省略号 [...] 在列表中是什么意思?

问题描述

I was playing around in python. I used the following code in IDLE:

p  = [1, 2]
p[1:1] = [p]
print p

The output was:

[1, [...], 2]

What is this […]? Interestingly I could now use this as a list of list of list up to infinity i.e.

p[1][1][1]....

I could write the above as long as I wanted and it would still work.

EDIT:

• How is it represented in memory?
• What's its use? Examples of some cases where it is useful would be helpful.
• Any link to official documentation would be really useful.

解决方案

It means that you created an infinite list nested inside itself, which can not be printed. p contains p which contains p ... and so on. The [...] notation is a way to let you know this, and to inform that it can't be represented! Take a look at @6502's answer to see a nice picture showing what's happening.

Now, regarding the three new items after your edit:

• This answer seems to cover it
• Ignacio's link describes some possible uses
• This is more a topic of data structure design than programming languages, so it's unlikely that any reference is found in Python's official documentation