Variables and Assignment

Variables and Assignment#

Note

Source: Drawn from the SE4ML Python chapter (chapter_python.rst, lines 442–484) and the C# edition (data/variables.rst). Augmented assignment, swap idiom, and no-declaration discussion are Python-specific additions based on the SE4ML presentation.

Programs need to store and refer to data. Variables are names that refer to values stored in memory.

No Declarations Needed#

In C# or Java, you must declare a variable before using it, specifying its type:

// C# — not Python
double width = 5.0;

In Python, you simply assign a value. The variable is created automatically and takes the type of whatever you assign to it:

>>> width = 5.0
>>> width
5.0

The value has a type; the variable does not. You can assign a different type to the same variable name later (though this is rarely a good idea):

>>> x = 10
>>> type(x)
<class 'int'>
>>> x = 3.14
>>> type(x)
<class 'float'>

Assignment is Right-to-Left#

An assignment statement evaluates the expression on the right side first, then stores the result in the variable on the left:

>>> area = 5 * 7
>>> area
35

The variable on the left receives the result. This works even when the same variable appears on both sides:

>>> n = 3
>>> n = n + 1
>>> n
4

The right side is evaluated first (n + 1 = 4), then that value replaces the old value of n. This is not a mathematical equation — it is an instruction to update a stored value.

Naming Rules and Conventions#

Variable names (called identifiers) must:

  • Start with a letter or underscore (_).

  • Contain only letters, digits, and underscores.

  • Not be a Python keyword (like if, for, while, def).

Python is case-sensitive: total, Total, and TOTAL are three different variables.

Convention: Use snake_case for variable names — all lowercase with underscores between words:

wall_area = 488.0
room_length = 20.5
student_count = 30

By convention, names written in ALL_CAPS indicate constants — values that are not intended to change:

HEIGHT = 8
PI = 3.14159

Python does not enforce this convention, but it is widely followed and helps readers understand your program.

Multiple and Augmented Assignment#

You can assign the same value to several variables at once:

>>> i = j = 0
>>> i
0
>>> j
0

You can also assign to multiple variables from multiple values in one line:

>>> x, y = 3, 4
>>> x
3
>>> y
4

This is called tuple unpacking (discussed further in the Tuples chapter). A particularly elegant use is swapping two values without a temporary variable:

>>> a, b = 10, 20
>>> a, b = b, a
>>> a
20
>>> b
10

Augmented assignment operators combine an operation with assignment:

>>> n = 5
>>> n += 3     # same as n = n + 3
>>> n
8
>>> n *= 2     # same as n = n * 2
>>> n
16

The operators +=, -=, *=, /=, //=, %=, and **= all work the same way.

Exercises#

Think what the printed result would be, then check in the shell:

x = 1
x = x + 1
x = x * 3
x = x * 5
print(x)

Another to try:

a = 5
a = a // 2
a = a + 1
a = a * 2
print(a)