The Meaning of True and False in Python

Notice that the test results returned in the last two examples represent true and false values. They print as the words True and False, but now that we’re using logical tests like these in earnest, I should be a bit more formal about what these names really mean.

In Python, as in most programming languages, an integer 0 represents false, and an integer 1 represents true. In addition, though, Python recognizes any empty data structure as false and any nonempty data structure as true. More generally, the notions of true and false are intrinsic properties of every object in Python—each object is either true or false, as follows:

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  • Numbers are true if nonzero.
  • Other objects are true if nonempty.

Table 9-4 gives examples of true and false objects in Python.

Table 9-4. Example object truth values

Object

Value

"spam"

True

""

False

[]

False

{}

False

1

True

0.0

False

None

False

As one application, because objects are true or false themselves, it’s common to see Python programmers code tests like if X:, which, assuming X is a string, is the same as if X != '':. In other words, you can test the object itself, instead of comparing it to an empty object. (More on if statements in Part III.)

The None object

As shown in the last item in Table 9-4, Python also provides a special object called None, which is always considered to be false. None was introduced in Chapter 4; it is the only value of a special data type in Python and typically serves as an empty placeholder (much like a NULL pointer in C).

For example, recall that for lists you cannot assign to an offset unless that offset already exists (the list does not magically grow if you make an out-of-bounds assignment). To preallocate a 100-item list such that you can add to any of the 100 offsets, you can fill it with None objects:

>>> L = [None] * 100
>>>
>>> L
[None, None, None, None, None, None, None, ... ]

This doesn’t limit the size of the list (it can still grow and shrink later), but simply presets an initial size to allow for future index assignments. You could initialize a list with zeros the same way, of course, but best practice dictates using None if the list’s contents are not yet known.

Keep in mind that None does not mean “undefined.” That is, None is something, not nothing (despite its name!)—it is a real object and piece of memory, given a built-in name by Python. Watch for other uses of this special object later in the book; it is also the default return value of functions, as we’ll see in Part IV.

The bool type

Also keep in mind that the Python Boolean type bool, introduced in Chapter 5, simply augments the notions of true and false in Python. As we learned in Chapter 5, the built-in words True and False are just customized versions of the integers 1 and 0—it’s as if these two words have been preassigned to 1 and 0 everywhere in Python. Because of the way this new type is implemented, this is really just a minor extension to the notions of true and false already described, designed to make truth values more explicit:

 

 
  • When used explicitly in truth test code, the words True and False are equivalent to 1 and 0, but they make the programmer’s intent clearer.
  • Results of Boolean tests run interactively print as the words True and False, instead of as 1 and 0, to make the type of result clearer.

You are not required to use only Boolean types in logical statements such as if; all objects are still inherently true or false, and all the Boolean concepts mentioned in this chapter still work as described if you use other types. Python also provides a bool built-in function that can be used to test the Boolean value of an object (i.e., whether it is True—that is, nonzero or nonempty):

>>> bool(1)
True
>>> bool('spam')
True
>>> bool({})
False

In practice, though, you’ll rarely notice the Boolean type produced by logic tests, because Boolean results are used automatically by if statements and other selection tools. We’ll explore Booleans further when we study logical statements in Chapter 12.