Variables and Assignment

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I store data in programs?

Objectives

• Write programs that assign scalar values to variables and perform calculations with those values.

• Correctly trace value changes in programs that use scalar assignment.

Use variables to store values.

• Variables are names for values.
• In Python the = symbol assigns the value on the right to the name on the left.
• The variable is created when a value is assigned to it.

• Here, Python assigns an age to a variable age and a name in quotes to a variable first_name.

  age = 42
  first_name = 'Ahmed'
  

• Variable names
  • can only contain letters, digits, and underscore _ (typically used to separate words in long variable names)
  • cannot start with a digit
  • are case sensitive (age, Age and AGE are three different variables)
  • There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.
• Variable names that start with underscores like __alistairs_real_age have a special meaning so we won’t do that until we understand the convention.

Use meaningful variable names.

• Python doesn’t care what you call variables as long as they obey the rules (alphanumeric characters and the underscore).

flabadab = 42

• Use meaningful variable names to help other people understand what the program does.
• The most important “other person” is your future self.

Use comments to add documentation to programs.

• If you want to add information to explain what your code is doing, you can use the # to indicate a comment.

  # This sentence isn't executed by Python.
  flabadab = 42   #  current age - anything after '#' is ignored.
  

• While using meaningful variable names helps avoid needing to explain with comments, if it is difficult to come up with a relatively short relevant variable name, this can help.

Use print to display values.

• Python has a built-in function called print that prints things as text.
• Call the function (i.e., tell Python to run it) by using its name.
• Provide values to the function (i.e., the things to print) in parentheses.
• To add a string to the printout, wrap the string in single or double quotes.
• The values passed to the function are called arguments

print(first_name, 'is', age, 'years old')

Ahmed is 42 years old

• print automatically puts a single space between items to separate them.
• And wraps around to a new line at the end.

Variables must be created before they are used.

• If a variable doesn’t exist yet, or if the name has been mis-spelled, Python reports an error. (Unlike some languages, which “guess” a default value.)

print(last_name)

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

NameError: name 'last_name' is not defined

• The last line of an error message is usually the most informative.

Syntax errors are another common error seen.

• Python reports a syntax error when it can’t understand the source of a program.
• Won’t even try to run the program if it can’t be parsed.

# Forgetting an end quote causes a syntax error.
last_name = 'Feng

  File "<ipython-input-56-f42768451d55>", line 2
    name = 'Feng
                ^
SyntaxError: EOL while scanning string literal

• Look more closely at the error message to see what went wrong.

Variables Persist Between Cells

Be aware that it is the order of execution of cells that is important in a Jupyter notebook, not the order in which they appear. Python will remember all the code that was run previously, including any variables you have defined, irrespective of the order in the notebook. Therefore if you define variables lower down the notebook and then (re)run cells further up, those defined further down will still be present. As an example, create two cells with the following content, in this order:

print(myval)

myval = 1

If you execute this in order, the first cell will give an error. However, if you run the first cell after the second cell it will print out 1.

Variables can be used in calculations.

• We can use variables in calculations just as if they were values.
  • Remember, we assigned the value 42 to age a few lines ago.

age = age + 3
print('Age in three years:', age)

Age in three years: 45

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.)

initial = 'left'
position = initial
initial = 'right'

Solution

print(position)

left

The initial variable is assigned the value 'left'. In the second line, the position variable also receives the string value 'left'. In third line, the initial variable is given the value 'right', but the position variable retains its string value of 'left'.

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.

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

first is 2 and second is 5

• The computer reads the value of first when doing the multiplication, creates a new value, and assigns it to second.
• After that, second does not remember where it came from.

Key Points

• Use variables to store values.

• Use print to display values.

• Variables persist between cells.

• Variables must be created before they are used.

• Variables can be used in calculations.

• Python is case-sensitive.

• Use meaningful variable names.