02-variables.md

title	teaching	exercises
Variables in Python	20	5

:::::::::::::::::::::::::::::::::::::: questions

• How do I run Python code?
• What are variables?
• How do I set variables in Python?

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::::::::::::::::::::::::::::::::::::: objectives

• Use the Python console to perform calculations.
• Assign values to variables in Python.
• Reuse variables in Python.

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::::::::::::::::::::::::::::::::::::: keypoints

• Python is an interpreted programming language, and can be used interactively.
• Values are assigned to variables in Python using =.
• You can use print to output variable values.
• Use meaningful variable names.

::::::::::::::::::::::::::::::::::::::::::::::::

Variables 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)
• 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 print to display values.

::: callout A string is the type which stores text in Python. Strings can be thought of as sequences or strings of individual characters (and using the words string this way actually goes back to a printing term in the pre-computer era). We will be learning more about types and strings in other lessons. :::

• 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

::: callout

Single vs. double quotes

In Python, you can use single quotes or double quotes to denote a string, but you need to use the same one for both the beginning and the end.

a = "mouse" # is a string
a = 'mouse' # is a string
a = "mouse'nt" # we can use a single quote inside double quotes without ending the string
a = 'mouse" #This doesn't work

:::

In Python,

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

Ahmed is 42 years old

• print automatically puts a single space between items to separate them.

In when using Jupyter notebooks, we can also simply write a variable name and its value will be displayed:

age

42

However, this will not work in other programming environments or when running scripts. We will be displaying variables using both methods throughout this workshop.

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.
• We will look at error messages in detail later.

::: callout

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. To prevent confusion, it can be helpful to use the Kernel -> Restart & Run All option which clears the interpreter and runs everything from a clean slate going top to bottom. :::

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

Python is case-sensitive.

• Python thinks that upper- and lower-case letters are different, so Name and name are 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.

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
ewr_422_yY = 'Ahmed'
print(ewr_422_yY, 'is', flabadab, 'years old')

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

::: challenge

Swapping Values

Fill the table showing the values of the variables in this program after each statement is executed.

# Command  # Value of x   # Value of y   # Value of swap #
x = 1.0    #              #              #               #
y = 3.0    #              #              #               #
swap = x   #              #              #               #
x = y      #              #              #               #
y = swap   #              #              #               #

::: hint

Try using this python visualization tool to visualize what happens in the code.

:::

::: solution

# Command  # Value of x   # Value of y   # Value of swap #
x = 1.0    # 1.0          # not defined  # not defined   #
y = 3.0    # 1.0          # 3.0          # not defined   #
swap = x   # 1.0          # 3.0          # 1.0           #
x = y      # 3.0          # 3.0          # 1.0           #
y = swap   # 3.0          # 1.0          # 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. ::: :::

::: challenge

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'. ::: :::

::: challenge

Choosing a Name

Which is a better variable name, m, min, or minutes? Why?

::: hint Hint: think about which code you would rather inherit from someone who is leaving the lab:

1. ts = m * 60 + s
2. tot_sec = min * 60 + sec
3. total_seconds = minutes * 60 + seconds ::: ::: solution minutes is better because min might mean something like "minimum" (and actually is an existing built-in function in Python that we will cover later). ::: :::