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string formats

Use strings to format strings

Many programs need to create strings with particular pieces of data
at particular places, so most computer languages try to make life
easier for you by letting you use string formats.
The best way to understand string formats is with an example. Open
up the Python Shell and type in this: The first value will
be inserted as a The second value will be
5-character number. inserted as a string.

>>> print("There are %5d %s available" % (17, "donuts"))

There are 17 donuts available
The string-formatting
The number has extra spaces operator
added at the front, to make
sure it takes up the required 5
characters.
OK, this string is
When Python sees a string followed by a percentage (%) symbol, followed by a % symbol.
it knows that it needs to consider the string as a string format. So... I‛ll need to replace
The values that follow the % operator are going to be inserted into %5d with the number 17,
the formatted string in the order specified, one value at a time. and %s with the string
Wherever Python finds a % inside the string, it knows to insert a “donuts”.
value.
A number format
specification %5d

When Python sees this, it will insert the value 17 as a 5-character
whole number—that’s because the “d” type specifier tells Python
to print the number as a decimal. Because 17 is not 5-characters
long, Python will left-pad it with 3 extra spaces at the front to make
it the correct length.
A string format %s
specification

This symbol tells Python to insert the value “donuts” as a string.
As you are not specifying a length here, the string is inserted as is.

These are just two examples of how to format
values in a string. There are lots more.

180 Chapter 6