Basic Types

Last updated on 2023-04-18 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • What kinds of data are there in Python?
  • How are different data types treated differently?
  • How can I identify a variable’s type?
  • How can I convert between types?

Objectives

  • Explain key differences between integers and floating point numbers.
  • Explain key differences between numbers and character strings.
  • Use built-in functions to convert between integers, floating point numbers, and strings.
  • Subset a string using slicing.

Key Points

  • Every value has a type.
  • Use the built-in function type to find the type of a value.
  • Types control what operations can be done on values.
  • Strings can be added and multiplied.
  • Strings have a length (but numbers don’t).

Every value has a type.

  • Every value in a program has a specific type.
  • Integer (int): represents positive or negative whole numbers like 3 or -512.
  • Floating point number (float): represents real numbers like 3.14159 or -2.5.
  • Character string (usually called “string”, str): text.
    • Written in either single quotes or double quotes (as long as they match).
    • The quote marks aren’t printed when the string is displayed.

Use the built-in function type to find the type of a value.

  • Use the built-in function type to find out what type a value has.
  • Works on variables as well.
    • But remember: the value has the type — the variable is just a label.

PYTHON 

print(type(52))

OUTPUT 

<class 'int'>

PYTHON 

fitness = 'average'
print(type(fitness))

OUTPUT 

<class 'str'>

Types control what operations (or methods) can be performed on a given value.

  • A value’s type determines what the program can do to it.

PYTHON 

print(5 - 3)

OUTPUT 

2

PYTHON 

print('hello' - 'h')

OUTPUT 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-67f5626a1e07> in <module>()
----> 1 print('hello' - 'h')

TypeError: unsupported operand type(s) for -: 'str' and 'str'

Simple and compound types.

We can broadly split different types into two categories in Python: simple types and compound types.

Simple types consist of a single value. In Python, these simple types include:

  • int
  • float
  • bool
  • NoneType

Compound types contain multiple values. Compound types include:

  • str
  • list
  • dictionary
  • tuple
  • set

In this lesson we will be learning about simple types and strings (str), which are made up of multiple characters. We will go into more detail on other compound types in future lessons.

You can use the “+” and “*” operators on strings.

  • “Adding” character strings concatenates them.

PYTHON 

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)

OUTPUT 

Ahmed Walsh

The empty string

We can initialize a string to contain no letter, empty = "". This is called the empty string, and is often used when we want to build up a string character by character

  • Multiplying a character string by an integer N creates a new string that consists of that character string repeated N times.
    • Since multiplication is repeated addition.

PYTHON 

separator = '=' * 10
print(separator)

OUTPUT 

==========

Strings have a length (but numbers don’t).

  • The built-in function len counts the number of characters in a string.

PYTHON 

print(len(full_name))

OUTPUT 

11
  • But numbers don’t have a length (not even zero).

PYTHON 

print(len(52))

OUTPUT 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-f769e8e8097d> in <module>()
----> 1 print(len(52))

TypeError: object of type 'int' has no len()

You must convert numbers to strings or vice versa when operating on them.

  • Cannot add numbers and strings.

PYTHON 

print(1 + '2')

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-fe4f54a023c6> in <module>()
----> 1 print(1 + '2')

TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • The result here would be 3 if both values were considered ints, and '12' if both values were considered strings. Due to this ambiguity addition is not allowed between strings and integers.
  • Some types can be converted to other types by using the type name as a function.

PYTHON 

print(1 + int('2'))
print(str(1) + '2')

OUTPUT 

3
12

You can mix integers and floats freely in operations.

  • Integers and floating-point numbers can be mixed in arithmetic.
    • Python 3 automatically converts integers to floats as needed.

PYTHON 

print('half is', 1 / 2.0)
print('three squared is', 3.0 ** 2)

OUTPUT 

half is 0.5
three squared is 9.0

Variables only change value when something is assigned to them.

  • If we make one cell in a spreadsheet depend on another, and update the latter, the former updates automatically.
  • This does not happen in programming languages.

