3.2 Expressions

In Python, you can use expressions as statements too. But since the result of the expression won't be saved, it makes sense to do so only if the expression does something useful as a side effect. Expressions are commonly used as statements in two situations:

For calls to functions and methods

Some functions and methods do lots of work without returning a value. Since you're not interested in retaining the value they return, you can call such functions with an expression statement. Such functions are sometimes called procedures in other languages; in Python, they take the form of functions that don't return a value.

For printing values at the interactive prompt

As we've already seen, Python echoes back the results of expressions typed at the interactive command line. Technically, these are expression statements too; they serve as a shorthand for typing print statements.

Table 3.3 lists some common expression statement forms in Python; we've seen most before. Calls to functions and methods are coded with a list of objects (really, expressions that evaluate to objects) in parentheses after the function or method.

Table 3.3. Common Python Expression Statements

Operation	Interpretation
spam(eggs, ham)	Function calls
spam.ham(eggs)	Method calls
spam	Interactive print
spam < ham and ham != eggs	Compound expressions
spam < ham < eggs	Range tests

The last line in the table is a special form: Python lets us string together magnitude comparison tests, in order to code chained comparisons such as range tests. For instance, the expression (A < B < C) tests whether B is between A and C; it's equivalent to the Boolean test (A < B and B < C) but is easier on the eyes (and keyboard). Compound expressions aren't normally written as statements, but it's syntactically legal to do so and can even be useful at the interactive prompt if you're not sure of an expression's result.

Although expressions can appear as statements in Python, statements can't be used as expressions. For instance, unlike C, Python doesn't allow us to embed assignment statements (=) in other expressions. The rationale for this is that it avoids common coding mistakes; you can't accidentally change a variable by typing = when you really mean to use the == equality test.