Python + and * operators

I'm working my way through some code examples and I开发者_C百科 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?

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.

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

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']

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']

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.

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

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

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