PYTHON 

variable_one = 1
variable_two = 5 * variable_one
variable_one = 2
print('first is', variable_one, 'and second is', variable_two)

OUTPUT 

first is 2 and second is 5
  • The computer reads the value of variable_one when doing the multiplication, creates a new value, and assigns it to variable_two.
  • Afterwards, the value of variable_two is set to the new value and not dependent on variable_one so its value does not automatically change when variable_one changes.

Fractions

What type of value is 3.4? How can you find out?

It is a floating-point number (often abbreviated “float”). It is possible to find out by using the built-in function type().

PYTHON 

print(type(3.4))

OUTPUT 

<class 'float'>

Automatic Type Conversion

What type of value is 3.25 + 4?

It is a float: integers are automatically converted to floats as necessary.

PYTHON 

result = 3.25 + 4
print(result, 'is', type(result))

OUTPUT 

7.25 is <class 'float'>

Choose a Type

What type of value (integer, floating point number, or character string) would you use to represent each of the following? Try to come up with more than one good answer for each problem. For example, in # 1, when would counting days with a floating point variable make more sense than using an integer?

  1. Number of days since the start of the year.
  2. Time elapsed from the start of the year until now in days.
  3. Serial number of a piece of lab equipment.
  4. A lab specimen’s age
  5. Current population of a city.
  6. Average population of a city over time.

The answers to the questions are: 1. Integer, since the number of days would lie between 1 and 365. 2. Floating point, since fractional days are required 3. Character string if serial number contains letters and numbers, otherwise integer if the serial number consists only of numerals 4. This will vary! How do you define a specimen’s age? whole days since collection (integer)? date and time (string)? 5. Choose floating point to represent population as large aggregates (eg millions), or integer to represent population in units of individuals. 6. Floating point number, since an average is likely to have a fractional part.

Division Types

In Python 3, the // operator performs integer (whole-number) floor division, the / operator performs floating-point division, and the % (or modulo) operator calculates and returns the remainder from integer division:

PYTHON 

print('5 // 3:', 5 // 3)
print('5 / 3:', 5 / 3)
print('5 % 3:', 5 % 3)

OUTPUT 

5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

Imagine that you are buying cages for mice. num_mice is the number of mice you need cages for, and num_per_cage is the maximum number of mice which can live in a single cage. Write an expression that calculates the exact number of cages you need to purchase.

PYTHON 

num_mice = 56
num_per_cage = 3

We want the number of cages to house all of our mice, which is the rounded up result of num_mice / num_per_cage. This is equivalent to performing a floor division with // and adding 1. Before the division we need to subtract 1 from the number of mice to deal with the case where num_mice is evenly divisible by num_per_cage.

PYTHON 

num_cages = ((num_mice - 1) // num_per_cage) + 1

print(num_mice, 'mice,', num_per_cage, 'per cage:', num_cages, 'cages')

OUTPUT 

56 mice, 3 per cage: 19 cages

Strings to Numbers

Where reasonable, float() will convert a string to a floating point number, and int() will convert a floating point number to an integer:

PYTHON 

print("string to float:", float("3.6"))
print("float to int:", int(3.6))

OUTPUT 

string to float: 3.4
float to int: 3

Note that converting a float to an int does not round the result, but instead truncates it by removing everything past the decimal point.

If the conversion doesn’t make sense, however, an error message will occur.

PYTHON 

print("string to float:", float("Hello world!"))

ERROR 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-df3b790bf0a2> in <module>
----> 1 print("string to float:", float("Hello world!"))
 ValueError: could not convert string to float: 'Hello world!'

Challenge

What do you expect the following program to do?

What does it actually do?

Why do you think it does that?

PYTHON 

print("fractional string to int:", int("3.6"))

What do you expect this program to do? It would not be so unreasonable to expect the Python 3 int command to convert the string “3.4” to 3.4 and an additional type conversion to 3. After all, Python 3 performs a lot of other magic - isn’t that part of its charm?

