Python from zero: a value has a type

Python still has types. You do not write them in front of the name. The value carries the type, and type reports it. A cup label needs three values. name is a str, the drink. cents is an int, the price as a whole number of cents. ready is a bool, whether you can hand the drink over. You can add ints. You cannot add a string to a bool.

The value carries the type#

Python does not store the type on the name. The name is a binding to an object, and the object knows its type. cents = 450 binds cents to an int object. A later cents = "latte" rebinds the name. It does not convert the int. type(cents).__name__ asks the object what it is. True is a bool, and bool is a subtype of int, which is why a bool can surprise you in arithmetic. This program prints the label a cup would carry, then the type of each value, so you can see both.

1. namestrlatte
2. centsint450
3. readyboolTrue

name = "latte"
cents = 450
ready = True
status = "ready" if ready else "sold out"
print(f"{name} ${cents // 100}.{cents % 100:02d} {status}")
print(type(name).__name__, type(cents).__name__, type(ready).__name__)

latte $4.50 ready
str int bool

The value decides the type#

name = "latte" stores a str. cents = 450 stores an int, 450 cents, which is $4.50. The int never holds the decimal point. cents // 100 is the dollars. cents % 100 is the leftover cents, and :02d prints them as two digits so 50 stays 50 and 5 would become 05. ready chooses the status word. The second print writes the type names, so you do not have to guess them.

A name can later point at a different type: cents = "450" is allowed, and cents is then a str. That freedom is why it helps to print the type while you are learning a function. If a result surprises you, print type(result).__name__ next to it.

Operators follow the type#

cents + 50 works while cents is an int. name + "!" joins text. name + cents raises TypeError, because a string and an int are not added. The error names both types. Read it. The label builds the dollars in the f-string, where you can see the conversion. The next lesson puts the name and the cents on one object and calls a method that returns the same kind of label.