ThinkPython: Lists

Now we have learned to save values to variables.

Then we can do whatever we want with the variable.

But what if you have a lot of values? You can just make a lot of variables...

thing1 = 'Hello World!'
thing2 = 'hi'
thing3 = 123
thing4 = 3.14
thing5 = 42
thing6 = 'This is yet another thing.'
thing7 = 'Python is fun!'

print(thing1)
print(thing2)
print(thing3)
print(thing4)
print(thing5)
print(thing6)
print(thing7)

...or you can use a list and keep everything in one variable.

things = ['Hello World!', 'hi', 123, 3.14, 42,
          'This is yet another thing.', 'Python is fun!']
for thing in things:
    print(thing)

Read more about lists in ThinkPython here. Skip the chapter "10.7 Map, filter and reduce", and all exercises. You would need to know how to define functions using the def keyword, but we haven't talked about that yet.

Summary

• Lists are a way to store multiple values in one variable. We can create lists by putting whatever we want inside [square brackets], for example, our_list = [] creates an empty list.

• Never do list = .... list is a built-in class, and it's used for converting other values to lists, like list_of_thingy = list(thingy). If we do list = something, then list(thingy) will probably do something else than we want it to do.

• When we have created a list, we can slice it. For example, our_list[2:] results in a new list with everything in the original list except the first two elements. Negative indexes start from the end of the list, for example, our_list[-2:] is a list of the last two elements.

• We can also index lists, our_list[0] is the first element in the list. Non-negative indexes start at zero, and negative indexes start at -1.

• You can assign to indexes and slices like some_list[0] = 'hi', or delete them like del some_list[:2].

• a = b does not create a copy of b.

  >>> a = []
  >>> b = a    # this does not copy anything, see the state diargams
  >>> b += [1, 2, 3]
  >>> a
  [1, 2, 3]
  >>> 

  If you want a copy, use the .copy() list method:

  >>> a = []
  >>> b = a.copy()
  >>> b += [1, 2, 3]
  >>> a
  []
  >>> 

You may use this tutorial freely at your own risk. See LICENSE.

Previous | Next | Back to the list of contents