PYTHON 

int("3.6")

ERROR 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-ec6729dfccdc> in <module>
----> 1 int("3.4")
ValueError: invalid literal for int() with base 10: '3.4'

However, Python 3 throws an error. Why? To be consistent, possibly. If you ask Python to perform two consecutive typecasts, you must convert it explicitly in code.

PYTHON 

int(float("3.6"))

OUTPUT 

3

Challenge

Arithmetic with Different Types

Which of the following will return the floating point number 2.0? Note: there may be more than one right answer.

PYTHON 

first = 1.0
second = "1"
third = "1.1"
  1. first + float(second)
  2. float(second) + float(third)
  3. first + int(third)
  4. first + int(float(third))
  5. int(first) + int(float(third))
  6. 2.0 * second

Answer: 1 and 4

Use an index to get a single character from a string.

Python uses 0-based indexing.
Python uses 0-based indexing.
  • The characters (individual letters, numbers, and so on) in a string are ordered. For example, the string 'AB' is not the same as 'BA'. Because of this ordering, we can treat the string as a list of characters.
  • Each position in the string (first, second, etc.) is given a number. This number is called an index or sometimes a subscript.
  • Indices are numbered from 0.
  • Use the position’s index in square brackets to get the character at that position.

PYTHON 

atom_name = 'sodium'
print(atom_name[0])

OUTPUT 

s

Use a slice to get a substring.

  • A part of a string is called a substring. A substring can be as short as a single character.
  • An item in a list is called an element. Whenever we treat a string as if it were a list, the string’s elements are its individual characters.
  • Slicing gets a part of a string (or, more generally, a part of any list-like thing).
  • Slicing uses the notation [start:stop], where start is the integer index of the first element we want and stop is the integer index of the element just after the last element we want.
  • The difference between stop and start is the slice’s length.
  • Taking a slice does not change the contents of the original string. Instead, taking a slice returns a copy of part of the original string.

PYTHON 

atom_name = 'sodium'
print(atom_name[0:3])

OUTPUT 

sod

Use the built-in function len to find the length of a string.

PYTHON 

print(len('sodium'))

OUTPUT 

6
  • Nested functions are evaluated from the inside out, like in mathematics.

Indexing

If you assign a = 123, what happens if you try to get the second digit of a via a[1]?

Numbers are not strings or sequences and Python will raise an error if you try to perform an index operation on a number. In the next lesson on types and type conversion we will learn more about types and how to convert between different types. If you want the Nth digit of a number you can convert it into a string using the str built-in function and then perform an index operation on that string.

PYTHON 

a = 123
print(a[1])

OUTPUT 

TypeError: 'int' object is not subscriptable

PYTHON 

a = str(123)
print(a[1])

OUTPUT 

2

Slicing practice

What does the following program print?

PYTHON 

atom_name = 'carbon'
print('atom_name[1:3] is:', atom_name[1:3])

OUTPUT 

atom_name[1:3] is: ar

Slicing concepts

Given the following string:

PYTHON 

species_name = "Acacia buxifolia"

What would these expressions return?

  1. species_name[2:8]
  2. species_name[11:] (without a value after the colon)
  3. species_name[:4] (without a value before the colon)
  4. species_name[:] (just a colon)
  5. species_name[11:-3]
  6. species_name[-5:-3]
  7. What happens when you choose a stop value which is out of range? (i.e., try species_name[0:20] or species_name[:103])
  1. species_name[2:8] returns the substring 'acia b'
  2. species_name[11:] returns the substring 'folia', from position 11 until the end
  3. species_name[:4] returns the substring 'Acac', from the start up to but not including position 4
  4. species_name[:] returns the entire string 'Acacia buxifolia'
  5. species_name[11:-3] returns the substring 'fo', from the 11th position to the third last position
  6. species_name[-5:-3] also returns the substring 'fo', from the fifth last position to the third last
  7. If a part of the slice is out of range, the operation does not fail. species_name[0:20] gives the same result as species_name[0:], and species_name[:103] gives the same result as species_name[:]