Python Data Types

String Data Types

Strings are a contiguous set of characters (alphabets, numbers, or symbols). 

Python identifies this contiguous set of characters as a string if these characters are enclosed in double quotes (“) as in the example below.

“I go to Bright Elementary School. I play soccer in my school.”

A string can also be defined by using single quotes (‘) instead of double quotes (“) as in the example below:

‘I go to Bright Elementary School. I play soccer in my school.’

Note you cannot mix single quotes and double quotes in the same string as in the example below.

“I love playing soccer.’ 

The above is not a string because the opening quote is a double quote (“) and the closing quote is a single quote (‘). The two quote symbols do not match.  Python will not like it and will give you an syntax error message. See the Program Example below.

PROGRAM EXAMPLE: SINGLE AND DOUBLE QUOTES IN THE SAME STRING – SYNTAX ERROR EXAMPLE

Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 21:26:53) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> "I love playing soccer.'
SyntaxError: EOL while scanning string literal
>>>

Strings are used to display a message on the computer screen.

How to Declare String Data Type

We declare strings just as we declared numbers by using the assignment operator equal sign (=). 

Let us use the facts of our solar system to explain how to declare and assign value to a string data type,

In the following program example, ‘X’ defines a string variable using single quotes (‘   ‘). ‘Y’ defines a string variable using double quotes (“  ‘’).

Python 3.10.7 (tags/v3.10.7:6cc6b13, Sep  5 2022, 14:08:36) [MSC v.1933 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.

X = 'moon distance from earth' # Using single quotes
Y = "moon distance from earth" # Using double quotes

Python will treat the above two variables, X and Y as string variables.

PROGRAM EXAMPLE: STRING DATA TYPE SYNTAX ERROR

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> # String Variables
>>> # Use single quotes (')to define a string variable.
>>> # and assign it a value 'mon distance from earth'.
>>> X = 'moon distance from earth'
>>> Python interpreter will treat X as a string variable.
>>>
>>> # Use double quotes (") to define string variable Y. 
>>> # and assign it a value "moon distance from earth". 
>>> # Python interpreter will treat “Y” means "moon distance from earth".
>>> Y = "moon distance from earth"
>>> 
>>> # You cannot define a string by starting with double quote and end with a single quote or vice versa.
>>> Z = "moon distance from earth'
SyntaxError: EOL while scanning string literal
>>> 

Facts About Earth And Moon

The Table below summarizes a few facts about the planet earth and the moon. The program examples below will use these facts.

The Earth We Live on Facts
Age	4.5 billion Years
Distance from Sun	93 million miles (150 million km)
Orbits around	Sun
Orbit period	365 days
Radius	4,000 miles (6,400 km)
Acceleration due to gravity	9.8 meter/ sec^2

Moon

Our Moon orbits around planet Earth and is Earth’s only natural satellite. It is the fifth-largest satellite in the Solar System. It takes the moon 27 earth-days to complete one orbit around the earth. 

Our Moon Facts
Age	4.5 billion Years
Distance from Earth	238,000 miles (385,000 km)
Orbits around	Earth
Orbit period	27 earth-days
Radius	1,079.4 miles
Acceleration due to gravity	1.62 meter/ sec^2

Strings with Embedded quotes

On the Printing in Python page, we learned how to print strings.  The earlier examples did not have quotes embedded within the strings. Strings that have quotes as part of the string would confuse the Python interpreter since Python uses quotes to identify a sequence of characters as a string. If there are quotes embedded in a string, Python will flag it as a syntax error.  In the Program Example below, the word Moon’s has an embedded quote. Python interpreter flags it as a syntax error.

PROGRAM EXAMPLE: STRINGS WITH EMBEDDED QUOTES – 1

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> # Example of Embedded quote in string literal causes syntax error.
>>> 'Moon's distance is 25000 miles from earth'
SyntaxError: invalid syntax
>>>

The reason why Python flags the above as a syntax error is because Python identifies the string ‘Moon’s distance is 25000 miles from earth‘ as a string when it sees the first single quote before the letter M. As the Python interpreter comes across the second quote after character ‘n’ in the word Moon’s, it will assume the end of the string. Python does not expect to see any more characters after the quote in the word Moon’s and gives a syntax error.

