assign those values to the left-hand variables: the first start is the function's start variable being updated to 0, and stop takes the original start value (10).
• That’s why replacing the second start with an integer (say 5) makes the list length equal to that integer—stop becomes 5, so the function generates values from 0 to 4. The second start is just capturing your original input to use as the new upper bound.

2. Why the Loop Runs When Passing start=10 #

Your confusion here stems from missing the variable reassignment that happens before the loop. Let’s walk through calling interval(10) step by step:

1. You pass start=10, stop is None, so the if block executes: start gets set to 0, stop gets set to 10.
2. The loop starts with i = start (now 0), and the condition is while 0 < 10—which is absolutely true!
3. The loop runs, appending 0, 1, 2, ..., 9 to the result list, incrementing i by step (1) each time, until i hits 10 (at which point 10 <10 is false, and the loop stops).
   You thought the loop was using the original start value (10) for the condition, but the if statement already changed start to 0 before the loop even begins.

内容的提问来源于stack exchange，提问作者Knovolt