How to avoid side effect in for loop using slicing operator in a list in python?

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.

How to avoid side effect in for loop using slicing operator in a list in python?

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