Constants, variables, and comments
Just printing hello world is not enough, is it? You want to do more than that - you want to take some input, manipulate it and get something out of it. We can achieve this in Python using constants and variables, and we’ll learn some other concepts as well in this chapter.
Literal Constants
An example of a literal constant is a number like 5, 1.23, or a string like 'This is a string' or "It's a string!".
It is called a literal because it is literal - you use its value literally. The number 2 always represents itself and nothing else - it is a constant because its value cannot be changed. Hence, all these are referred to as literal constants.
Numbers
Numbers are mainly of two types - integers and floats.
An example of an integer is 2 which is just a whole number.
Examples of floating point numbers (or floats for short) are 3.23 and 52.3E-4. The E notation indicates powers of 10. In this case, 52.3E-4 means 52.3 * 10^-4^.
Note for Experienced Programmers
There is no separate
longtype. Theinttype can be an integer of any size.
Strings
A string is a sequence of characters. Strings are basically just a bunch of words.
You will be using strings in almost every Python program that you write, so pay attention to the following part.
Single Quote
You can specify strings using single quotes such as 'Quote me on this'.
All white space i.e. spaces and tabs, within the quotes, are preserved as-is.
Double Quotes
Strings in double quotes work exactly the same way as strings in single quotes. An example is "What's your name?".
Triple Quotes
You can specify multi-line strings using triple quotes - (""" or '''). You can use single quotes and double quotes freely within the triple quotes. An example is:
'''This is a multi-line string. This is the first line.
This is the second line.
"What's your name?," I asked.
He said "Bond, James Bond."
'''Strings Are Immutable
This means that once you have created a string, you cannot change it. Although this might seem like a bad thing, it really isn’t. We will see why this is not a limitation in the various programs that we see later on.
Note for C/C++ Programmers
There is no separate
chardata type in Python. There is no real need for it and I am sure you won’t miss it.
Note for Perl/PHP Programmers
Remember that single-quoted strings and double-quoted strings are the same - they do not differ in any way.
The format method
Sometimes we may want to construct strings from other information. This is where the format() method is useful.
Save the following lines as a file str_format.py:
age = 20
name = 'Swaroop'
print('{0} was {1} years old when he wrote this book'.format(name, age))
print('Why is {0} playing with that python?'.format(name))Output:
$ python str_format.py
Swaroop was 20 years old when he wrote this book
Why is Swaroop playing with that python?How It Works
A string can use certain specifications and subsequently, the format method can be called to substitute those specifications with corresponding arguments to the format method.
Observe the first usage where we use {0} and this corresponds to the variable name which is the first argument to the format method. Similarly, the second specification is {1} corresponding to age which is the second argument to the format method. Note that Python starts counting from 0 which means that first position is at index 0, second position is at index 1, and so on.
Notice that we could have achieved the same using string concatenation:
name + ' is ' + str(age) + ' years old'but that is much uglier and more error-prone. Second, the conversion to string would be done automatically by the format method instead of the explicit conversion to strings needed in this case. Third, when using the format method, we can change the message without having to deal with the variables used and vice-versa.
Also note that the numbers are optional, so you could have also written as:
age = 20
name = 'Swaroop'
print('{} was {} years old when he wrote this book'.format(name, age))
print('Why is {} playing with that python?'.format(name))which will give the same exact output as the previous program.
We can also name the parameters:
age = 20
name = 'Swaroop'
print('{name} was {age} years old when he wrote this book'.format(name=name, age=age))
print('Why is {name} playing with that python?'.format(name=name))which will give the same exact output as the previous program.
Python 3.6 introduced a shorter way to do named parameters, called “f-strings”:
age = 20
name = 'Swaroop'
print(f'{name} was {age} years old when he wrote this book') # notice the 'f' before the string
print(f'Why is {name} playing with that python?') # notice the 'f' before the stringwhich will give the same exact output as the previous program.
