Type Conversions (Casting)

Earlier in this chapter of the notes, we learned about different types and doing operations on them, such as adding ints together (17 + 32).

Some of these operations result in a value of the same type (17 + 32 is 49, which is also an int), while some operations change the type. For instance, 17 + 32.0 changes the result into 49.0, which is a float.

There are two general ways to convert values from one type to another:

  1. With math.
  2. With casting functions.

Before we get into the details of these, let’s take a look at the type() function. This is a function like the print() function. It takes one argument of any value and tells you what type that value is:

# TODO: run this code
type(7)  # the output should be "int"


# TODO: try out the type function on some different values
type("hi")

We can also pass a variable to the type() function. When we pass a variable to the type() function, we are asking “what is the type of the value stored in this variable?”.

# TODO: run this code, then play around with it more.
# we can also pass variables to the type 
# function as its argument
x = 12.4
type(x)

Now that we know another way (with jupyter notebooks we have the whos notebook tool) to find the type of variables, let’s learn about actually changing data types.

The Math Way

When we are talking about numbers, we can convert from int to float just by doing math.

Whenever we multiply, divide, add, or subtract a float with an int, the result will always be a float.

For example:

x = 3 + 4 
print(type(x))  # int
y = 3.0 + 4
print(type(y))  # float

This means that if you have an int but you want the same number as a float, if you multiply it by 1.0, you will have your desired result.

# TODO: convert the variable z to a float using math.
z = 12

When we use math to convert types in python, we can change from int to float but we can’t do the reverse! Even if we use integer divide //, this won’t give us an int if any of the numbers started as floats!

The Casting Way

The most general way to convert values to a different type is by using casting functions. These are functions that take one value and give you back that same value, but as a different type. There are 4 basic casting functions that match our 4 basic types: int(), float(), str(), bool().

The int() function takes any value and returns it as an int. For example, int(7.0) gives you the value 7.

What do you think happens if you do int(7.2)? What about int(7.9)?

# TODO: run this code. play with the int() function

print(int(7.2))
print(int(7.9))

Notice that we put the int(7.2) function inside a print() function. This is so that we can print out the results and can see them.

With casting functions, if you want to be able to use the results, you must always save them in a variable.

x = 7.9
x = int(x) # now I can access the results!
print(x)

People frequently make the mistake of forgetting to save their results:

x = 7.9
int(x) # DANGER!!!! Not saved!
print(x)

The other three casting functions work the same as the int() function.

# TODO: play with float()
val = 8
val = float(val)


# TODO: play with str()
val = 8
val = str(val)


# TODO: play with bool()
# We'll talk more about booleans later but they have behavior that you
# might not expect when casting, so they are fun to play with
val = 8
val = bool(val)

val2 = 0
val2 = bool(val)

Lastly, it’s important to note that these casting functions don’t always work! Sometimes, they can cause errors.

# TODO: run this code
int("hello")  # What does this even mean anyways?