<p>Learn the fundamentals of Python programming language and how to start using it for finance.​</p>



<p>This course provides an introduction to the python programming language for data science. Python is a general-purpose, versatile, and popular programming language. Python is also a great language for data analysis and has been consistently gaining popularity.</p>



<p>In this course we will help you setup python on your system and introduce the most important programming concepts in Python, such as variables and various data types, how to write functions, how to use methods, and how to use packages and modules in Python. In future courses, we will deep dive into more complex things and apply our knowledge to data science problems.</p>

<p>There is a lot of debate in the industry and amongst data science professionals as to which is the best language for data science. The short answer is that no one language is good and your best bet is to learn R as well as Python. And adding SQL skills to it will significantly increase your chances to become a data scientist in demand. A good skillset to aim for is R + Python + SQL. And of course these skills will be complimented with the tools you use for the specific data science projects you are involved in.</p>



<p>At Finance Train, our objective is to help you with this skill set and combine it with the finance and quants knowledge you need become a financial data scientist.</p>



<p>This course provides an introduction to the python programming language for data science. Python is a general-purpose, versatile, and popular programming language. Python is also a great language for data analysis and has been consistently gaining popularity.</p>



<p>Python has several features that make it well suited for data science:</p>



<ol><li>It is open-source and free to use</li><li>It is very simple to learn and code</li><li>Python brings people with different backgrounds together</li><li>Lots of useful data-science related libraries</li></ol>

Introduction to Python

<p>You can start working with Python by downloading and installing it on your system.</p>



<p>Python itself is available in two versions, namely, 2.x and 3.x versions. Python 2.x is legacy, while Python 3.x is the present and future of the language. While many people swear by Python 2.x versions, the whole development and community effort is moving towards version 3.x. In this course, we will be using python 3.x version.</p>



<h3>Install Python</h3>



<p>You can download Python from python.org (https://www.python.org/downloads/). There are separate downloadables available for different operating systems such as Windows, Mac OS, and Linux. Once downloaded, start the installer by double-clicking it and follow the prompts.</p>



<p>If you face any problem in installation, feel free to discuss any issues in the forum.</p>



<p>OS X and Linux come with Python already installed.</p>



<h3>Install a Text Editor</h3>



<p>When you create python programs, they are saved in files with the extension&nbsp;<code>.py</code>. While you can create Python programs in Notepad or TextEdit, you will find it much easier to read and write the code using a specialized text editor. There are a variety of free editors to choose from such as Notepad++ (Windows), TextWrangler (Mac), or Atom. There are also advanced Integrated Development Environments (IDEs) such as PyCharm which makes working on python projects very easy. To start with, I would suggest to just use Notepad++, Textwrangler, or Atom. Once you've become comfortable with writing python programs and are working on complex projects, you can move to using an IDE. My personal favourite is Atom.</p>



<h2>Python Interface</h2>



<p>Once you've installed Python, you can start using it. To do so, open Command Prompt (Windows) or Terminal (Mac/Linux) and type&nbsp;<code>python</code>. To open Command Prompt in windows, Press&nbsp;<code>Windows key+R</code>&nbsp;to open the&nbsp;<code>Run</code>&nbsp;dialog box, type&nbsp;<code>cmd</code>&nbsp;in the empty box, and then click&nbsp;<code>OK</code>&nbsp;or hit&nbsp;<code>Enter</code>&nbsp;to open it</p>



<p>Python will load and the version number will be displayed. You will be taken to the Python interpreter command prompt, shown as&nbsp;<code>&gt;&gt;&gt;</code>.</p>



<p>Type <code>print("Hello, World!")</code> and press <code>↵ Enter</code>. You should see the text <code>Hello, World!</code> displayed beneath the Python command line.</p>



<h3>Use Python Interpreter as a Calculator</h3>



<p>The best way to start getting used to the python interpretor is to use it as a calculator. You can use Python to perform basic arithmetic with ease. Below are some examples on how to use the calculator functions. Note: <code>#</code> designates comments in Python code, and they are executed by the interpreter.</p>



<pre class="wp-block-code"><code>>>> 2 + 5
7
>>> 100 - 5*3
85
>>> (100 - 5*2) / 3 # Division will always return a floating point (decimal) number
30.0
>>> 25 % 6 # This calculates the remainder of the division
1
>>> 7 ** 3 # Use ** to calculate power (7 raised to the power of 3)
343</code></pre>



