Variables and Types

Last updated on 2024-04-15 | Edit this page

Overview

Questions

  • How can I store data in Python?
  • What are some types of data that I can work with in Python?

Objectives

  • Write Python to assign values to variables.
  • Print outputs to a Jupyter notebook.
  • Use indexing to manipulate string elements.
  • View and convert the data types of Python objects.

Use variables to store values.

Variables are names given to certain values. In Python the = symbol assigns a value to a variable. Here, Python assigns the number 42 to the variable age and the name Ahmed in single quote to a variable name.

PYTHON 

age = 42
name = 'Ahmed'

Naming variables

Variable names:

  • cannot start with a digit
  • cannot contain spaces, quotation marks, or other punctuation
  • may contain an underscore (typically used to separate words in long variable names)
  • are case sensitive. name and Name would be different variables.

Use print() to display values.

You can print Python objects to the Jupyter notebook output using the built-in function, print(). Inside of the parentheses we can add the objects that we want print, which are known as the print() function’s arguments.

PYTHON 

print(name, age)

OUTPUT 

Ahmed 42

In Jupyter notebooks, you can leave out the print() function for objects – such as variables – that are on the last line of a cell. If the final line of Jupyter cell includes the name of a variable, its value will display in the notebook when you run the cell.

PYTHON 

name
age

OUTPUT 

42

Format output with f-strings

F-strings provide a concise and readable way to format strings by embedding Python expressions within them. You can format variables as text strings in your output using an f-string. To do so, start a string with f before the open single (or double) quote. Then add any replacement fields, such as variable names, between curly braces {}. (Note the f string syntax can only be used with Python 3.6 or higher.)

PYTHON 

f'{name} is {age} years old'

OUTPUT 

'Ahmed is 42 years old'

Variables must be created before they are used.

If a variable doesn’t exist yet, or if the name has been misspelled, Python reports an error called a NameError.

PYTHON 

print(eye_color)

ERROR 

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-c1fbb4e96102> in <module>()
----> 1 print(eye_color)

NameError: name 'eye_color' is not defined

The last line of an error message is usually the most informative. In this case it tells us that the eye_color variable is not defined. NameErrors often refer to variables that haven’t been created or assigned yet.

Variables can be used in calculations.

We can use variables in calculations as if they were values. We assigned 42 to age a few lines ago, so we can reference that value within a new variable assignment.

PYTHON 

age = age + 3
f'Age equals: {age}'

OUTPUT 

Age equals: 45

Every Python object has a type.

Everything in Python is some type of object and every Python object will be of a specific type. Understanding an object’s type will help you know what you can and can’t do with that object.

You can use the built-in Python function type() to find out an object’s type.

PYTHON 

print(type(140.2), 
      type(age), 
      type(name), 
      type(print))

OUTPUT 

<class 'float'> <class 'int'> <class 'str'> <class 'builtin_function_or_method'>
  1. 140.2 is an example of a floating point number or float. These are fractional numbers.
  2. The value of the age variable is 45, which is a whole number, or integer (int).
  3. The name variable refers to the string (str) of ‘Ahmed’.
  4. The built-in Python function print() is also an object with a type, in this case it’s a builtin_function_or_method. Built-in functions refer to those that are included in the core Python library.

Types control what operations (or methods) can be performed on objects.

An object’s type determines what the program can do with it.

PYTHON 

5 - 3

OUTPUT 

2

We get an error if we try to subtract a letter from a string:

PYTHON 

'hello' - 'h'

ERROR 

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

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

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

We can reference the specific location of a character (individual letters, numbers, and so on) in a string by using its index position. In Python, each character in a string (first, second, etc.) is given a number, which is called an index. Indexes begin from 0 rather than 1. We can use an index in square brackets to refer to the character at that position.

PYTHON 

library = 'Alexandria'
library[0]

OUTPUT 

A

Use a slice to get multiple characters from a string.

A slice is a part of a string that we can reference using [start:stop], where start is the index of the first character we want and stop is the last character. Referencing a string slice does not change the contents of the original string. Instead, the slice returns a copy of the part of the original string we want.

