Table of Contents
- -Python provides an intuitive way to represent a group items, called - Lists. The items of a - List are called its elements. Unlike C/C++, elements can be of any type. A - List is represented as a list of comma-sepated elements with square brackets around them: -
->>> a = [10, 'Python programming', 20.3523, 23, 3534534L] ->>> a -[10, 'Python programming', 20.3523, 23, 3534534L] - - --
The following are some of the most commonly used operations on - Lists. -
-Individual elements of a - List can be accessed using an index to the element. The indices start at 0. One can also access the elements of the - List in reverse using negative indices.: -
->>> a[1] -'Python programming' ->>> a[-1] -3534534L - --
It is important to note here that the last element of the - List has an index of -1. -
-Two or more - Lists can be concatenated using the + operator: -
->>> a + ['foo', 12, 23.3432, 54] -[10, 'Python programming', 20.3523, 'foo', 12, 23.3432, 54] ->>> [54, 75, 23] + ['write', 67, 'read'] -[54, 75, 23, 'write', 67, 'read'] - - --
A - List can be sliced off to contain a subset of elements of the - List. Slicing can be done by using two indices separated by a colon, where the first index is inclusive and the second index is exclusive. The resulting slice is also a - List.: -
->>> num = [1, 2, 3, 4, 5, 6, 7, 8, 9] ->>> num[3:6] -[4, 5, 6] ->>> num[0:1] -[1] ->>> num[7:10] -[7, 8, 9] - --
The last example showed how to access last 3 elements of the - List. There is a small catch here. The second index 10 actually refers to the 11th element of the - List which is still valid, even though it doesn't exist because the second index is exclusive and tells the Python interpreter to get the last element of the - List. But this can also be done in a much easier way using negative indices: -
->>> num[-3:-1] -[7, 8, 9] - --
Excluding the first index implies that the slice must start at the beginning of the - List, while excluding the second index includes all the elements till the end of the - List. A third parameter to a slice, which is implicitly taken as 1 is the step of the slice. It is specified as a value which follows a colon after the second index: -
->>> num[:4] -[1, 2, 3, 4] ->>> num[7:] -[8, 9] ->>> num[-3:] -[7, 8, 9] ->>> num[:] -[1, 2, 3, 4, 5, 6, 7, 8, 9] ->>> num[4:9:3] -[5, 8] ->>> num[3::2] -[4, 6, 8] ->>> num[::4] -[1, 5, 9] - - --
A - List can be multiplied with an integer to repeat itself: -
->>> [20] * 5 -[20, 20, 20, 20, 20] ->>> [42, 'Python', 54] * 3 -[42, 'Python', 54, 42, 'Python', 54, 42, 'Python', 54] - - --
- in operator is used to find whether an element is part of the - List. It returns - True if the element is present in the - List or - False if it is not present. Since this operator returns a Boolean value it is called a Boolean operator: -
->>> names = ['Guido', 'Alex', 'Tim'] ->>> 'Tim' in names -True ->>> 'Adam' in names -False - - --
Length of a - List can be found out using the len function. The max function returns the element with the largest value and the min function returns the element with the smallest value: -
->>> num = [4, 1, 32, 12, 67, 34, 65] ->>> len(num) -7 ->>> max(num) -67 ->>> min(num) -1 - - --
Unlike Strings - Lists are mutable, i.e. elements of a - List can be manipulated: -
->>> a = [1, 3, 5, 7] ->>> a[2] = 9 ->>> a -[1, 3, 9, 7] - - --
An element or a slice of a - List can be deleted by using the - del statement: -
->>> a = [1, 3, 5, 7, 9, 11] ->>> del a[-2:] ->>> a -[1, 3, 5, 7] ->>> del a[1] ->>> a -[1, 5, 7] - - --
In the same way, values can be assigned to individual elements of the - List, a - List of elements can be assigned to a slice: -
->>> a = [2, 3, 4, 5] ->>> a[:2] = [0, 1] -[0, 1, 4, 5] ->>> a[2:2] = [2, 3] ->>> a -[0, 1, 2, 3, 4, 5] ->>> a[2:4] = [] ->>> a -[0, 1, 4, 5] - --
The last two examples should be particularly noted carefully. The last but one example insert elements or a list of elements into a - List and the last example deletes a list of elements from the - List. -
-An - Empty List is a - List with no elements and is simply represented as []. A - None List is one with all elements in it being - None. It serves the purpose having a container list of some fixed number of elements with no value: -
->>> a = [] ->>> a -[] ->>> n = [None] * 10 ->>> n -[None, None, None, None, None, None, None, None, None, None] - - --
As mentioned earlier, a List can contain elements of any data type. This also implies a - List can have a - Lists themselves as its elements. These are called as - Nested Lists. There is no limit on the depth of the - Nested Lists: -
->>> a = [1, [1, 2, 3], 3, [1, [1, 2, 3]], 7] - - --
A method is a function that is coupled to an object. More about objects and its methods are discussed in Advanced Python module. In general, a method is called like:
-object.method(arguments) - --
For now, it is enough to know that a list of elements is an object and so - List methods can be called upon them. Also some of the methods change the - List in-place, meaning it modifies the existing list instead of creating a new one, while other methods don't. It must be noted as we run through the - List methods. -
-Some of the most commonly used - List methods are as follows: -
-The - append method is used to append an object at the end of the list: -
->>> prime = [2, 3, 5] ->>> prime.append(7) ->>> prime -[2, 3, 5, 7] - --
It is important to note that append changes the - List in-place. -
-The - count method returns the number of occurences of a particular element in a list: -
->>> [1, 4, 4, 9, 9, 9].count(9)
-3
->>> tlst = ['Python', 'is', 'a', 'beautiful', 'language']
->>> tlst.count('Python')
-1
-
-
-
-The - extend method extends the list on which it is called by the list supplied as argument to it: -
->>> a = [1, 2, 3] ->>> b = [4, 5, 6] ->>> a.extend(b) -[1, 2, 3, 4, 5, 6] - --
This is an in-place method. This method is equivalent to using the + operator, but using the + operator returns a new list.
