Casting
It is very common that you will need to convert one data type to another.
Type the following into the terminal:
print("The meaning of life is " + 42)
You will the following TypeError:
TypeError: can only concatenate str (not "int") to str
This is because you are trying to add a str and an int. Python does not know how to do this. To correct this you need to do something called casting.
The following code casts (converts) the int to a str using the constructor function str():
print("The meaning of life is " + str(42))
If you try this you will see that it works.
Type str(42) into the terminal on its own and you will see that it returns the str '42' (note it uses single quotes, but that is the same as double quotes in Python!).
Note that we could have done the following:
print(f"The meaning of life is {42}")
This is because the python f-string knows how to convert the number 42 to play nicely with the string. Try it in the terminal.
1. How to Cast
Casting in python is therefore done using constructor functions:
| Function | Description |
|---|---|
int() | Constructs an integer number from an integer, a float (by removing all decimals), or a string (providing the string represents a whole number) |
float() | Constructs a float number from an integer, a float, or a string (providing the string represents a float or an integer) |
str() | Constructs a string from a wide variety of data types, including strings, integers, floats, and booleans |
bool() | Constructs a boolean from a wide variety of data types, including strings, integers, floats, and booleans |
1.1 Examples
Try these in the terminal:
int(1) # Creates an int with value 1
int("2") # Creates an int with value 2
int(4.3) # Creates an int with value 4
int(True) # Creates an int with value 1
int(False) # Creates an int with value 0
float(1) # Creates a float with value 1.0
float("2") # Creates a float with value 2.0
float("3.142") # Creates a float with value 3.142
float(4.3) # Creates a float with value 4.3
float(True) # Creates a float with value 1.0
float(False) # Creates a float with value 0.0
str(1) # Creates a str with value '1'
str("3.142") # Creates a str with value '3.142'
str(4.3) # Creates a str with value '4.3'
str(True) # Creates a str with value 'True'
str(False) # Creates a str with value 'False'
bool() can throw up some unexpected results unless you understand what it is doing.
You might think that bool("0") would result in False, it doesn't!
Python treats everything as True other than False, 0, an empty string "" and some other things we have yet to encounter, empty lists, dictionaries, tuples, and the None keyword which represents no value at all (more on that later).
bool(1) # Creates a bool with value True
bool("3.142") # Creates a bool with value True
bool(4.3) # Creates a bool with value True
bool("0") # Creates a bool with value True
bool(True) # Creates a bool with value True
bool(False) # Creates a bool with value False
bool(0) # Creates a bool with value False
bool("") # Creates a bool with value False
bool([]) # Creates a bool with value False
bool({}) # Creates a bool with value False
bool(()) # Creates a bool with value False
bool(None) # Creates a bool with value False
=== TASK ===
Create a new Python file and write a program that outputs the following to the terminal for a given X and Y.
The result of multiplying X by Y is X*Y
For example, for 2.1 and 3:
Please enter a number:
2.1
Please enter another number:
3
The result of multiplying 2.1 by 3.0 is 6.3
For example, for 5.2 and 3.4:
Please enter a number:
5.2
Please enter another number:
3.4
The result of multiplying 5.2 by 3.4 is 17.68