<h3>Creating Python Programs</h3>



<p>In Python, you can write your programs and then save them as Python (.py) files which can them be open and executed by the Python interpreter. To understand the files concept, let's write our own small program.</p>



<h4>Windows</h4>



<ol><li>Create a folder in your system where you would like to store your programs. For example, <code>pythondatascience</code>.</li><li>Open up your text editor and create a new file called<code> hello.py</code> containing just this line: <code>print('Welcome to Data Science!')</code></li><li>Save your <code>hello.py</code> program in folder you just created.</li><li>In the Start menu, select <code>"Run...",</code> and type in cmd. This will cause the Windows terminal to open. Type <code>cd \pythondatascience </code>to change directory to your <code>pythondatascience</code> folder, and hit Enter.</li><li>Type <code>python hello.py</code> to run your program!</li></ol>



<h4>Mac</h4>



<ol><li>Create a folder on your computer to use for your Python programs. For example, name it <code>pythondatascience</code> and place it in your Home folder (the one that contains folders for Documents, Movies, Music, Pictures, etc).</li><li>Open up your text editor and create a new file called <code>hello.py</code> containing just this line:<code>print('Welcome to Data Science!')</code></li><li>Save your <code>hello.py</code> program in the folder you just created.</li><li>Open the Applications folder, go into the Utilities folder, and open the Terminal program.</li><li>Type <code>cd pythondatascience</code> to change directory to your <code>pythondatascience</code> folder, and hit Enter.</li><li>Type <code>python ./hello.py</code> to run your program!</li></ol>



<h3>Result</h3>



<p>The program should print:</p>



<pre class="wp-block-code"><code>Welcome to Data Science!</code></pre>



<div class="wp-block-image"><figure class="aligncenter size-large"><img src="https://financetrain.com/wp-content/uploads/27206-python-file-run.png" alt="" class="wp-image-27492"/></figure></div>



<p>Congratulations! You've written your first Python program.</p>



<h3>Exercises</h3>



<ul><li>Modify the <code>hello.py</code> program to greet someone.</li><li>Modify the program to print two statements instead of one.</li><li>Modify the program to print the statement <code>'The sum of 3 and 7 is: '</code> and then write the math formula <code>3+7</code> to print the final</li></ul>



<p><strong>Note: </strong>In Python 3+, You need to put the calculation within the print bracket, as shown below: print ("The sum of 3 and 5 is: ", (3+5)) In Python 2+, there are no brackets. So, it will be like below: print "The sum of 3 and 5 is:" + str(3+5)</p>

Setting up Python on your Computer

<p>In Python, a variable is a case-sensitive name that can hold a value. A variable can hold all kinds of values such as numbers, strings, etc. Below is an example of a variable storing a number.</p>



<pre class="wp-block-code"><code>>>> price = 200
>>> print(price)
200</code></pre>



<p>The above code creates a variable called&nbsp;<code>price</code>&nbsp;and assigns the integer value&nbsp;<code>200</code>&nbsp;to it. We can ask Python to tell us what is stored in the variable&nbsp;<code>price</code>&nbsp;using the<code>print</code>&nbsp;statement and it will return the number&nbsp;<code>200</code>.</p>



<p>We can change the value stored in the variable by just reassigning a new value to it.</p>



<pre class="wp-block-code"><code>>>> price = 200
>>> print(price)
200
>>> price = 300
>>> print(price)
300</code></pre>



<h3>Variables in Calculations</h3>



<p>We can use variables in calculations instead of directly using values. Let's take an example. Let's say you have $100 in savings and you put it in a bank account that pays a simple interest of 6% per year. We can create a formula in Python that can help us calculate the simple interest you will earn on your savings.</p>



<pre class="wp-block-code"><code>p = 100
r = 6
t = 2
I = p *(r/100) * t
print(I)</code></pre>



<ul><li>p is the principal amount, $100.00.</li><li>r is the interest rate, 6% per year, or in decimal form, 6/100=0.06.</li><li>t is the time involved, 2 years time periods.</li></ul>



<p>Save this script in your folder with the name&nbsp;<code>simpleinterest.py</code>. Run the script on the command prompt using&nbsp;<code>python simpleinterest.py</code></p>



<p>It will produce the following result:</p>



<pre class="wp-block-code"><code>C:\pythondatascience>python simpleinterest.py
12.0</code></pre>