-The - index method returns the index position of the element in the list specified as argument: -
->>> a = [1, 2, 3, ,4, 5] ->>> a.index(4) -3 - - --
The - insert method is used to insert an element specified as the second argument to the list at the position specified by the first argument: -
->>> a = ['Python', 'is', 'cool'] ->>> a.insert(2, 'so') ->>> a -['Python', 'is', 'so', 'cool'] - -- -
The - pop method removes an element from the list. The index position of the element to be removed can be specified as an argument to the - pop method, if not it removes the last element by default: -
->>> a = [1, 2, 3, 4, 5] ->>> a.pop() ->>> a -5 ->>> a.pop(2) ->>> a -3 - -- -
The - remove method removes the first occurence of an element supplied as a parameter: -
->>> a = [1, 2, 3, 4, 2, 5, 2] ->>> a.remove(2) ->>> a -[1, 3, 4, 2, 5, 2] - - --
The - reverse method reverses elements in the list. It is important to note here that - reverse method changes the list in-place and doesn't return any thing: -
->>> a = ['guido', 'alex', 'tim'] ->>> a.reverse() ->>> a -['tim', 'alex', 'guido'] - - --
The - sort method is used to sort the elements of the list. The - sort method also sorts in-place and does not return anything: -
->>> a = [5, 1, 3, 7, 4] ->>> a.sort() ->>> a -[1, 3, 4, 5, 7] - --
In addition to the sort method on a - List object we can also use the built-in - sorted function. This function takes the - List as a parameter and returns a sorted copy of the list. However the original list is left intact: -
->>> a = [5, 1, 3, 7, 4] ->>> b = sorted(a) ->>> b -[1, 3, 4, 5, 7] ->>> a -[5, 1, 3, 7, 4] - - --
- Tuples are sequences just like - Lists, but they are immutable. In other words - Tuples provides a way to represent a group of items, where the group of items cannot be changed in any way. The syntax of a - Tuple is also very similar to - List. A - Tuple is represented with the list of items, called elements of the - Tuple separated by comma, with the entire list being enclosed in parenthesis. It is not compulsory to use parenthesis around a - Tuple but it may be necessary in some of the cases: -
->>> a = 1, 2, 3 ->>> a -(1, 2, 3) ->>> b = 1, ->>> b -(1,) - --
It is interesting to note the second example. Just a value followed by a comma automatically makes that an element of a - Tuple with only one element. It is also important to note that, irrespective of input having a parenthesis, the output always has a parenthesis. -
-The first example is also known as - Tuple packing, because values are being packed into a tuple. It is also possible to do - Tuple unpacking which is more interesting. It is better to understand that by example. Say we have a co-ordinate pair from which we need to separate x and y co-ordinates: -
->>> a = (1, 2) ->>> x, y = a ->>> x -1 ->>> y -2 - --
Tuple unpacking also has several other use-cases of which the most interesting one is to swap the values of two variables. Using programming languages like C would require anywhere around 10 lines of code and an extra temporary variable to do this (including all the #include stuff). Python does it in the most intuitive way in just one line. Say we want to swap the co-ordinates in the above example: -
->>> x, y = y, x ->>> x -2 ->>> y -1 - --
There is no need to introduce all the - Tuple operations again, since - Tuples support the following operations that - List supports in exactly the same way: -
- -The following examples illustrate the above operations:
->>> a = (1, 2, 3, 4, 5, 6) ->>> a[5] -6 ->>> b = (7, 8, 9) ->>> a + b -(1, 2, 3, 4, 5, 6, 7, 8, 9) ->>> a[3:5] -(4, 5) ->>> 5 in a -True ->>> c = (1,) ->>> c * 5 -(1, 1, 1, 1, 1) ->>> len(a) -6 ->>> max(a) -6 ->>> min(a) -1 - --
However the following - List operations are not supported by - Tuples because - Tuples cannot be changed once they are created: -
- -Similarity to - Lists leads to the questions like, why not - Lists only? Why do we even want - Tuples? Can we do the same with - Lists? And the answer is - Yes we can do it, but - Tuples are helpful at times, like we can return Tuples from functions. They are also returned by some built-in functions and methods. And also there are some use cases like co-ordinate among other things. So - Tuples are helpful. -
-The following additional syntax are introduced to make it easier to operate on - Lists. -
-The - range function takes at least one argument and 2 additional optional arguments. If two or more arguments are specified, the range function returns a list of natural numbers starting from the first argument passed to it to the second argument. The third argument, if specified is used as a step. Suppose only one argument is specified, then - range function returns a list of natural numbers starting from 0 upto the argument specified: -
->>> range(5, 10, 2) -[5, 7, 9] ->>> range(2, 15) -[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] ->>> range(12) -[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] - --
The - for keyword is used as a part of the looping construct. Unlike for loops in other languages, Python's for is used to iterate through the elements of sequences like - Lists, - Tuples, - Dictionaries, etc. The syntax of the for loop consists of - for, followed by a variable to hold the individual or the current element of the list during iteration and - in, followed by the sequence and a semicolon(':') The next line which is part of the - for loop, i.e the statements that are part of the loop should start with a new intend: -
->>> names = ['Guido', 'Alex', 'Tim'] ->>> for name in names: -... print "Name =", name -... -Name = Guido -Name = Alex -Name = Tim - - --