Python + and * operators.

Question

Hello,

I'm working my way through some code examples and I stumbled upon this:

endings = ['st', 'nd', 'rd'] + 17 * ['th'] + ['st', 'nd', 'rd'] + 7 * ['th']
+ ['st']

I understand that for numbers after 4 and until 20 they end in 'th' and I can see that we are adding 17 more items to the list, and I understand that '17 * ['th'] is adding 'th' to the list 17 times, however, I don't understand how this works.

Can you shed some light on this?

Answer 1

Consider twentieth twenty first twenty second twenty third ... thirtieth thirty first

Or is it something else that you don't understand about it?

Answer 2

17 * ['th'] generates ['th', 'th', ..., 'th'] (17 items).

In addition it's worth noting 2 behaviours:

• That this is only really useful because the contents 'th' is immutable (unless of course you never intended to modify the ending list).
• The list object ['th'] is only created once, however it is extended by iterating over the original copy 17 times, appending each entry to the final ['th', ...] list. This in turn is merged with the surrounding endings via the + operator.

I don't normally shed my light. Only about once every 6 months. If you see it lying about don't tell anyone it's mine.

Answer 3

The + operator returns the 'sum' of 2 list, or both of them concatenated together. The * operator returns a list added to itself X times.

http://www.linuxtopia.org/online_books/programming_books/python_programming/python_ch14s03.html

Answer 4

The part of the code 17 * ['th'] creates a list with 17 items that are all 'th' and the + operator concatenates the list together, so ['st', 'nd', 'rd'] + 17 * ['th'] would become ['st', 'nd', 'rd', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th']

Answer 5

When multiplying a list, you're creating a new list containing the elements of the list that many times. In this case, 17 * ['th'] creates a list containing seventeen strings 'th'. When adding lists together, you're creating a new list containing the elements of all operands.

>>> a = [1, 2]
>>> a * 2
[1, 2, 1, 2]

>>> a = ['th']
>>> b = ['st']
>>> 3 * a + b
['th', 'th', 'th', 'st']

Answer 6

That makes the following list:

endings = [ "st", "nd", "rd", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "th", "st", "nd", "rd", "th", "th", "th", "th", "th", "th", "th" ]

So if you want to write "21st", do

"{0}{1}".format( 21, endings[ 20 ] )

Notice that the list is off by one, since endings[0] is the first element.

Answer 7

Additionally, you can override the operators of your own objects e.g.

#!/usr/bin/env python
# encoding: utf-8

class SomeClass(object):
    def __init__(self, v):
        self.value = v

    def __add__(self, b):
        return self.value + b.value

def main():
    a = SomeClass(1)
    b = SomeClass(2)
    print a + b


if __name__ == '__main__':
    main()

see http://docs.python.org/library/operator.html for more details