<h2>Common Data Types</h2>



<p>The most common data types in Python are:</p>



<ul><li><code>int</code>, or integer. These are positive or negative whole numbers with no decimal point</li><li><code>float</code>, or floating point. They are real numbers and are written with a decimal point between the integer and fractional parts.</li><li><code>str</code>, or string. A string holds any combination of letters and numbers you would like it to hold. Use double or single quotes to build strings.</li><li><code>bool</code>, or boolen. Python provides the boolean type that can be either set to False or True.</li></ul>



<p>Below are a few examples:</p>



<pre class="wp-block-code"><code>>>> course = "Financial Data Science"  # String example
>>> print (course)
Financial Data Science
>>> age = 7.5 # Float example
>>> print(age)
7.5
>>> score = 103 # Integer example
>>> print(score)
103
>>> valuable = False # Boolen example (True or False)
>>> print(valuable)
False</code></pre>



<p>You can check that these variables have the correct data type using the <code>type()</code> function.</p>



<pre class="wp-block-code"><code>>>> type(course)
&lt;class 'str'>
>>> type(age)
&lt;class 'float'>
>>> type(score)
&lt;class 'int'>
>>> type(valuable)
&lt;class 'bool'>
>>></code></pre>



<h2>Operations on Data Types</h2>



<p>We have already seen that we can perform math operations on integer variable. Python behaves differently when you perform these operations on other data types.</p>



<p><strong>Adding Two Strings</strong></p>



<p>If you try to add two strings, Python will concatenate the two strings as shown below:</p>



<pre class="wp-block-code"><code>>>> the_text = "The " + "Quick " + "Brown " + "Fox"
>>> print(the_text)
The Quick Brown Fox</code></pre>



<p><strong>Adding Two Boolens</strong></p>



<p>Internally, Python treats <code>True</code> as 1 and <code>False</code> as 0. So, if you perform any mathematical calculation on boolens, it will do the operation on their numeric counterpart.</p>



<pre class="wp-block-code"><code>>>>True + True # Think 1 + 1
2
>>>True + False # Think 1 + 0
1
>>>False + False # Think 0 + 0
0</code></pre>



<p>For Boolen variables, only the words&nbsp;<code>True</code>&nbsp;and&nbsp;<code>False</code>&nbsp;are recognized. Other variations, such as&nbsp;<code>TRUE</code>,&nbsp;<code>true</code>, or&nbsp;<code>T</code>&nbsp;are incorrect.</p>



<p><strong>Adding Variables of Different Types</strong></p>



<p>What if we try to add variables of different types, for example, string and integer?</p>



<pre class="wp-block-code"><code>>>> text = "My total portfolio value is: "
>>> value = 5000
print(text+value)</code></pre>



<p>As you can guess, this is not possible and will throw an error. If we want to add these two things, we can convert <code>value</code> to string type using the <code>str()</code> function as shown below:</p>



<pre class="wp-block-code"><code>>>> text+str(value)
'My total portfolio value is: 5000'
>>></code></pre>



<p>You can use the functions str(), int(), float() and bool() to convert Python values into any type.</p>



<h2>Exercise</h2>



<ul><li>Create the simple interest formula as we did in the above example, and present the results in a nicely formatted way, e.g., "I will earn an interest of 12".</li><li>Create a&nbsp;<code>boolen</code>&nbsp;variable with value&nbsp;<code>False</code>. Then convert it into a string, integer, and float and print the results. Notice the difference in the formats.</li></ul>

Variables in Python

<p>Lists are the most significant and flexible sequence types in Python. A list in Python is simply an ordered collection of items each of which can be of any type. A list is a dynamic mutable data structure which means that items can be added to and deleted from it.</p>



<p>List constants are surrounded by square brackets and the elements in the list are separated by commas. The individual elements of the list can be accessed using the square brackets by specifying the item's index. The first element is at index 0. Indexes range from 0 to n-1 (where n is the length of the list) and from -1 to -n.</p>



<p>Lists are heterogeneous, i.e., values can be of any type (strings are homogeneous because their elements are characters).</p>



<p>The following is an example of a list and how we can access elements of the list.</p>



<pre class="wp-block-code"><code>>>> vals = &#91;1, 2, 3, 4, 5, 6]    # Define a new list
>>> vals            # Print List
&#91;1, 2, 3, 4, 5, 6] 
>>> vals&#91;0]            # Print first item in list
1
>>> vals&#91;3]            # print fourth item in list
4
>>> i = 2            # Create a Variable
>>> vals&#91;2]            # Pass the index value using the variable
3
>>></code></pre>