Another example of string with embedded quote:

PROGRAM EXAMPLE: STRINGS WITH EMBEDDED QUOTES – 2

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> # Another example of embedded quotes in string Variable causes syntax error.
>>> Moon_distance = "My teacher said, "Moon's distance from earth is 250,000 miles.""
SyntaxError: invalid syntax
>>>

In the above example, Python identifies “My teacher said, “Moon’s distance from earth is 250,000 miles.” as a string on seeing the first double quote before the word My. On seeing the second double quote before the word Moon’s, the Python interpreter will assume the end of the string at the second double before the word Moon’s. All characters after the second double quote cause a syntax error.

How Do We Handle Strings That Have Quotes As Part Of String’s Text?

The solution to the above problem is to use what is called an escape character in strings that have embedded quotes.  The escape character is the symbol backslash (\).  If you put a backslash before the quote embedded in the string, Python ignores the embedded slash and treats the embedded quote as a valid character. This is explained in the Program below using five different program examples.

Program 1 shows the syntax error due to the embedded quote in “isn’t”.

Program 2 uses backslash (\) escape character before the quote in isn’t and replaces it with (isn\’t). The backslash eliminates the syntax error.

Program 3 uses backslash (\) escape character before the embedded quote in the word “The. This escape character will ignore the embedded quote (“) in the word “The.

Programs 3a and program 4 uses triple single quotes (‘’’) to bypass the embedded quote issue.

PROGRAM EXAMPLE: ESCAPE CHARACTER USE IN STRINGS WITH EMBEDDED QUOTES

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
# Program 1
>>> water_boiling_point = 'My teacher said, "The water boiling point is'nt 90 degrees Celsius"'
SyntaxError: invalid syntax
>>> 
>>>
# Program 2
>>> water_boiling_point = 'My teacher said, "The water boiling point isn\'t 90 degrees Celsius."'
>>> print(water_boiling_point)
My teacher said, "The water boiling point isn't 90 degrees Celsius."
>>> 
>>>
>>>
# Program 3
>>> water_boiling_point = "My teacher said,\"The water boiling point is 100 degrees Celsius.\""
>>> print(water_boiling_point)
My teacher said, "The water boiling point is 100 degrees Celsius."
>>> 
# Program 3a
>>> water_boiling_point = '''My teacher said, "The water boiling point isn't 90 degrees Celsius"'''
>>> print(water_boiling_point)
My teacher said, "The water boiling point isn't 90 degrees Celsius"
>>> 
# Program 4
>>> water_boiling_point = '''My teacher said, "The water boiling point is 100 degrees Celsius."'''
>>> print(water_boiling_point)
My teacher said, "The water boiling point is 100 degrees Celsius."
>>>

Multi-line Strings

From the above program, we saw how triple quotes (‘’’) can be used to print strings that have embedded quotes (‘) or (“).  Triple single quotes can also be used for multi-line strings.

We introduced the multi-line strings on the Printing in Python page. Let us write a program that prints a multi-line string using triple single quotes. The multi-line string used in the program describes the earth-moon facts. Since the earth-moon facts string is very long, we continue the string onto several lines.

The string uses triple single quotes (‘’’) as below.

PROGRAM EXAMPLE: TRIPLE QUOTES FOR MULTI-LINE STRINGS

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> '''Moon's distance from our planet Earth is 250,000 miles. Moon is planet earth's only satellite. It orbits around planet earth in about 27 days. Its radius is about 1,080 miles.'''
"Moon's distance from our planet Earth is 250,000 miles. Moon is planet earth's only satellite. It orbits around planet earth in about 27 days. Its radius is about 1,080 miles."
>>>

Concatenation of String Literals

We defined literals earlier in this chapter.  To recap, literals are text or numbers whose value is directly visible from their name. String literals are text enclosed in quotes.