PYTHON 

library[0:3]

OUTPUT 

Ale

Note that in the example above, library[0:3] begins with zero, which refers to the first element in the string, and ends with a 3. When working with slices the end point is interpreted as going up to, but not including the index number provided. In other words, the character in the index position of 3 in the string Alexandria is x, so the slice [0:3] will go up to but not include that character, and therefore give us Ale.

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

The len()function will tell us the length of an item. In the case of a string, it will tell us how many characters are in the string.

PYTHON 

len('Babel')

OUTPUT 

5

Variables only change value when something is assigned to them.

Once a Python variable is assigned it will not change value unless the code is run again. The value of older_age below does not get updated when we change the value of age to 50, for example:

PYTHON 

age = 42
older_age = age + 3
age = 50
f'Older age is {older_age} and age is {age}'

OUTPUT 

Older age is 45 and age is 50

A variable in Python is analogous to a sticky note with a name written on it: assigning a value to a variable is like putting a sticky note on a particular value. When we assigned the variable older_age, it was like we put a sticky note with the name older_age on the value of 45. Remember, 45 was the result of age + 3 because age at that point in the code was equal to 42. The older_age sticky note (variable) was never attached to (assigned to) another value, so it doesn’t change when the age variable is updated to be 50.

F-string Syntax

Use an f-string to construct output in Python by filling in the blanks with variables and f-string syntax to tell Christina how old she will be in 10 years.

Tip: You can combine variables and mathematical expressions in an f-string in the same way you can in variable assignment. We’ll see more examples of dynamic f-string output as we go through the lesson.

PYTHON 

name = 'Christina'
age = 23

f'{____}, you will be ______ in 10 years.'

PYTHON 

f'{name}, you will be {age + 10} in 10 years.'

OUTPUT 

'Christina, you will be 33 in 10 years.'

Swapping Values

Draw a table showing the values of the variables in this program after each statement is executed. In simple terms, what do the last three lines of this program do?

PYTHON 

x = 1.0
y = 3.0
swap = x
x = y
y = swap
swap = x  #  x = 1.0 y = 3.0 swap = 1.0
x = y     #  x = 3.0 y = 3.0 swap = 1.0
y = swap  #  x = 3.0 y = 1.0 swap = 1.0

These three lines exchange the values in x and y using the swap variable for temporary storage. This is a fairly common programming idiom.

Predicting Values

What is the final value of position in the program below? (Try to predict the value without running the program, then check your prediction.)

PYTHON 

initial = "left"
position = initial
initial = "right"

PYTHON 

initial = "left"  # Initial is assigned the string "left"
position = initial  # Position is assigned the variable initial, currently "left"
initial = "right"  # Initial is assigned the string "right"
print(position)

OUTPUT 

left

The last assignment to position was “left”

Can you slice integers?

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

Numbers are not stored in the written representation, so they can’t be treated like strings.

PYTHON 

a = 123
print(a[1])

ERROR 

TypeError: 'int' object is not subscriptable

Slicing

We know how to slice using an explicit start and end point:

PYTHON 

library_name = 'Library of Babel'
f'library_name[1:3] is: {library_name[1:3]}'

OUTPUT 

'library_name[1:3] is: ib'

But we can also use implicit and negative index values when we define a slice. Try the following (replacing low and high with index positions of your choosing) to figure out how these different forms of slicing work:

  1. What does library_name[low:] (without a value after the colon) do?
  2. What does library_name[:high] (without a value before the colon) do?
  3. What does library_name[:] (just a colon) do?
  4. What does library_name[number:negative-number] do?
  1. It will slice the string, starting at the low index and stopping at the end of the string.
  2. It will slice the string, starting at the beginning on the string, and ending an element before the high index.
  3. It will print the entire string.
  4. It will slice the string, starting the number index, and ending a distance of the absolute value of negative-number elements from the end of the string.

Fractions

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

It is a floating-point number (often abbreviated “float”).

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'>

Key Points

  • Use variables to store values.
  • Use print to display values.
  • Format output with f-strings.
  • Variables persist between cells.
  • Variables must be created before they are used.
  • Variables can be used in calculations.
  • Use an index to get a single character from a string.
  • Use a slice to get a portion of a string.
  • Use the built-in function len to find the length of a string.
  • Python is case-sensitive.
  • Every object has a type.
  • Use the built-in function type to find the type of an object.
  • Types control what operations can be done on objects.
  • Variables only change value when something is assigned to them.