<p>An index value of −1 is used to reference the last item of a list and an index value of −2 is used to reference the previous to last item of the list. Examples below:</p>



<pre class="wp-block-code"><code>>>> vals&#91;-1]        # Access last item of the list
6
>>> vals&#91;-2]        # Access second last item of the list
5
>>></code></pre>



<h3>Lists are Mutable</h3>



<p>Lists are mutable, which means that the items of the list can change value by performing assignment on them. In the following example, we assign a new value of 12 to item at index 3.</p>



<pre class="wp-block-code"><code>>>> vals
&#91;1, 2, 3, 4, 5, 6]
>>> vals&#91;3] = 12    # Assign new value to item at index 3
>>> vals        # Notice the change in value of vals&#91;3]
&#91;1, 2, 3, 12, 5, 6]
>>></code></pre>



<h2>Slicing Operations</h2>



<p>The slicing operations are used to access a sublist of the list. The colon notation is used to specify the range of index values of the items. The first index value is written before the colon and the last index value is written after the colon. This indicates the range of index values from the start index value (inclusive) up to but not including the last index value (exclusive) specified.</p>



<p>In the following example, we first define a list called&nbsp;<code>mylist</code>&nbsp;which contains 6 elements. The second command specifies a sublist of list&nbsp;<code>mylist</code>, that includes the items starting with index value 0 up to but not including the item with index value 4. The third command assigns the sublist mylist[2:5] to variable&nbsp;<code>mysublist</code>.</p>



<pre class="wp-block-code"><code>>>> mylist = &#91;1,2,3,4,5,6]
>>> mylist&#91;0:4]
&#91;1, 2, 3, 4]
>>> mysublist = mylist&#91;2:5]
>>> mysublist
&#91;3, 4, 5]</code></pre>



<p>In a list, we can update a range of items using slicing and assignment. For example, the following command changes the values of items with index 0 and up to but not including the item with index value 2 with another set of values.</p>



<pre class="wp-block-code"><code>>>> mylist&#91;0:2]=mylist&#91;1:3]
>>> mylist
&#91;2, 3, 3, 4, 5, 6]</code></pre>



<p>Using slicing, we can leave the second index value which implies that the range will be upto the last item. Similarly, we can leave the first index value which implies that the range of items starts with the first item of the list.</p>



<pre class="wp-block-code"><code>>>> mylist&#91;2:]
&#91;3, 4, 5, 6]
>>> mylist&#91;:4]
&#91;2, 3, 3, 4]</code></pre>



<p>Sometimes you may need to count the number of items in a list. This can be done using the&nbsp;<code>len()</code>&nbsp;method as shown below:</p>



<pre class="wp-block-code"><code>>>> len('mylist')
6
>>></code></pre>



<p>Just like the&nbsp;<code>len()</code>&nbsp;method, all Python objects have many methods attached to them which allow us to perform various actions on the objects. We will learn about these methods in the coming lessons.</p>

Lists in Python

<p>In Python a function is some reusable code that takes arguments(s) as input does some computation and then returns a result. We have already seen functions in previous lessons. For example, the&nbsp;<code>print()</code>&nbsp;command that we used is actually a function that allows us to print something.</p>



<p>In Python, there are two kinds of functions:</p>



<ul><li>Built-in functions that are provided as part of Python -&nbsp;<code>print(), type(), float(), int() ...</code></li><li>Functions that we define ourselves and then use</li></ul>



<p>The built-in function names are treated as "new" reserved words, and should not be used as variable names. We can define a new function using the&nbsp;<code>def</code>&nbsp;reserved word (We will see this soon).</p>



<p>We call/invoke the function by using the function name, parenthesis and arguments in an expression.</p>



<h3>Example 1 - Max()</h3>



<p>The following is an example of the in-built <code>max()</code> function in Python. The <code>max()</code> returns the largest of its arguments.</p>



<pre class="wp-block-code"><code>>>> max("hello world")  #Returns the largest letter in the string
'w'
>>> mylist = &#91;1,2,3,4,5,6] # returns the largest item in the list
>>> max(mylist)
6
>>></code></pre>



