Variables and types

In Python, a variable is simply a name we bind a value to. You don't need to declare it with a keyword like in JavaScript: just assign it something with the = symbol.

saluto = "Ciao, mondo!"
anni = 30
pi_greco = 3.14
attivo = True

From that moment on, every time you write saluto Python replaces it with the string "Ciao, mondo!".

Built-in types you'll always encounter

The five basic types you'll use in the first weeks are:

• int — integers, with no size limit: 42, -7, 1_000_000.
• float — numbers with a decimal point: 3.14, -0.5, 2e10.
• str — text, in single or double quotes: "ciao", 'mondo'.
• bool — truth values: True or False (capitalized!).
• NoneType — its only instance is None, the equivalent of "no value".

You can always ask for the type of a value with the type(...) function:

type(42)        # <class 'int'>
type(3.14)      # <class 'float'>
type("ciao")    # <class 'str'>
type(True)      # <class 'bool'>
type(None)      # <class 'NoneType'>

Reassignment and dynamic types

Python is dynamically typed: the same variable can point over time to values of different types. It's not a good practice to abuse it, but it's legal:

x = 10        # ora x è int
x = "dieci"   # ora x è str

The last expression is displayed

In our exercises (and in the Playground) the last expression of a block is captured as the "return value" and shown next to the output panel, REPL-style. So you can simply write the name of a variable as the last line to display its value.

Conventions and strong typing

Python is a strongly typed language: it does not perform implicit type conversions between incompatible types. For instance, summing a string and an integer ("Years: " + 30) will raise a TypeError. To achieve this, you must explicitly convert the number to a string using str(30). Additionally, variables in Python are references to objects in memory: when you write a = 5 and then b = a, both variables point to the same integer in memory.

Try it yourself

Review exercise

Additional challenge