What Python does in the format method is that it substitutes each argument value into the place of the specification. There can be more detailed specifications such as:
# decimal (.) precision of 3 for float '0.333'
print('{0:.3f}'.format(1.0/3))
# fill with underscores (_) with the text centered
# (^) to 11 width '___hello___'
print('{0:_^11}'.format('hello'))
# keyword-based 'Swaroop wrote A Byte of Python'
print('{name} wrote {book}'.format(name='Swaroop', book='A Byte of Python'))Output:
0.333
___hello___
Swaroop wrote A Byte of PythonSince we are discussing formatting, note that print always ends with an invisible “new line” character (\n) so that repeated calls to print will all print on a separate line each. To prevent this newline character from being printed, you can specify that it should end with a blank:
print('a', end='')
print('b', end='')Output is:
abOr you can end with a space:
print('a', end=' ')
print('b', end=' ')
print('c')Output is:
a b cPrinting Extra Line Breaks
Since the default end parameter is set to \n it always prints one line break by default. We can change end to \n\n to print two line breaks – that creates an empty line after whatever the print statement output.
print("This is a line", end = "\n\n") We can also use print with no arguments to print just an empty line:
print() # prints an empty line Escape Sequences
Suppose, you want to have a string which contains a single quote ('), how will you specify this string? For example, the string is "What's your name?". You cannot specify 'What's your name?' because Python will be confused as to where the string starts and ends. So, you will have to specify that this single quote does not indicate the end of the string. This can be done with the help of what is called an escape sequence. You specify the single quote as \' : notice the backslash. Now, you can specify the string as 'What\'s your name?'.
Another way of specifying this specific string would be "What's your name?" i.e. using double quotes. Similarly, you have to use an escape sequence for using a double quote itself in a double quoted string. Also, you have to indicate the backslash itself using the escape sequence \\.
What if you wanted to specify a two-line string? One way is to use a triple-quoted string as shown previously or you can use an escape sequence for the newline character - \n to indicate the start of a new line. An example is:
'This is the first line\nThis is the second line'Another useful escape sequence to know is the tab: \t. There are many more escape sequences but I have mentioned only the most useful ones here.
One thing to note is that in a string, a single backslash at the end of the line indicates that the string is continued in the next line, but no newline is added. For example:
"This is the first sentence. \
This is the second sentence."is equivalent to
"This is the first sentence. This is the second sentence."Raw String
If you need to specify some strings where no special processing such as escape sequences are handled, then what you need is to specify a raw string by prefixing r or R to the string. An example is:
r"Newlines are indicated by \n"Note for Regular Expression Users
Always use raw strings when dealing with regular expressions. Otherwise, a lot of backwhacking may be required. For example, backreferences can be referred to as
'\\1'orr'\1'.
Variable
Using just literal constants can soon become boring - we need some way of storing any information and manipulate them as well. This is where variables come into the picture. Variables are exactly what the name implies - their value can vary, i.e., you can store anything using a variable. Variables are just parts of your computer’s memory where you store some information. Unlike literal constants, you need some method of accessing these variables and hence you give them names.
Identifier Naming
Variables are examples of identifiers. Identifiers are names given to identify something. There are some rules you have to follow for naming identifiers:
- The first character of the identifier must be a letter of the alphabet (uppercase ASCII or lowercase ASCII or Unicode character) or an underscore (
_). - The rest of the identifier name can consist of letters (uppercase ASCII or lowercase ASCII or Unicode character), underscores (
_) or digits (0-9). - Identifier names are case-sensitive. For example,
mynameandmyNameare not the same. Note the lowercasenin the former and the uppercaseNin the latter. - Examples of valid identifier names are
i,name_2_3. Examples of invalid identifier names are2things,this is spaced out,my-nameand>a1b2_c3.
Data Types
Variables can hold values of different types called data types. The basic types are numbers and strings, which we have already discussed.
Object
Remember, Python refers to anything used in a program as an object. This is meant in the generic sense. Instead of saying “the something”’, we say “the object”.
Note for Object Oriented Programming users:
Python is strongly object-oriented in the sense that everything is an object including numbers, strings and functions.
We will now see how to use variables along with literal constants. Save the following example and run the program.
Summary
Now that we have gone through many nitty-gritty details, we can move on to more interesting stuff such as functions. Be sure to become comfortable with what you have read in this chapter.
Comments
Comments are any text to the right of the
#symbol and is mainly useful as notes for the reader of the program.For example:
or:
Use as many useful comments as you can in your program to:
Code tells you how, comments should tell you why.
This is useful for readers of your program so that they can easily understand what the program is doing. Remember, that person can be yourself after six months!