<h3>Example 2 - Round()</h3>



<p>Let's take another example of an in-built function <code>round()</code>. The <code>round()</code> function rounds a given number to specific digits from the decimal point. The round function takes as first argument the number and the second argument is the precision. In the following example, the number <code>4.758</code> is rounded to two decimal points.</p>



<pre class="wp-block-code"><code>>>> round(4.758,2)
4.76</code></pre>



<h3>Getting Help on Functions</h3>



<p>While there are many ways to get help on Python functions (including Google), the first thing you should do when you want to know about a particular function is to use the inbuild <code>help()</code> function. The following example shows how we can get help on the <code>round()</code> function using the <code>help()</code> function.</p>



<pre class="wp-block-code"><code>>>> help(round)
Help on built-in function round in module builtins:
round(...)
    round(number&#91;, ndigits]) -> number
    Round a number to a given precision in decimal digits (default 0 digits).
    This returns an int when called with one argument, otherwise the
    same type as the number. ndigits may be negative.</code></pre>



<h2>Finding a Function</h2>



<p>You now know that you can use the Python functions such as&nbsp;<code>max()</code>&nbsp;and&nbsp;<code>round()</code>. But how do you know that a function exists in the first place. As you will work more and more with Python code, you will learn more about a variety of functions. Whenever you want to perform a specific task, it's a good idea to do a search on the internet to know if a function already exists (there's a high chance it is actually does). If a function does exist, your first choice should be to use it rather than writing your own code. You can also ask the Finance Train community for your requirements.</p>



<h2>Exercises</h2>



<ul><li>Define a list with six numbers and assign it to variable&nbsp;<code>myvar</code>. Use the&nbsp;<code>type()</code>&nbsp;function to find the variable's type and print it. Try this both in interactive and file mode. Note that if you're creating a file, you will have to use the&nbsp;<code>print()</code>&nbsp;function to print the results.</li><li>Use the&nbsp;<code>len()</code>&nbsp;function to get and print the length of the list.</li><li>Get the 2nd element of the list and convert it to a float using the&nbsp;<code>float()</code>&nbsp;function.</li><li>Get help on&nbsp;<code>sorted()</code>&nbsp;function using the&nbsp;<code>help()</code>&nbsp;function. Use the&nbsp;<code>sorted()</code>&nbsp;function to sort the list we created above.</li></ul>

Functions in Python

<p>In Python, we define a function using the&nbsp;<code>def</code>&nbsp;reserved word followed by optional parameters in parenthesis. The body of the function is indented (very important!).</p>



<p>This defines the function but does not execute the body of the function.</p>



<p>In the below example, we define a new function called <code>greet()</code> which will print <code>Hello Dolly</code> when called.</p>



<pre class="wp-block-code"><code>def greet():
    print("Hello Dolly")</code></pre>



<p>Once we have defined a function, we can call (or invoke) it as many times as we like.</p>



<pre class="wp-block-code"><code>greet()</code></pre>



<p>Calling the function executes the function's body, in our case, it prints the text "Hello Dolly".</p>



<pre class="wp-block-code"><code>Hello Dolly</code></pre>



<h3>Function Arguments</h3>



<p>Often a function will take its arguments, do some computation and return a value to be used as the value of the function call in the calling expression. The return keyword is used for this.</p>



<p>In the above example, we had a fixed output from the function ("Hello Dolly"). We can customize this using arguments. In the following revised code, we update the greeting based on the language specified at the time of calling the function:</p>



<pre class="wp-block-code"><code>def greet(lang):
    if lang == 'es':
        return 'Hola'
    elif lang == 'fr':
        return 'Bonjour'
    else:
        return 'Hello'</code></pre>



<p>There are a few important things to notice here:</p>



<ul><li>The function has one parameter&nbsp;<code>lang</code>. A parameter is a variable which we use in the function definition that is a "handle" that allows the code in the function to access the arguments for a particular function invocation.</li><li>We use the&nbsp;<code>if</code>&nbsp;condition to decide what the function should return depending on the argument supplied. If the argument passed is&nbsp;<code>es</code>, then the function will return&nbsp;<code>Hola</code>. If the argument is&nbsp;<code>fr</code>, then the function will return&nbsp;<code>Bonjour</code>&nbsp;and so on.</li><li>Notice that the function 'returns' a value not 'print' it, i.e., the value will be supplied back to the calling function and then the calling function can do whatever it wants with it.</li></ul>



