Data types and type conversion

A variable's type decides what you can do with the value inside it. This week we go through the built-in types and how to move between them.

The built-in types

text = "CSC 102"          # str
count = 102               # int
price = 2.33              # float
scores = [90, 40, 80]     # list  (mutable)
point = (3, 4)            # tuple (immutable)
grades = {"CSC": 89}      # dict
passed = True             # bool

Numbers, strings, lists, tuples, booleans

• A string is text in quotes. You can index and slice it.
• A number is an int or a float; mixing them in arithmetic gives a float.
• A list holds an ordered, changeable collection: ["a", "b", "c"].
• A tuple is like a list but cannot be changed once made: (1, 2).
• A boolean is True or False, which is what conditions evaluate to.

Type conversion

Sometimes the data you have is not the type you need. The classic case: input from a user always arrives as a string, so "5" + "3" is "53", not 8. To convert, call the type's name as a function.

# string to number
age = int("19")        # 19
gpa = float("4.33")    # 4.33
 
# number to string
label = str(102)       # "102"
 
# between collections
letters = list("abc")  # ['a', 'b', 'c']
unique = set([1, 1, 2])  # {1, 2}

A common bug, and its fix:

n = input("Enter a number: ")   # n is a string, e.g. "5"
total = int(n) + 10             # convert before doing maths
print(total)

Practice

In a Week_3 notebook:

1. Create one value of each built-in type and print its type(...).
2. Ask the user for two numbers with input, convert them, and print the sum, difference, and product.
3. Turn the string "112358" into a list of its digits as integers.