Python has a great built-in list type named "list". List literals are written within square brackets [ ]. Lists work similarly to strings -- use the len() function and square brackets [ ] to access data, with the first element at index 0. (See the official python.org list docs.)
colors = ['red', 'blue', 'green']print colors ## redprint colors ## greenprint len(colors) ## 3
Assignment with an = on lists does not make a copy. Instead, assignment makes the two variables point to the one list in memory.
b = colors ## Does not copy the list
The "empty list" is just an empty pair of brackets [ ]. The '+' works to append two lists, so [1, 2] + [3, 4] yields [1, 2, 3, 4] (this is just like + with strings).
Python's *for* and *in* constructs are extremely useful, and the first use of them we'll see is with lists. The *for* construct -- for var in list -- is an easy way to look at each element in a list (or other collection). Do not add or remove from the list during iteration.
squares = [1, 4, 9, 16]sum = 0for num in squares:sum += numprint sum ## 30
If you know what sort of thing is in the list, use a variable name in the loop that captures that information such as "num", or "name", or "url". Since python code does not have other syntax to remind you of types, your variable names are a key way for you to keep straight what is going on.
The *in* construct on its own is an easy way to test if an element appears in a list (or other collection) -- value in collection -- tests if the value is in the collection, returning True/False.
list = ['larry', 'curly', 'moe']if 'curly' in list:print 'yay'
The for/in constructs are very commonly used in Python code and work on data types other than list, so you should just memorize their syntax. You may have habits from other languages where you start manually iterating over a collection, where in Python you should just use for/in.
You can also use for/in to work on a string. The string acts like a list of its chars, so for ch in s: print ch prints all the chars in a string.
The range(n) function yields the numbers 0, 1, ... n-1, and range(a, b) returns a, a+1, ... b-1 -- up to but not including the last number. The combination of the for-loop and the range() function allow you to build a traditional numeric for loop:
## print the numbers from 0 through 99for i in range(100):print i
There is a variant xrange() which avoids the cost of building the whole list for performance sensitive cases (in Python 3000, range() will have the good performance behavior and you can forget about xrange()).
Python also has the standard while-loop, and the *break* and *continue* statements work as in C++ and Java, altering the course of the innermost loop. The above for/in loops solves the common case of iterating over every element in a list, but the while loop gives you total control over the index numbers. Here's a while loop which accesses every 3rd element in a list:
## Access every 3rd element in a listi = 0while i < len(a):print a[i]i = i + 3
Here are some other common list methods.
Notice that these are *methods* on a list object, while len() is a function that takes the list (or string or whatever) as an argument.
list = ['larry', 'curly', 'moe']list.append('shemp') ## append elem at endlist.insert(0, 'xxx') ## insert elem at index 0list.extend(['yyy', 'zzz']) ## add list of elems at endprint list ## ['xxx', 'larry', 'curly', 'moe', 'shemp', 'yyy', 'zzz']print list.index('curly') ## 2list.remove('curly') ## search and remove that elementlist.pop(1) ## removes and returns 'larry'print list ## ['xxx', 'moe', 'shemp', 'yyy', 'zzz']
Common error: note that the above methods do not *return* the modified list, they just modify the original list.
list = [1, 2, 3]print list.append(4) ## NO, does not work, append() returns None## Correct pattern:list.append(4)print list ## [1, 2, 3, 4]
One common pattern is to start a list a the empty list , then use append() or extend() to add elements to it:
list =  ## Start as the empty listlist.append('a') ## Use append() to add elementslist.append('b')
Slices work on lists just as with strings, and can also be used to change sub-parts of the list.
list = ['a', 'b', 'c', 'd']print list[1:-1] ## ['b', 'c']list[0:2] = 'z' ## replace ['a', 'b'] with ['z']print list ## ['z', 'c', 'd']