Python for Everyone: The PreTeXt Interactive Edition

The format operator, % allows us to construct strings, replacing parts of the strings with the data stored in variables. When applied to integers, % is the modulus operator. But when the first operand is a string, % is the format operator.

The first operand is the format string, which contains one or more format sequences that specify how the second operand is formatted. The result is a string.

For example, the format sequence %d means that the second operand should be formatted as an integer (“d” stands for “decimal”):

>>>camels = 42
>>>print('%d' % camels)
42

The result is the string “42”, which is not to be confused with the integer value 42.

A format sequence can appear anywhere in the string, so you can embed a value in a sentence:

If there is more than one format sequence in the string, the second argument has to be a tuple [A tuple is a sequence of comma-separated values inside a pair of parentheses. We will cover tuples in Chapter 10]. Each format sequence is matched with an element of the tuple, in order.

The following example uses %d to format an integer, %g to format a floating-point number (don't ask why), and %s to format a string:

The number of elements in the tuple must match the number of format sequences in the string. The types of the elements also must match the format sequences:

>>> '%d %d %d' % (1, 2)
TypeError: not enough arguments for format string
>>> '%d' % 'dollars'
TypeError: %d format: a number is required, not str

In the first example, there aren't enough elements; in the second, the element is the wrong type.

The format operator is powerful, but it can be difficult to use. You can read more about it at

https://docs.python.org/library/stdtypes.html#printf-style-string-formatting¹.