如何在python列表中使用切片运算符避免for循环中的副作用？

Question

This is quoted from the link : http://www.python-course.eu/for_loop.php - To avoid these side effects, it's best to work on a copy by using the slicing operator, as can be seen in the next example:

colours = ["red"]
for i in colours[:]:
    if i == "red":
        colours += ["black"]
    if i == "black":
        colours += ["white"]
print colours

The output is :

['red', 'black']

My question is: does the statement colours[:] makes a copy of the colours list and the for loop works on the copy instead of the original list? If it is so, then how the ["balck"] is appended to the original colour list?

Answer 1

does the statement colours[:] makes a copy of the colours list : Yes.

and the for loop works on the copy instead of the original list? Yes, but be careful what you mean by "works on". The variable i takes its values from a copy of list colours . However, the references to colours such as in the line colours += ["black"] refer to the original list. That is just what the code wants, so it works.

If it is so, then how the ["balck"] is appended to the original colour list? This is because the line that does the appending refers to the original list and not the copy of the list.

Answer 2

colours[:] makes a copy of the original list and iterates over it, but colours remains the original list. The following code would be equivalent:

copy = colours.copy()
for i in copy:
    if i == "red":
        colours.append("black")  # append is O(1)
    if i == "black":
        colours.append("white")

Answer 3

This is the code you are referring to in the link provided:

If you loop over a list, it's best to avoid changing the list in the loop body. To give you an example, what can happen, have a look at the following example:

colours = ["red"]
for i in colours:
    if i == "red":
        colours += ["black"]
    if i == "black":
        colours += ["white"]
print colours

What will be printed by "print colours"?

['red', 'black', 'white']

To avoid these side effects, it's best to work on a copy by using the slicing operator, as can be seen in the next example :

colours = ["red"]
for i in colours[:]:
    if i == "red":
        colours += ["black"]
    if i == "black":
        colours += ["white"]
print colours
Now the output looks like this:
['red', 'black']

We still might have done something, what we shouldn't have done. We changed the list "colours", but our change hasn't had any effect on the loop anymore. The elements to be looped remained the same during the iterations.

So:

colours = ["red"]
for i in colours[:]: # iterates over a copy so we only evaluate what is in the list originally
    if i == "red":
        colours += ["black"]
    if i == "black":
        colours += ["white"]
print(colours) # ["red", "black"]

Now no copy:

colours = ["red"]
for i in colours:
    if i == "red":
        colours += ["black"] # black gets added, colours -> ["red", "black"]
    if i == "black": # now because of ^^, white gets added.
        colours += ["white"]
print(colours) # -> ['red', 'black', 'white']

Now an even worse case:

colours = ["red"]
for i in colours:
    if i == "red":
        colours += ["red"] # red gets add, colours = ["red", "red"]
                          # 2nd iteration, red gets added, colours ->  colours = ["red", "red", "red"]
                          # infinite loop....
    if i == "black":
        colours += ["white"]
print(colours) 

What you are doing is completely different to what the example is trying to show, it is trying to avoid adding elements to the list you are iterating over so you only evaluate elements from the original list not newly added elements. You are trying to create a new list:

colours = ["red"]
new = colours[:] # assign new list/copy to a name.
for i in colours:
    if i == "red":
        new += ["red"] # add to new list
    if i == "black":
        new += ["white"]
print(colours)

You should also append not += :

colours = ["red"]
new = colours[:]
for i in colours:
    if i == "red":
        new.append("red")
    if i == "black":
        new.append("white")
print(colours)