Expression Statements

In Python, you can use an expression as a statement, too—that is, on a line by itself. But because the result of the expression won’t be saved, it usually 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

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Some functions and methods do lots of work without returning a value. Such functions are sometimes called procedures in other languages. Because they don’t return values that you might be interested in retaining, you can call these functions with expression statements.

For printing values at the interactive prompt

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 11-4 lists some common expression statement forms in Python. Calls to functions and methods are coded with zero or more argument objects (really, expressions that evaluate to objects) in parentheses, after the function/method name.

Table 11-4. Common Python expression statements

Operation

Interpretation

spam(eggs, ham)

Function calls

spam.ham(eggs)

Method calls

spam

Printing variables in the interactive interpreter

print(a, b, c, sep='')

Printing operations in Python 3.0

yield x ** 2

Yielding expression statements

The last two entries in Table 11-4 are somewhat special cases—as we’ll see later in this chapter, printing in Python 3.0 is a function call usually coded on a line by itself, and the yield operation in generator functions (discussed in Chapter 20) is often coded as a statement as well. Both are really just instances of expression statements.

For instance, though you normally run a print call on a line by itself as an expression statement, it returns a value like any other function call (its return value is None, the default return value for functions that don’t return anything meaningful):

>>> x = print('spam')         # print is a function call expression in 3.0
spam
>>> print(x)                  # But it is coded as an expression statement
None

Also keep in mind that although expressions can appear as statements in Python, statements cannot be used as expressions. For example, Python doesn’t allow you 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. You’ll see how to code around this when you meet the Python while loop in Chapter 13.