If you want to concatenate several literals together, you can do this by typing them next to each other. Python will join them as in the following example program.

PROGRAM EXAMPLE: CONCATENATION (ADDITION) OF STRING LITERALS

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> # Example program showing how string literals can be joined together just by typing them next to each other.
>>>
>>> 'Moon distance from ' 'earth is 250' '000 miles.'
'Moon distance from earth is 250000 miles.'
>>> 

Concatenation of String Variables

We saw in the previous section how we can concatenate string literals just by typing them next to each other. Now we will concatenate two string variables together, using the plus (+) symbol. Think of plus symbol as a concatenation operation. We will use the interesting facts about the planet Jupiter to write a program to show how we can concatenate several string variables.

Planet Jupiter Facts:

The Romans named the planet after the Roman God, Jupiter.  It is mainly composed of hydrogen and helium with a rocky core of heavier elements. It has 79 moons, more than any other planet in the solar system. The four largest moons are called Galilean Moons since they were discovered by Galileo (Italian mathematician, physicist, philosopher, and astronomer) in the year 1610.  Jupiter has a diameter of about 88,695 miles (142,800 kilometers), which is more than 11 times the diameter of Earth. Jupiter has the strongest magnetic field of any planet in the solar system; it is 14 times the magnetic field of the earth.

Source: Wikipedia

Jupiter Facts

JUPITER QUICK FACTS	DISTANCE FROM SUN	UNITS
Distance from Sun	484 million miles (778 million km)
Order from Sun	5th
Orbit duration	11.84	Earth-years
Rotation	10	Earth-hours
Size	1,300 times volume of earth
Diameter	88,695 miles (142,800 km)
Moons	79

In the program example of string variables concatenation, we will define three string variables ‘planet_name’, ‘size’, and ‘order’ as below.

planet_name = ‘Jupiter’

size= ‘ is 142,984 kilometer diameter’

order = ‘ and is fifth from Sun.’

We will use the plus (+) symbol to concatenate the above three string variables together.

PROGRAM EXAMPLE: CONCATENATION (ADDITION) OF STRING VARIABLES

>>> # String Variable concatenation
>>> planet_name = 'Jupiter'
>>> size = ' is 142,984 kilometer in diameter'
>>> order = ' and is fifth planet from Sun.'
>>> planet_name + size + order
'Jupiter is 142,984 kilometer in diameter and is fifth planet from Sun.'
>>> 

Concatenation of String Literal and String Variable

Sometimes you may want to concatenate (add) string literal and string variable. You can again use the plus (+) symbol to concatenate a string literal with a string variable.  This time let us use planet Mars facts for the program example.

Source: nasa.gov

MARS QUICK FACTS	DISTANCE FROM SUN	UNITS
Distance from Sun	142 million miles (225 million km)
Order from Sun	4th
Orbit duration	687	Earth-days
Rotation period	24 hours 39 min	Earth-hours
Size	1/10 of Earth
Moons	2	Phobos and Deimos
Diameter	4220 miles (6752 km)

For our program example, we will define four string variables ‘planet_name’, ‘size’, ‘distance’, and ‘year’. We will use the concatenation operator (+) to concatenate three string variables and one string literal ‘6752 kilometers’ together.

planet_name = ‘Mars’

distance = ‘ Its distance from Sun is 225 million kilometers. ’

Year = ‘ Mars year is 687 earth days.’

PROGRAM EXAMPLE: STRING LITERAL AND STRING VARIABLE CONCATENATION (ADDITION)

>>> # String literal and string variable concatenation Example
>>> planet_name = 'Mars'
>>> distance = ' distance from Sun is 225 kilometers. '
>>> diameter = 'Mars diameter is '

>>> planet_name + distance  + diameter + '6752 kilometers.'
'Mars distance from Sun is 225 kilometers. Mars diameter is 6752 kilometers.'
>>>

Note that this program concatenates three string variables ‘planet_name’, ‘distance’, and ‘diameter’, and one string literal ‘6752 kilometers’.

