Learn Python in 7 Days

One of the most famous platforms where Python is extensively used is YouTube. The other places where you will find Python being extensively used are the special effects in Hollywood movies, drug evolution and discovery, traffic control systems, ERP systems, cloud hosting, e-commerce platform, CRM systems, and whatever field you can think of.

At the time of writing this book, two main versions of the Python programming language were available in the market, which are Python 2.x and Python 3.x. The stable release as of writing the book were Python 2.7.13 and Python 3.6.0.

Major implementations include CPython, Jython, IronPython, MicroPython, and PyPy.

Python 2.x can be downloaded from https://www.python.org/downloads. The installer is simple and easy to install. Perform the following steps to install the setup:

These days most of the Linux-based systems come preloaded with Python, so in most cases, you do not need to install it separately. However, if you do not find your desired version of Python on the Linux platform, you can download your desired version for a particular Linux platform from the site https://www.python.org/downloads/source/. Perform the following steps:

      [root@localhost Python-2.7.12]# ./configure
      [root@localhost Python-2.7.12]# make
      [root@localhost Python-2.7.12]# make install

For Mac OS, you will get the installer from the site https://www.python.org/downloads/mac-osx/. 

For the new Mac OS X, Sierra comes loaded with Python 2.7 and it's good for learning but the same cannot be used to develop advanced-level programs. You do not need to separately install Python on the new Mac OS X. However, a word of caution is that the version of Python packed with the Mac OS could be out of date and might require updating. If you still want to install a separate version, you can follow the simple steps mentioned as follows:

      /usr/bin/ruby -e "$(curl -fsSL
      https://raw.githubusercontent.com/Homebrew/install/master/install)"

      $ brew install python

Every language understands a file format, for example, like the C language file extension is .c likewise java language has a file extension .java. The Python file extension is .py while bytecode file extension is .pyc.

Python interactive shell is also known as Integrated Development Environment (IDLE). With the Python installer, two interactive shells are provided: one is IDLE (Python GUI) and the other is Python (command line). Both can be used for running simple programs.

For complex programs and executing large files, the windows command prompt is used, where after the system variables are set automatically, large files are recognized and executed by the system.

The preceding screenshot is what we call Python IDLE, which comes bundled with the Python installation. The next screenshot is of the command line that also comes bundled with the Python installation, or we can simply launch the Python command through the windows command line and get Python command line. For most of our programming instructions, we will be using the Python command line:

If you remember the installation steps explained earlier, you might be still thinking what are system variables? They are a set of predefined variables, which are available to all programming units. If it's not set, then each and every time you want to run your program or execute a Python file, you will have to manually provide path for each Python executable, but if python.exe is set as a system variable, then the system automatically recognizes your programming instruction and starts executing.

Setting environment variables in Windows

Here, if python.exe is not provided to the path variable, then the system does not recognize python as a command, as shown in the following screenshot:

The Windows command prompt does not recognize python as shown in the previous screenshot. Once Python has been updated in the path variables or system variables, the windows command line recognizes the python command and executes as shown here: 

 

During installation, the installer provides an option to set system variables, which we have seen in our installation steps. But in case you missed that out, you need not worry. You can manually set the system variables. Simply follow these steps:

1. Just right-click on My Computer (older version of Windows PC) or Computer (win 7 onwards) and select Properties. Once you have selected the properties, you will get the following screenshot where you need to select Advanced system settings:

2. After you have clicked on Advanced system settings, you will get System Properties. Click on the Environment Variables button. 
3. After completing Step 2, a window will pop up. From the System variables pane, select Path and click on the Edit button.
4. Your Python installation will look something like the following screenshot, where you need to right-click on the address bar and select the Copy address as text option:

 

 

5. Add the Python installation folder path to the Path variable.
6. Click on OK and then again on OK to close all the windows. Our system variable is set:

$ sudo edit /etc/launchd.conf
setenv PYTHONPATH /Users/vesper/temp:/Users/vesper/misc

In Python, there are two types of comments--one is a single-line comment and the other is multiline comment. For a single-line comment, # is used, while for a multiline comment, triple quotes """ are used:

#This is a single line comment in Python

print "Hello World" #This is a single comment in Python

""" For multi-line
comment use three
double quotes
...
"""
print "Hello World!"

