What is an expression in Python?

Question

I have some confusion about its meaning or definition.

Isn\'t that some code that produce or calculate new data values? (Says Zelle in his book)

Then I wond

Answer 1

TL;DR: Expressions are combinations of values and operators and always evaluate down to a single value. A statement is every other instruction. Some statements contain expressions.

An expression is an instruction that combines values and operators and always evaluates down to a single value.

For example, this is an expression:

>>> 2 + 2

The 2s are integer values and the + is the mathematical operator. This expression evaluates down to the single integer value 4.

Technically, this is also an expression:

>>> 4

As an expression, it evaluates down to the single value 4.

When I say values and operators, this isn't limited to math problems:

>>> 'You will be ' + str(int(myAge) + 1) + ' next year.'

The myAge variable evaluates to the value inside it. The function call int('5') evaluates to the function's return value, 5. All these string values are combined with the + operator (in this case, it's the string concatenation operator). No matter how big an expression is, it evaluates down to a single value: in this case, the string value 'You will be 6 next year.'

Contrast this with a statement, which is a Python instruction that does not evaluate down to a value. A Python statement is pretty much everything else that isn't an expression. Here's an assignment statement:

>>> spam = 2 + 2

Here's an if statement:

>>> if spam == 4:

Here's a while statement for an infinite loop:

>>> while True:

Note that both of these statements contain expressions (even True, which evaluates down to the single value True). But not all statements use expressions in them. Here's a break statement:

>>> break