Variables and Data Types

What is a variable?

So far you have written numbers directly into print(). Real programs need to remember values and use them several times. A variable is a name that refers to a value stored in the computer's memory. You create one with an assignment statement: the name, an equals sign, and the value.

temperature = 298print(temperature)

Read temperature = 298 as "temperature is assigned the value 298", not as an equation. Python first works out the value on the right of =, then attaches the name on the left to it. After that, wherever you use the name, Python uses the value.

A variable can be given a new value at any time. The old value is replaced. This is why they are called variables: their value can vary while the program runs.

temperature = 298print(temperature)temperature = 350   # replaces the old valueprint(temperature)

Because the right-hand side is worked out first, a variable can even appear on both sides. count = count + 1 means "take the current value of count, add 1, and store the result back in count".

count = 10count = count + 1print(count)

Using variables in calculations

Variables make calculations readable. Instead of a line full of unexplained numbers, each value has a name that says what it is. If a value changes, you only update it in one place.

The examples below use round(value, 2), a built-in function that rounds a number to 2 decimal places. You will learn more ways to format numbers in the Strings lesson.

lattice_constant = 3.615   # ångströmsvolume = lattice_constant ** 3print("Unit cell volume:", round(volume, 2), "ų")

Naming variables

Python has a few strict rules for variable names, and some widely followed conventions that make code easier to read.

• Use descriptive names: melting_point is far clearer than mp or x.
• Write names in lower case with underscores between words. This style is called snake_case.
• Consider putting units in the name when they matter, for example mass_g or temperature_k.

Assigning several variables at once

You can assign several variables in one line by separating the names and values with commas. The first name gets the first value, and so on.

a, b, c = 4.05, 4.05, 4.05   # lattice parameters in Åprint(a, b, c)

Data types

Every value in Python has a type, which decides what you can do with it. You can add two numbers, for example, but not a number and a word. These are the four basic types you will use constantly:

Python works out the type from the value, so you don't have to declare it. The built-in type() function tells you the type of any value.

atoms_per_cell = 4density = 8.96element = "Cu"is_metal = Trueprint(type(atoms_per_cell))print(type(density))print(type(element))print(type(is_metal))

Scientific notation

Very large and very small numbers are written with e, which means times ten to the power of. 6.022e23 is 6.022 × 10²³ and 1.602e-19 is 1.602 × 10⁻¹⁹. Python also uses this notation when it prints very large or very small floats.

avogadro = 6.022e23          # particles per moleelectron_charge = 1.602e-19  # coulombsprint(avogadro)print(electron_charge)

Converting between types

Sometimes a value has the wrong type for what you want to do. Text read from a file or typed by a user is always a string, even if it looks like a number. The functions int(), float() and str() create a new value of the type you ask for. This is called type conversion or casting.

print(float("2.70"))   # text to a floatprint(int("12"))       # text to an integerprint(int(3.9))        # float to integer: the decimal part is droppedprint(str(4) + " atoms")