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Variables, Lists, and Dictionaries (1/3)

Variables

You can think of variables as containers that store some form of data. You can use variables in Python to store pieces of information, and then later recall them when you need them. Variables can be declared and assigned freely in Python, as opposed to other languages where you have to explicitly state the kind of data they will be storing. To assign a value to a variable, use the = operator:

a = 2

Here, a is the name of the variable, and the number '2' is the data I am assigning it. From here on out, a will be associated with the number '2', until it is assigned another value, or the program ends. Try this code:

a = 2
b = 3
print(a + b)

This should print out the number 5, since a is storing the number '2', and b is storing the number '3'. You can use many other common arithmetic operators in the same way. Some of the most common are:

  • + (addition)
  • - (subtraction)
  • * (multiplication)
  • / (division)
  • ** (raise to a power)
  • % (modulo)

In Python, you can name your variables anything, as long as the name starts with a letter, does not contain spaces, and is not a reserved keyword (such as 'print'). In practice, to enhance readability most programmers follow some conventions for naming variables. One common approach is to use ‘camel case’ to make variables composed of multiple words readable without spaces.

withCamelCaseTheFirstWordIsLowerCaseWhileAllSubsequentWordsAreUpperCase

It is also common to substitute underscores ('_') for spaces in variable names. In general, variable names should not be too long, but should err on the side of description rather than brevity to facilitate the code's readability. For instance, if you are keeping track of the number of blueberries, it is better to call the variable 'numBlueberries' than simply 'n' or 'b'.

Variables can hold data of different types. Although Python does not make you explicitly declare the type of data you will be using, it is important to know the types because they will each behave differently in your code. Although there are many different types of data supported by Python, the most common are:

  • int (means integer, or a whole number)
  • float (means floating point, or decimal number)
  • bool (means boolean, or a True/False)
  • str (means string, or ‘a piece of text’)

In Python you can use the type() function to get the type for any piece of data. Try to run the following code:

print(type(12))
print(type(12.1))
print(type(True))
print(type('blueberries'))

You can see that it prints the four types described above. Notice also the particular way in which the data must be written so that Python does not confuse it with the name of a variable. Numbers can be written directly because you cannot name variables with only a number. Booleans must be written capitalized (True or False) as these are reserved key words in Python (notice that the Python editor gives a special color to the 'True' keyword). Strings are always contained within quotes. You can use either single (') or double (") quotes, but they must match on either side of the string. If you try to write:

print(type(blueberries))

without the quotes, you will get the following error:

NameError: name 'blueberries' is not defined

This error tells you that the name ‘blueberries’ is not defined as a variable. However, if you write:

blueberries = 5
print(type(blueberries))

it will tell you that it's a type int, because 'blueberries' is now a variable with an int stored inside of it.

In Python, many operators are 'over-loaded', which means that they function differently depending on the data type that they are used on. For instance, if we run:

print(2 + 2)

we get '4'. When given two numbers, the + operator performs arithmetic addition. However, if we run:

print('First ' + 'Last')

we get 'First Last'. When given two strings, the + operator 'concatenates' or merges them together into one string. Over-loading is useful because it produces clean and readable code without having a special function for each type of variable. You have to be careful, however, because mismatching different types of variables can lead to errors. For instance, running this line:

numBerries = 5
print('Number of Blueberries: ' + numBerries)

will produce an error because it is trying to perform a concatenation of a string and an integer. Instead, you can use the str() function to convert the 5 to a string before using it with the + operator:

numBerries = 5
print('Number of Blueberries: ' + str(numBerries))
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