在Django中自动分配模型字段值(Automatically assigning model field values in Django)

 如果您想一次访问3个字符的字符串，则需要使用切片 。 
 
 您可以使用像这样的列表理解来获得字符串的3个字符长的部分列表： 
 
>>> x = 'this is a string'
>>> step = 3
>>> [x[i:i+step] for i in range(0, len(x), step)]
['thi', 's i', 's a', ' st', 'rin', 'g']
>>> step = 5
>>> [x[i:i+step] for i in range(0, len(x), step)]
['this ', 'is a ', 'strin', 'g']
 
 重要的是： 
 
[x[i:i+step] for i in range(0, len(x), step)]
 
 range(0, len(x), step)我们提供了每个step字符切片开始的索引。 for i in将会遍历这些指数。 x[i:i+step]获得从索引i开始的x的片段，并且是step长字符。 
 
 如果你知道每次都会得到四件，那么你可以这样做： 
 
a, b, c, d = [x[i:i+step] for i in range(0, len(x), step)]
 
 如果3 * step < len(x) <= 4 * step则会发生这种情况。 
 
 如果你没有完整的四部分，那么Python会给你一个ValueError试图解压这个列表。 因此，我会认为这种技术非常脆弱，不会使用它。 
 
 你可以简单地做 
 
x_pieces = [x[i:i+step] for i in range(0, len(x), step)]
 
 现在，您曾经访问a ，您可以访问x_pieces[0] 。 对于b ，可以使用x_pieces[1]等。 这可以让你更灵活。 

If you'd like to access a string 3 characters at a time, you're going to need to use slicing.
 
You can get a list of the 3-character long pieces of the string using a list comprehension like this:
 
>>> x = 'this is a string'
>>> step = 3
>>> [x[i:i+step] for i in range(0, len(x), step)]
['thi', 's i', 's a', ' st', 'rin', 'g']
>>> step = 5
>>> [x[i:i+step] for i in range(0, len(x), step)]
['this ', 'is a ', 'strin', 'g']
 
The important bit is:
 
[x[i:i+step] for i in range(0, len(x), step)]
 
range(0, len(x), step) gets us the indices of the start of each step-character slice. for i in will iterate over these indices. x[i:i+step] gets the slice of x that starts at the index i and is step characters long.
 
If you know that you will get exactly four pieces every time, then you can do:
 
a, b, c, d = [x[i:i+step] for i in range(0, len(x), step)]
 
This will happen if 3 * step < len(x) <= 4 * step.
 
If you don't have exactly four pieces, then Python will give you a ValueError trying to unpack this list. Because of this, I would consider this technique very brittle, and would not use it.
 
You can simply do
 
x_pieces = [x[i:i+step] for i in range(0, len(x), step)]
 
Now, where you used to access a, you can access x_pieces[0]. For b, you can use x_pieces[1] and so on. This allows you much more flexibility.