<p>Now that the function is defined, we can call it in the way we like:</p>



<pre class="wp-block-code"><code>print(greet('es'),"Dolly")
print(greet('fr'),"Dolly")
print(greet('en'),"Dolly")</code></pre>



<p>These calls will output the following:</p>



<pre class="wp-block-code"><code>Hola Dolly
Bonjour Dolly
Hello Dolly</code></pre>



<h3>Multiple Arguments</h3>



<p>A function can take multiple arguments. To do so, we define more than one parameter in the function definition. While calling the function, we simply add more arguments. The number and order of arguments and parameters must match. The following custom function calculates the area of a rectangle when its length and breadth are passed to it:</p>



<pre class="wp-block-code"><code>def rectangle_area(l,b):
    area = l*b
    return area</code></pre>



<p>We can then call the function along with print to output the results.</p>



<pre class="wp-block-code"><code>print(rectangle_area(5,4))</code></pre>



<p>This will print '20'.</p>



<p>While writing programs, you will often think about whether to convert a part of your code into a function or not. There are a few guiding principles to help you decide whether to create a function or not to.</p>



<ul><li>Don't repeat yourself - make it work once and then reuse it</li><li>If something gets too long or complex, break up logical chunks and put those chunks in functions</li></ul>



<h3>Exercises</h3>



<ul><li>Make a function&nbsp;<code>reverse_string</code>&nbsp;that, given a string, returns that string in reverse.</li><li>Make a function that returns 'apples' if given a string, 'oranges' if given an integer, and 'bananas' if given anything else.</li></ul>

Define Custom Functions in R

<p>We earlier learned about various data types in Python such as <code>int</code>,<code>float</code>, and <code>str</code>. We also learned about other data structures such as lists. When we create a variable with an integer value, or a list with some elements, what we are actually creating is a python object of that type.</p>



<pre class="wp-block-code"><code># a is a python object of type integer 
a = 15
# b is a python object of type string
b = "Data Science"
# c is a python object of type list
c = &#91;"GOOG","MSFT","INTL"]</code></pre>



<p>We also learned about built-in functions in Python. For example, we can use the&nbsp;<code>max()</code>&nbsp;function to return the maximum value, or the&nbsp;<code>sum()</code>&nbsp;function to calculate the sum of numbers. Similarly, we also saw that we can create our own custom functions.</p>



<p>Python also has special types of functions which define some characteristic of an object. These are called Methods. Each type of object will have its own set of methods that can be called upon those objects. For example, if you have a string object, you can use the string methods to perform specific tasks on your string object. Two examples are the&nbsp;<code>capitalize()</code>&nbsp;method and the&nbsp;<code>replace()</code>&nbsp;method. Similarly, a list object will have its own set of methods available to it, for example&nbsp;<code>index()</code>,<code>count()</code>&nbsp;and&nbsp;<code>append()</code>.</p>



<h3>Examples - String Methods</h3>



<p>Below are some examples showing the application of string methods.</p>



<pre class="wp-block-code"><code># course is a string object 
course = "data science"
# The capitalize() method will capitalize the string
course.capitalize()
# the upper() method will make the string uppercase
course.upper()
# The replace() method will replace a part of the string with new str.replace(old, new&#91;, max])
# the word "science" will be replaced with "analytics"
course.replace("science", "analytics")</code></pre>



<h3>Examples - List Methods</h3>



<pre class="wp-block-code"><code># Define a new list
prices = &#91;25,67,56,78,89]
#The index method gets the index of the element of a list that matches its input
#In the following example, the index for the number 56 will be 2.
prices.index(56)
#The append method adds new elements to the list
# After appending the new list will be &#91;25,67,56,78,89,56]
prices.append(56)
#The count method gets the number of times an element appears in a list
#Our list contains the number 56 two times
prices.count(56)
#The remove() method removes the first element of a list that matches the input
#After removing the new list will be &#91;67,56,78,89,56]
prices.remove(25)
#The reverse() method reverses the order of the elements in the list
prices.reverse()
#After reversing the list would have been reversed &#91;56,89,78,56,67]
print(prices)</code></pre>



<h3>Exercise</h3>



<ul><li>Create a list with the names of your favourite stocks.</li><li>Use the&nbsp;<code>append()</code>&nbsp;method to add one more stock and print the updatd list</li><li>Use the&nbsp;<code>index()</code>&nbsp;method to find the index of any stock of your choice.</li></ul>

