Expressions

An expression is a combination of constants, variables and operators. The defining property of an expression is that it always evaluates to a value — Python works it out and produces a single result.

Two boundary cases complete the definition:

• A value on its own (like 100) or a standalone variable (like num) is also considered an expression — it trivially evaluates to itself (or to the value it holds).
• A standalone operator is not an expression — + or * by itself has nothing to operate on and produces no value.

Examples of valid expressions

100                      # a value alone is an expression
num                      # a standalone variable is an expression
num - 20.4               # variable combined with a constant
3.0 + 3.14               # two constants and an operator
23/3 - 5 * 7 * (14 - 2)  # a longer arithmetic expression
"Global" + "Citizen"     # a string expression - evaluates to 'GlobalCitizen'

Each of these, when typed at the interpreter prompt, is evaluated down to one value:

>>> 3.0 + 3.14
6.[PII:card]
>>> "Global" + "Citizen"
'GlobalCitizen'

Notice that expressions are not limited to numbers — the last example is a string expression, where + acts as the concatenation operator and the result is a new string. …