String Multiplication (Repetition)

We now know how to add (concatenate) string literals and string variables. How about if you want to multiply (repeat) a string multiple times to emphasize what you want to say?

Python can do string multiplication (repetition) using the multiply (*) symbol.

Let us read about planet Mercury facts below and then write a program to show how string multiplication works.

Source: http://www.nasa.gov

MERCURY QUICK FACTS		UNITS
Mercury Quick Facts
Average distance from Sun	58	Million km
Order from Sun	First
Orbit duration (length of year)	87.97	Earth-days
Rotation period (length of day)	58.65	Earth-days
Diameter (km)	4879.4	km
Maximum surface temperature	430	°C
Minimum surface temperature	-170	°C

Using the Mercury facts, we will write a program to show how you can use the multiplication (*) symbol to repeat the string any number of times. 

This program combines string multiplication and string addition.

PROGRAM EXAMPLE: STRING MULTIPLICATION (REPETITION)

>>> # String Multiplication Example Program
>>> # Use of multiply (*) symbol to do string repetition
>>> 'Planet Mercury gets ' +5*'very ' + 'cold (-170C) during the night and gets ' +4*'extremely ' + 'hot (430C) during the day' + 3*'!'
'Planet Mercury gets very very very very very cold (-170C) during the night and gets extremely extremely extremely extremely hot (430C) during the day!!!'
>>>

This program repeats (multiplies) the string “very” five times for cold, the string “extremely” 4 times for hot, and the string ‘!’ three times.  String multiplication saves you some typing.

String Indexing

As discussed above, a string is a series of characters. Python numbers these characters in the string starting with 0 for the first character, 1 for the second character, 2 for the third character, and so on. These numbers are called index. Spaces are also characters and are numbered just as characters. Let us explain the concept of string indexing by using planet Venus facts.

In 1990, the Magellan spacecraft began orbiting Venus. It performed advanced radar image mapping to obtain finer details, similar to that of the Pioneer Venus Orbiter. What Magellan found were approximately 1000 impact craters.

Example of String Indexing

Now, let us explain the concept of the index using planet Venus facts. We define a string variable called ‘planet’ as below. Each character in the string has a number (called an index) associated with it. The table below explains the indexing concept.

>>> planet = ‘venus_temp_462’

String	v	e	n	u	s	_	t	e	m	p	_	4	6	2
Index	0	1	2	3	4	5	6	7	8	9	10	11	12	13

From the table, you can see the first character ‘v’ has an index of ‘0’. The second character ‘e’ has an index of ‘1’, and so on.

Why Use Index

Indexes are useful if you want to select and print a specified character from a string.  For example, if you want to print character ‘s’ in the above string, you would write the following statement:

>>> print(string[4])

In the above statement, ‘4’ is the index of character ‘s’ in ‘venus’.

 The procedure to select and print a few contiguous characters from a string is

• use the index of the first character to be printed
• followed by a colon (:)
• followed by the index of the last character to be printed + 1.

Printing a bunch of contiguous characters in a string is called slicing.

>>> print(string[1:5])

This program will print characters from index 1 to 4.  Note that the last character printed is the one before the last index in the statement. The last index in the statement is never printed.

The program below further explains the concept of string indexing and slicing.

PROGRAM EXAMPLE: INDEXING CONCEPT

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 19:29:22) 
[MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> # String Index and string slicing example.
>>> planet = 'venus_temp_462'
>>> print(planet[4])
s
>>> print(planet[6])
t
>>> # Print a few contiguous characters in string ‘planet’ (Called string slicing).
>>> print(planet[6:10])
temp
>>> # The next statement uses index larger than the string maximum index. Python gracefully handles this.
>>> print(planet[0:20])
venus_temp_462
>>>

Changing Strings

Strings are immutable, that is they cannot be changed.  Any attempt to change a character in the string will cause a syntax error.