Lists: Holding Many Things

So far each variable held one thing — one name, one number. But what if you have a whole class of names, or a week of temperatures? You could make name1, name2, name3… but that’s miserable. Python has a better way: a list, which holds many values in one variable. Lists are how programs handle data, so this lesson opens a big door toward Phase III.

Making a list

Put values inside square brackets [ ], separated by commas:

 
  

len(...) works on lists too — it tells you how many items are inside. A list can hold strings just as well:


   
   
 
 
  

Grabbing one item by position


Each item has a position number, called an index — and here’s the surprise: counting starts at 0, not 1. So the first item is [0]:


   
   
 
 
  

friends[0] is "Ada", friends[1] is "Linus". This zero-based counting trips up everyone at first — the first item is number 0.


Try it 🎯


    

  1. Make a list of your three favorite foods and print the second one ([1]).
    2. 

  2. Print the last one. (In a 3-item list, that’s index 2.)
    3. 



Adding to a list


A list can grow. .append(...) adds a new item to the end:


   
   
 
 
  

You can start with an empty list [] and build it up:


   
   
 
 
  

Is it in there?


The in keyword checks whether a value is in a list — True or False:


   
   
 
 
  

Predict it 🔮


What does this print? Remember where counting starts:


   
   
 
 
  

(It prints green — index 1 is the second item, because index 0 is red.)


Fix the bug 🐞


This tries to print the third item but crashes with an “index out of range” error. There are 3 items, so the valid indexes are 0, 1, 2:


   
   
 
 
  

(There’s no index 3 — the items are at 0, 1, 2. The third item is pets[2].)


Your mission 🚀


Build a “top 3” list: start with an empty list, append your three favorite movies (or games, or songs), then print the whole list and how many are in it:


   
   
 
 
  

What you learned today


    

  • A list holds many values: [90, 85, 100] or ["Ada", "Linus"].
    • 

  • len(list) counts items; indexes start at 0, so the first item is [0].
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  • .append(x) adds an item to the end.
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  • value in list checks membership (True/False).
    • 



A list is great, but right now you have to read items one by one. Next time, a loop will march through an entire list for you — and that’s where data really comes to life. 🔁
   
 
 
 
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