Practice these examples using Repl.it to become more familiar with some methods commonly applied to lists. As you go through these examples, you should begin to see how powerful Python can be as a programming language.
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.
colors = ['red', 'blue', 'green'] print(colors) ## red print(colors) ## green print(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).
FOR and INPython'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.
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.
squares = [1, 4, 9, 16] sum = 0 for num in squares: sum += num print(sum) ## 30
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.
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.
list = ['larry', 'curly', 'moe'] if 'curly' in list: print('yay')
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 chprints all the chars in a string.
RangeThe 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:
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()).
## print the numbers from 0 through 99 for i in range(100): print(i)
While LoopPython 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 list i = 0 while i < len(a): print(a[i]) i = i + 3
List MethodsHere are some other common list methods.
- list.append(elem) -- adds a single element to the end of the list. Common error: does not return the new list, just modifies the original.
- list.insert(index, elem) -- inserts the element at the given index, shifting elements to the right.
- list.extend(list2) adds the elements in list2 to the end of the list. Using + or += on a list is similar to using extend().
- list.index(elem) -- searches for the given element from the start of the list and returns its index. Throws a ValueError if the element does not appear (use "in" to check without a ValueError).
- list.remove(elem) -- searches for the first instance of the given element and removes it (throws ValueError if not present)
- list.sort() -- sorts the list in place (does not return it). (The sorted() function shown later is preferred).
- list.reverse() -- reverses the list in place (does not return it)
- list.pop(index) -- removes and returns the element at the given index. Returns the rightmost element if index is omitted (roughly the opposite of append()).
Common error: note that the above methods do not *return* the modified list, they just modify the original list.
list = ['larry', 'curly', 'moe'] list.append('shemp') ## append elem at end list.insert(0, 'xxx') ## insert elem at index 0 list.extend(['yyy', 'zzz']) ## add list of elems at end print(list) ## ['xxx', 'larry', 'curly', 'moe', 'shemp', 'yyy', 'zzz'] print(list.index('curly')) ## 2 list.remove('curly') ## search and remove that element list.pop(1) ## removes and returns 'larry' print(list) ## ['xxx', 'moe', 'shemp', 'yyy', 'zzz']
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]
List Build UpOne 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 list list.append('a') ## Use append() to add elements list.append('b')
List SlicesSlices 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']
Source: Google, https://developers.google.com/edu/python/lists#list-methods
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