Methods in Python

<p>We learned that Python functions and methods are really useful and can be used to solve many of our problems. We can even take functions written by others to solve our problems. When new functions and methods are written, they can be standardized and distributed so that a large numebr of Python developers can use them in their projects. However, it's not a good idea to distribute all these functions as a part of the core Python distribution. This is where packages come into play. A Python package is like a directory of Python scripts. Each script is a module. Modules in Python are simply Python files with the .py extension, which implement a set of functions. Each Module is designed to solve a particular set of problems.</p>



<p>Python is heavily dependent on it’s vast array of packages. The Python Package Index lists 90k+ packages available for download. Programmers can also create their own packages and make them available through Python Package Index from where others can download and use them.</p>



<p>Packages are used by importing the package at the beginning of your script, which allows you to then access</p>



<p>their specialized functions or objects. Below is a short list of the most widely used packages:</p>



<ul><li><strong>NumPy:</strong>&nbsp;"the fundamental package for scientific computing with Python," the source of most linear algebra functions, data import functions, and general computation</li><li><strong>SciPy:</strong>&nbsp;builds on top of Numpy to include more specific, higher level functionality including image processing, statistics, and optimization</li><li><strong>Matplotlib:</strong>&nbsp;the primary way to generate figures in Python, replicates much of the functionality of plotting in MATLAB (hence the name)</li></ul>



<p>Again, things are very google-able if you’re looking for something specific, there’s almost certainly a python package somewhere that has it.</p>



<h3>Using Inbuilt Modules</h3>



<p>Python already has some very useful inbuilt modules. These modules are considered to be a part of Python's standard environment. An example of an inbuilt module is the <code>math</code> module which contains many useful math functions. To use a module, we need to first import it. Following are some examples of the <code>math</code> package.</p>



<pre class="wp-block-code"><code>import math
#Calculate square root of the number 25
x = math.sqrt(25)
print(x)
#We can calculate the area of a circle using the formula πr^2
#We can access the constant π using math.pi
r = 5
Area = math.pi*5**2
print(Area)</code></pre>



<p>Apart from&nbsp;<code>math</code>, some other useful inbuilt modules are listed below:</p>



<ul><li>sys - contains tools for system arguments, OS information etc.</li><li>os - for handling files, directories, executing external programs</li><li>re - for parsing regular expressions</li><li>datetime - for date and time conversions etc.</li><li>csv - for reading CSV tables</li><li>and many more</li></ul>



<h3>Different Ways of Importing</h3>



<p>Depending on our needs, we can import a module in different ways.</p>



<h4>import x</h4>



<p>You can import a module by it's name like <code>import math</code>. When we import a module X in this manner, we need to use X.name to refer to an item called name that is defined in the module X.</p>



<pre class="wp-block-code"><code>import math
math.sqrt(25)
math.pi</code></pre>



<h4>from X import *</h4>



<p>In this case, you can directly refer to items in the module X, without using the “X.” prefix.</p>



<pre class="wp-block-code"><code>from math import *
sqrt(25)</code></pre>



<h4>from x import y</h4>



<p>This is useful, if there is an object you specifically use frequently and would like to make it a part of your main namespace. So, only object y will be imported, nothing else.</p>



<pre class="wp-block-code"><code>from math import sqrt
print(sqrt(25))</code></pre>



<h4>Aliases for a package</h4>



<p>If you have decided to access a module's objects from its own namespace, you can choose to alias the module with a name. For example the module <code>math</code> can be given alias <code>m</code>.</p>



<pre class="wp-block-code"><code>import math as m
print(m.sqrt(25))</code></pre>



<h3>Installing Third-party Package</h3>



<p>If a module is not inbuilt, it can be installed as a part of a third-part package.</p>



<p>We use <code>pip</code> a Python package management system to install and manage software packages written in Python. If you have installed Python 3 from python.org, you will already have <code>pip</code> installed along with it.</p>



<pre class="wp-block-code"><code>Installing PIP
Visit http://pip.readthedocs.org/en/stable/installing/
Download get-pip.py
On terminal execute the command python get-pip.py</code></pre>



<p>To install a package, you will use the command <code>pip install &lt;i>Package Name&lt;/i></code>. For example, you can install the <code>numpy</code> package using the following command.</p>



<pre class="wp-block-code"><code>pip install numpy</code></pre>



<p>This will download and install the <code>numpy</code> package. Once the package is installed, you can import it in your Python program to start using it.</p>

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