Python doesn't care if you use single quotes or double quotes to print a single statement. But, surely, both has some significance while printing complex statements, which we will see soon.

print "Hello World!" and print 'Hello World!' will give the same output Hello World! two times:

How will you print something like this:

I am mad in love  do you think  I am doing the right thing? One way is to enclose the complete thing within the triple quotes as shown here:

print '''I am mad in love
                     do you think
                                 I am doing
                                           the right thing '''

Alternatively, you can also use double quotes three times to achieve the same thing:

print """I am mad in love
                     do you think
                                 I am doing
                                                                                                                             the right thing """

Let's try another example. What should be the outcome of the following statement?

print 'Hey there it's a cow'

The preceding piece of code gives the following results:

C:pydev>python hello.py
File "hello.py", line 1
print 'Hey there it's a cow'
^
SyntaxError: invalid syntax

Python simply interprets that the statement terminated with a single quote after it. The solution is to enclose the complete sentence within double quotes as shown:

print "Hey there it's a cow"

Adding double quotes (") gives an error-free output as shown:

C:pydev>python hello.py
Hey there it's a cow

The Python back slash is used for continuation of the print statement. You can stretch a single statement across multiple lines:

print "Hello
  world "

This gives the following output:

C:pydev>python hello.py
Hello world

For printing a string, either a pair of single (' ') quotes or pair of double quotes (" ") can be used as shown in the succeeding examples: 

print "Hello World 'Mr' Bond"
print 'old world "but" still good'

This gives the following results:

C:pydev>python hello.py
Hello World 'Mr' Bond
old world "but" still good

The escape sequence is used to insert the tab, the newline, the backspace, and other special characters into your code. They give you greater control and flexibility to format your statements and code:

print 'a' 
print 'tHermit' 
print "i know , they are 'great'"

The output is as follows:

C:pydev>python hello.py
Hermit
i know , they are 'great'

The preceding code executes with a beep sound. If you did not hear the beep sound, check your speakers.

Two strings can be joined using the + operator:

print "Only way to join" + "two strings"

The following is the output of the preceding code:

C:pydev>python string_concatenation.py
Only way to join two strings

Consider an example where you would want to print the name, marks, and the age of the person:

print "Name", "Marks", "Age" 
print "John Doe", 80.67, "27" 
print "Bhaskar", 76.908, "27"
print "Mohit", 56.98, "25" 

The output will be as follows:

C:pydev>python hello.py
Name Marks Age
John Doe 80.67 27
Bhaskar 76.908 27
Mohit 56.98 25

You can see the output, but the output that is displayed is not formatted. Python allows you to set the formatted output. If you have done some coding in C language, then you should be familiar with %d, %f, %s. In order to represent an integer %d is used, %f is used for float, and %s is used for string. If you used %5d, it means 5 spaces. If you used %5.2f, it means 5 spaces and .2 means precision. The decimal part of the number or the precision is set to 2. Let's use the formatting on the preceding example:

print "Name Marks Age" 
print ( "%s %14.2f %11d" % ("John Doe", 80.67, 27)) 
print ( "%s %12.2f %11d" %("Bhaskar" ,76.901, 27))
print ( "%s %3.2f %11d" %("Mohit", 56.98, 25)) 

The output we get is as follows:

C:pydev>python hello.py
Name Marks Age
John Doe 80.67 27
Bhaskar 76.90 27
Mohit 56.98 25

The preceding output is much better than the previous one. You can see Marks76.901 set to 76.90 automatically.

The most unique characteristic of Python, unlike other programming languages, is indentation. Indentation not only makes Python code readable, but also distinguishes each block of code from the other. Let's explain this with an example:

def fun():
    pass
for each in "Australia":
    pass

While writing the code, a new block of code starts with a colon followed by a tab. Here, after the function fun(), a colon is provided which will start the function body, pass is part of the function fun() and it is placed at one tab space. Likewise, the for loop starts with a colon. Here, most people get confused whether to use a tab or space. It is advisable to stick to only one type and follow the same across the whole code. If the indentation is not strictly implemented, then code execution will throw an error.